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Freescale Semiconductor Technical Data
An Energy Efficient Solution by Freescale
MPR121 Rev 2, 04/2010
Advanced Information Proximity Capacitive Touch Sensor Controller
MPR121 OVERVIEW
The MPR121 is the second generation sensor controller after the initial release of the MPR03x series devices. The MPR121 will feature increased internal intelligence in addition to Freescale's second generation capacitance detection engine. Some of the major additions include an increased electrode count, a hardware configurable I2C address, an expanded filtering system with debounce, and completely independent electrodes with auto-configuration built in. The device also features a 13th simulated electrode that represents the simultaneous charging of all the electrodes connected together to allow for increased proximity detection in a touch panel or touch screen array. Features * * * * * * * * * * * * * * * * * 1.71 V to 3.6 V operation 29 A supply current at 16 ms sample period 3 A shutdown current 12 electrodes Continuous independent auto-calibration for each electrode input Separate touch and release trip thresholds for each electrode, providing hysteresis and electrode independence I2C interface, with IRQ output to advise electrode status changes 3 mm x 3 mm x 0.65 mm 20 lead QFN package LED driver functionality with 8 shared LEDs -40C to +85C operating temperature range Switch Replacements Touch Pads PC Peripherals MP3 Players Remote Controls Mobile Phones Lighting Controls ORDERING INFORMATION
Device Name MPR121QR2 Temperature Range -40C to +85C Case Number 2059 (20-Pin QFN) Touch Pads 12-pads
MPR121
Capacitive Touch Sensor Controller
Bottom View
20-PIN QFN CASE 2059-01
Top View
ELE11 ELE10 ELE9 ELE8 15 14 13 12 11 6 VSS 7 REXT 8 ELE0 9 ELE1 10 ELE2 VDD 1 2 3 4 5
20 19 18 17 16 IRQ SCL SDA ADDR VREG ELE7 ELE6 ELE5 ELE4 ELE3
Implementations
Typical Applications
Pin Connections
IRC Address 0x5A - 0x5D
Shipping Tape & Reel
This document contains a product under development. Freescale Semiconductor reserves the right to change or discontinue this product without notice.
(c) Freescale Semiconductor, Inc., 2009, 2010. All rights reserved.
PIN DESCRIPTION
Pin Description
Pin No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Pin Name IRQ SCL SDA ADDR VREG VSS REXT ELE0 ELE1 ELE2 ELE3 ELE4 (LED0) ELE5 (LED1) ELE6 (LED2) ELE7 (LED3) ELE8 (LED4) ELE9 (LED5) ELE10 (LED6) ELE11 (LED7) VDD Description Open Collector Interrupt pin I2C Clock I2C Data I2C Address Select VREG - 0.1 F cap connect Ground External Resistor - 75 k Electrode 0 Electrode 1 Electrode 2 Electrode 3 Electrode 4 Electrode 5 Electrode 6 Electrode 7 Electrode 8 Electrode 9 Electrode 10 Electrode 11 VDD
MPR121 2 Sensors Freescale Semiconductor
SCHEMATIC DRAWINGS AND IMPLEMENTATION
VDD 1.71 V to 2.75 V VDD 1.71 V to 2.75 V 0.1 F
20 6 5 1 2 3 4 7
VDD VSS VREG IRQ SCL SDA ADDR REXT
ELE11/LED7 ELE10/LED6 ELE9/LED5 ELE8/LED4 ELE7/LED3 ELE6/LED2 ELE5/LED1 ELE4/LED0 ELE3
19 18 17 16 15 14 13 12 11 10 9 8
75 k 1% GND GND
ELE2 ELE1 ELE0 MPR121Q TOUCH SENSOR
Figure 1. Configuration 1: MPR121 runs from a 1.71 V to 2.75 V supply.
VDD 2.5 V to 3.6 V VDD 2.5 V to 3.6 V 0.1 F
20 6 5 1 2 3 4 7
VDD VSS VREG IRQ SCL SDA ADDR REXT
ELE11/LED7 ELE10/LED6 ELE9/LED5 ELE8/LED4 ELE7/LED3 ELE6/LED2 ELE5/LED1 ELE4/LED0 ELE3
19 18 17 16 15 14 13 12 11 10 9 8
0.1 F 75 k 1% GND GND GND
ELE2 ELE1 ELE0 MPR121Q TOUCH SENSOR
Figure 2. Configuration 2: MPR121 runs from a 2.5 V to 3.6 V supply. Capacitance Sensing The MPR121 uses a constant current touch sensor system with two primary types of control. It can measure capacitances ranging from 10 pF to 2000 pF by varying the current and the amount of time supplied to each electrode. The electrodes are controlled independently allowing for a great deal of flexibility in electrode pattern design. To make setup of the device easier, an automatic configuration system can be used to set the ideal capacitance of each electrode. For information on how to set up this system refer to application note AN3889. Once capacitance is calculated, it runs through a couple of levels of digital filtering allowing for good noise immunity in different environments without sacrificing response time or power consumption. The MPR121 can be configured for sample rates between 1 ms and 128 ms. For information on how to set up this system refer to application note AN3890.
MPR121 3 Sensors Freescale Semiconductor
Touch Sensing Once the capacitance is determined at any given moment, this information must then be translated into intelligent touch recognition. The MPR121 has a couple of systems that have improved over the previous generation in the MPR03x series devices. A baseline tracking system allows the system to track the untouched capacitance in the system. For information on how to set up the baseline capacitance system refer to application note AN3891. The baseline value is then compared with the current value to determine if a touch has occurred. A designer has the ability to set both the rising and falling thresholds in addition to a debounce to eliminate jitter and false touches due to noise. These elements are described in application note AN3892. Proximity Sensing A new feature of the MPR121 is the use of a proximity sensing system whereby all of a system's electrodes can be shorted together internally and create a single large electrode. The capacitance of this electrode is larger and projected capacitance can be measured. When enabled, this "13th" electrode will be included at the end of a normal detection cycle and will have its own independent set of configuration registers. This system is described in application note AN3893. LED Driver The MPR121 includes eight shared LED driving pins. When these pins are not configured as electrodes, they may be used to drive LEDs. The system allows for both pull up and pull down LED configurations as well as general GPIO push/pull functionality. The configuration of the LED driver system is described in application note AN3894. Serial Communication The MPR121 is an Inter-Integrated Circuit (I2C) compliant device with an additional interrupt that is triggered any time a touch or release of a button is detected. The device has a configurable I2C address by connecting the ADDR pin to the VSS, VDD, SDA or SCL lines. The resulting I2C addresses are 0x5A, 0x5B, 0x5C and 0x5D respectively. The specific details of this system are described in AN3895. For reference the register map of the MPR121 is included in Table 1.
MPR121 4 Sensors Freescale Semiconductor
Table 1. Register Map
REGISTER ELE0 - ELE7 Touch Status ELE8 - ELE11, ELEPROX Touch Status ELE0-7 OOR Status ELE8-11, ELEPROX OOR Status ELE0 Electrode Filtered Data LSB ELE0 Electrode Filtered Data MSB ELE1 Electrode Filtered Data LSB ELE1 Electrode Filtered Data MSB ELE2 Electrode Filtered Data LSB ELE2 Electrode Filtered Data MSB ELE3 Electrode Filtered Data LSB ELE3 Electrode Filtered Data MSB ELE4 Electrode Filtered Data LSB ELE4 Electrode Filtered Data MSB ELE5 Electrode Filtered Data LSB ELE5 Electrode Filtered Data MSB ELE6 Electrode Filtered Data LSB ELE6 Electrode Filtered Data MSB ELE7 Electrode Filtered Data LSB ELE7 Electrode Filtered Data MSB ELE8 Electrode Filtered Data LSB ELE8 Electrode Filtered Data MSB ELE9 Electrode Filtered Data LSB ELE9 Electrode Filtered Data MSB ELE10 Electrode Filtered Data LSB ELE10 Electrode Filtered Data MSB ELE11 Electrode Filtered Data LSB ELE11 Electrode Filtered Data MSB ELEPROX Electrode Filtered Data LSB ELEPROX Electrode Filtered Data MSB ELE0 Baseline Value ELE1 Baseline Value ELE2 Baseline Value ELE3 Baseline Value ELE4 Baseline Value ELE5 Baseline Value ELE6 Baseline Value ELE7 Baseline Value ELE8 Baseline Value ELE9 Baseline Value ELE10 Baseline Value ELE11 Baseline Value ELEPROX Baseline Value MHD Rising NHD Amount Rising NCL Rising FDL Rising MHD Falling NHD Amount Falling NCLR FDLR MHDF NHDF E0BV E1BV E2BV E3BV E4BV E5BV E6BV E7BV E8BV E9BV E10BV E11BV EPROXBV MHDR NHDR EFDPROXLB EFDPROXHB EFD11LB EFD11HB EFD10LB EFD10HB EFD9LB EFD9HB EFD8LB EFD8HB EFD7LB EFD7HB EFD6LB EFD6HB EFD5LB EFD5HB EFD4LB EFD4HB EFD3LB EFD3HB EFD2LB EFD2HB EFD1LB EFD1HB ELE7 OVCF ELE7 ARFF ELE6 ACFF ELE5 ELE6 ELE5 ELE4 ELEPROX ELE4 ELEPROX Fields ELE3 ELE11 ELE3 ELE11 ELE2 ELE10 ELE2 ELE10 ELE1 ELE9 ELE1 ELE9 ELE0 ELE8 ELE0 ELE8 Register Address 0x00 0x01 0x02 0x03 0x04 EFD0HB 0x05 0x06 0x07 0x08 0x09 0x0A 0x0B 0x0C 0x0D 0x0E 0x0F 0x10 0x11 0x12 0x13 0x14 0x15 0x16 0x17 0x18 0x19 0x1A 0x1B 0x1C 0x1D 0x1E 0x1F 0x20 0x21 0x22 0x23 0x24 0x25 0x26 0x27 0x28 0x29 0x2A 0x2B 0x2C 0x2D 0x2E 0x2F 0x30 Initial Value 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 Register Address + 1 Auto Increment Address
EFD0LB
MPR121 Sensors Freescale Semiconductor 5
Table 1. Register Map
REGISTER NCL Falling FDL Falling NHD Amount Touched NCL Touched FDL Touched ELEPROX MHD Rising ELEPROX NHD Amount Rising ELEPROX NCL Rising ELEPROX FDL Rising ELEPROX MHD Falling ELEPROX NHD Amount Falling ELEPROX NCL Falling ELEPROX FDL Falling ELEPROX NHD Amount Touched ELEPROX NCL Touched ELEPROX FDL Touched ELE0 Touch Threshold ELE0 Release Threshold ELE1 Touch Threshold ELE1 Release Threshold ELE2 Touch Threshold ELE2 Release Threshold ELE3 Touch Threshold ELE3 Release Threshold ELE4 Touch Threshold ELE4 Release Threshold ELE5 Touch Threshold ELE5 Release Threshold ELE6 Touch Threshold ELE6 Release Threshold ELE7 Touch Threshold ELE7 Release Threshold ELE8 Touch Threshold ELE8 Release Threshold ELE9 Touch Threshold ELE9 Release Threshold ELE10 Touch Threshold ELE10 Release Threshold ELE11 Touch Threshold ELE11 Release Threshold ELEPROX Touch Threshold ELEPROX Release Threshold Debounce Touch & Release AFE Configuration Filter Configuration Electrode Configuration ELE0 Electrode Current ELE1 Electrode Current ELE2 Electrode Current CL FFI CDT EL CDC0 CDC1 CDC2 SFI EleEn DR CDC ESI NCLPROXT FDLPROXT E0TTH E0RTH E1TTH E1RTH E2TTH E2RTH E3TTH E3RTH E4TTH E4RTH E5TTH E5RTH E6TTH E6RTH E7TTH E7RTH E8TTH E8RTH E9TTH E9RTH E10TTH E10RTH E11TTH E11RTH EPROXTTH EPROXRTH DT NCLPROXF FDLPROXF NHDPROXT NCLPROXR FDLPROXR MHDPROXF NHDPROXF NCLT FDLT MHDPROXR NHDPROXR Fields NCLF FDLF NHDT Register Address 0x31 0x32 0x33 0x34 0x35 0x36 0x37 0x38 0x39 0x3A 0x3B 0x3C 0x3D 0x3E 0x3F 0x40 0x41 0x42 0x43 0x44 0x45 0x46 0x47 0x48 0x49 0x4A 0x4B 0x4C 0x4D 0x4E 0x4F 0x50 0x51 0x52 0x53 0x54 0x55 0x56 0x57 0x58 0x59 0x5A 0x5B 0x5C 0x5D 0x5E 0x5F 0x60 0x61 Initial Value 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x10 0x04 0x00 0x00 0x00 0x00 Register Address + 1 Auto Increment Address
MPR121 6 Sensors Freescale Semiconductor
Table 1. Register Map
REGISTER ELE3 Electrode Current ELE4 Electrode Current ELE5 Electrode Current ELE6 Electrode Current ELE7 Electrode Current ELE8 Electrode Current ELE9 Electrode Current ELE10 Electrode Current ELE11 Electrode Current ELEPROX Electrode Current ELE0, ELE1 Charge Time ELE2, ELE3 Charge Time ELE4, ELE5 Charge Time ELE6, ELE7 Charge Time ELE8, ELE9 Charge Time ELE10, ELE11 Charge Time ELEPROX Charge Time GPIO Control Register 0 GPIO Control Register 1 GPIO Data Register GPIO Direction Register GPIO Enable Register GPIO Data Set Register GPIO Data Clear Register GPIO Data Toggle Register AUTO-CONFIG Control Register 0 AUTO-CONFIG Control Register 1 AUTO-CONFIG USL Register AUTO-CONFIG LSL Register AUTO-CONFIG Target Level Register SCTS USL LSL TL CTL011 CTL111 DAT11 DIR11 EN11 SET11 CLR11 TOG11 AFES CTL010 CTL110 DAT10 DIR10 EN10 SET10 CLR10 TOG10 CTL09 CTL19 DAT9 DIR9 EN9 SET9 CLR9 TOG9 CTL08 CTL18 DAT8 DIR8 EN8 SET8 CLR8 TOG8 CTL07 CTL17 DAT7 DIR7 EN7 SET7 7CLR7 TOG7 BVA OORIE CTL06 CTL16 DAT6 DIR6 EN6 SET6 CLR6 TOG6 CDT1 CDT3 CDT5 CDT7 CDT9 CDT11 CDC3 CDC4 CDC5 CDC6 CDC7 CDC8 CDC9 CDC10 CDC11 CDCPROX CDT0 CDT2 CDT4 CDT6 CDT8 CDT10 CDTPROX CTL05 CTL15 DAT5 DIR5 EN5 SET5 CLR5 TOG5 ARE ARFIE CTL04 CTL14 DAT4 DIR4 EN4 SET4 CLR4 TOG4 ACE ACFIE Fields Register Address 0x62 0x63 0x64 0x65 0x66 0x67 0x68 0x69 0x6A 0x6B 0x6C 0x6D 0x6E 0x6F 0x70 0x71 0x72 0x73 0x74 30x75 0x76 0x77 0x78 0x79 0x7A 0x7B 0x7C 0x7D 0x7E 0x7F Initial Value 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 Register Address + 1 Auto Increment Address
RETRY
MPR121 Sensors Freescale Semiconductor 7
ELECTRICAL CHARACTERISTICS
Absolute Maximum Ratings Absolute maximum ratings are stress ratings only, and functional operation at the maxima is not guaranteed. Stress beyond the limits specified in Table 2 may affect device reliability or cause permanent damage to the device. For functional operating conditions, refer to the remaining tables in this section. This device contains circuitry protecting against damage due to high static voltage or electrical fields; however, it is advised that normal precautions be taken to avoid application of any voltages higher than maximum-rated voltages to this high-impedance circuit. Table 2. Absolute Maximum Ratings - Voltage (with respect to VSS)
Rating Supply Voltage Supply Voltage Input Voltage Symbol VDD VREG VIN TO iGPIO iGPIO TS Value -0.3 to +3.6 -0.3 to +2.75 VSS - 0.3 to VDD + 0.3 -40 to +85 12 1.2 -40 to +125 Unit V V V C mA mA C
SCL, SDA, IRQ
Operating Temperature Range GPIO Source Current per Pin GPIO Sink Current per Pin Storage Temperature Range
ESD AND LATCH-UP PROTECTION CHARACTERISTICS
Normal handling precautions should be used to avoid exposure to static discharge. Qualification tests are performed to ensure that these devices can withstand exposure to reasonable levels of static without suffering any permanent damage. During the device qualification ESD stresses were performed for the Human Body Model (HBM), the Machine Model (MM) and the Charge Device Model (CDM). A device is defined as a failure if after exposure to ESD pulses the device no longer meets the device specification. Complete DC parametric and functional testing is performed per the applicable device specification at room temperature followed by hot temperature, unless specified otherwise in the device specification. Table 3. ESD and Latch-up Test Conditions
Rating Human Body Model (HBM) Machine Model (MM) Charge Device Model (CDM) Latch-up current at TA = 85C Symbol VESD VESD VESD ILATCH Value 2000 200 500 100 Unit V V V mA
MPR121 8 Sensors Freescale Semiconductor
DC CHARACTERISTICS
This section includes information about power supply requirements and I/O pin characteristics. Table 4. DC Characteristics (Typical Operating Circuit, VDD and VREG = 1.8 V, TA = 25C, unless otherwise noted.)
Parameter High Supply Voltage Low Supply Voltage Average Supply Current Average Supply Current Average Supply Current Average Supply Current Average Supply Current Average Supply Current Average Supply Current Average Supply Current Measurement Supply Current Idle Supply Current Input Leakage Current ELE_ Input Capacitance ELE_ Input High Voltage SDA, SCL Input Low Voltage SDA, SCL Input Leakage Current SDA, SCL Input Capacitance SDA, SCL Output Low Voltage SDA, IRQ Output High Voltage ELE4 - ELE11 (GPIO mode) Output Low Voltage ELE4 - ELE11 (GPIO mode) Power On Reset VOL VOHGPIO IOL = 6mA VDD = 2.7 V to 3.6 V: IOHGPIO = -10 mA VDD = 2.3 V to 2.7 V: IOHGPIO = -6 mA VDD = 1.8 V to 2.3 V: IOHGPIO = -3 mA IOLGPIOD = 1 mA VDD rising VDD falling 1.08 0.88 1.35 1.15 VDD - 0.5 VIH VIL IIH, IIL 0.025 0.7 x VDD 0.3 x VDD 1 7 0.5V Symbol VDD VREG IDD IDD IDD IDD IDD IDD IDD IDD IDD IDD IIH, IIL Run1 Mode @ 1 ms sample period Run1 Mode @ 2 ms sample period Run1 Mode @ 4 ms sample period Run1 Mode @ 8 ms sample period Run1 Mode @ 16 ms sample period Run1 Mode @ 32 ms sample period Run1 Mode @ 64 ms sample period Run1 Mode @ 128 ms sample period Peak of measurement duty cycle Stop Mode Conditions Min 2.0 1.71 Typ 3.3 1.8 393 199 102 54 29 17 11 8 1 3 0.025 15 Max 3.6 2.75 Units V V A A A A A A A A mA A A pF V V A pF V V
VOLGPIO VTLH VTHL
0.5 1.62 1.42
V V V
AC CHARACTERISTICS
Table 5. AC CHARACTERISTICS
(Typical Operating Circuit, VDD and VREG = 1.8 V, TA = 25C, unless otherwise noted.)
Parameter 8 MHz Internal Oscillator 1 kHz Internal Oscillator Symbol fH fL Conditions Min 7.44 0.65 Typ 8 1 Max 8.56 1.35 Units MHz kHz
MPR121 Sensors Freescale Semiconductor 9
I2C AC CHARACTERISTICS
Table 6. I2C AC Characteristics
(Typical Operating Circuit, VDD and VREG = 1.8 V, TA = 25C, unless otherwise noted.)
Parameter Serial Clock Frequency Bus Free Time Between a STOP and a START Condition Hold Time, (Repeated) START Condition Repeated START Condition Setup Time STOP Condition Setup Time Data Hold Time Data Setup Time SCL Clock Low Period SCL Clock High Period Rise Time of Both SDA and SCL Signals, Receiving Fall Time of Both SDA and SCL Signals, Receiving Fall Time of SDA Transmitting Pulse Width of Spike Suppressed Capacitive Load for Each Bus Line Symbol fSCL tBUF tHD, STA tSU, STA tSU, STO tHD, DAT tSU, DAT tLOW tHIGH tR tF tF.TX tSP Cb 100 1.3 0.7 20+0.1Cb 20+0.1Cb 20+0.1Cb 25 400 300 300 250 1.3 0.6 0.6 0.6 0.9 Conditions Min Typ Max 400 Units kHz s s s s s ns s s ns ns ns ns pF
MPR121 10 Sensors Freescale Semiconductor
AN3889: MPR121 Capacitance Sensing Settings
INTRODUCTION Touch acquisition takes a few different parts of the system in order to detect touch. The first stage of this process is to capture the pad capacitance. Freescale's MPR121 utilizes the principle that a capacitor holds a fixed amount of charge at a specific electric potential. Both the implementation and the configuration will be described in this application note.
AFE AQUISITION
RAW DATA 1 - 32 s
1st FILTER 1 - 128 s
2nd FILTER 4 - 2048 s
BASELINE FILTER
TOUCH
STATUS REGISTER
IRQ
Figure 3. Data Flow in the MPR121 CAPACITANCE MEASUREMENT The basic measurement technique used by the MPR121 is to charge up the capacitor C on one electrode input with a DC current I for a time T (the charge time). Before measurement, the electrode input is grounded, so the electrode voltage starts from 0 V and charges up with a slope, Equation 1, where C is the pad capacitance on the electrode (Figure 4). All of the other electrodes are grounded during this measurement. At the end of time T, the electrode voltage is measured with a 10 bit ADC. The voltage is inversely proportional to capacitance according to Equation 2.The electrode is then discharged back to ground at the same rate it was charged.
dV = --I-----dt C IxT V = ---------C
Equation 1
Equation 2
MPR121 Sensors Freescale Semiconductor 11
Electrode Voltage
V
Electrode voltage measured here
Electrode Charging
Electrode Discharging
Electrode Charge Time
T
Electrode Discharge Time
2T
Figure 4. MPR121 Electrode Measurement Charging Pad Capacitance When measuring capacitance there are some inherent restrictions due to the methodology used. On the MPR121 the voltage after charging must be in the range that is shown in Figure 5.
Valid ADC Values vs. V DD 900 800 ADChigh 700 600 ADC Counts ADCmid 500 400 ADClow 300 200 100 0 1.71
1.91
2.11 V DD (V)
2.31
2.51
2.71
Figure 5. The valid operating range of the electrode charging source is 0.7 V to (VDD - 0.7 V). This means that for a given VDD the valid ADC (voltage visible to the digital interface) range is given by
0.7 ADC low = ------------ ( 1024 ) , V DD
and
Equation 3
V DD - 0.7 ADC high = ------------------------- ( 1024 ) . V DD
Equation 4
These equations are represented in the graph. In the nominal case of VDD = 1.8 V the ADC range is shown below in Table 7. Table 7. VDD 1.8 ADChigh 625.7778 ADClow 398.2222 ADCmid 512
MPR121 12 Sensors Freescale Semiconductor
Any ADC counts outside of the range shown are invalid and settings must be adjusted to be within this range. If capacitance variation is of importance for an application after the current output, charge time and supply voltage are determined then the following equations can be used. The valid range for capacitance is calculated by using the minimum and maximum ADC values in the capacitance equation. Substituting the low and high ADC equations into the capacitance equation yields the equations for the minimum and maximum capacitance values which are
IxT IxT C low = ------------------------- and C high = ---------- . V - 0.7 0.7 DD
SENSITIVITY
Equation 5
The sensitivity of the MPR121 is relative to the capacitance range being measured. Given the ADC value, current and time and settings capacitance can be calculated,
I x T x 1024 C = ------------------------------- . V x ADC DD
Equation 6
For a given capacitance the sensitivity can be measured by taking the derivative of this equation. The result of this is the following equation, representing the change in capacitance per one ADC count, where the ADC in the equation represents the current value.
dC I x T x 1024 --------------- = ---------------------------------dADC 2 V DD x ADC
Equation 7
This relationship is shown in the following graph by taking the midpoints off all possible ranges by varying the current and time settings. The midpoint is assumed to be 512 for ADC and the nominal supply voltage of 1.8 V is used.
Sensitivity vs. Midpoint Capacitance for VDD = 1.8 V 0 0 500 1000 1500 2000 2500
-0.5 -1 Sensitivity (pF/ADC Count) -1.5 -2 -2.5 -3 -3.5 -4 -4.5 -5 Midpoint Capacitance (pF) dC/dADC @cmid (pF/1 ADC Count)
Figure 6. Smaller amounts of change indicate increased sensitivity for the capacitance sensor. Some sample values are shown in Table 8. Table 8. pF 10 100 Sensitivity (pF/ADC count) -0.01953 -0.19531
In the previous cases, the capacitance is assumed to be in the middle of the range for specific settings. Within the capacitance range the equation is nonlinear, thus the sensitivity is best with the lowest capacitance. This graph shows the sensitivity derivative reading across the valid range of capacitances for a set I, T, and VDD. For simple small electrodes (that are approximately 21 pF) and a nominal 1.8 V supply. Figure 7 is representative of this effect.
MPR121 Sensors Freescale Semiconductor 13
Sensitivity vs. Capacitance for VDD = 1.8 V and I =36 A and T = .5 S 0.1 0.09 0.08 Sensitivity (pF/ADC Count) 0.07 0.06 0.05 C/ADC 0.04 0.03 0.02 Minimum 0.01 0 10 Maximum
12
14
16
18
20 Capacitance
22
24
26
28
30
Figure 7. CONFIGURATION From the implementation above, there are two elements that can be configured to yield a wide range of capacitance readings ranging from 0.455 pF to 2874.39 pF. The two configurable components are the electrode charge current and the electrode charge time. The electrode charge current can be configured to equal a range of values between 1 A and 63 A. This value is set in the Charge Discharge Current (CDC) in the Analog Front End AFE Configuration register. The electrode charge time can be configured to equal a range of values between 500 ns and 32 S. This value is set in the Charge Discharge Time (CDT) in the Filter Configuration Register. AFE CONFIGURATION REGISTER The AFE Configuration Register is used to set both the CDC and the number of samples taken in the lowest level filter. The address of the AFE Configuration Register is 0x5C.
7 R W Reset: 0 FFI
6
5
4
3 CDC
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 8. AFE Configuration Register Table 9. AFE Configuration Register Field Descriptions Field 7:6 FFI Description First Filter Iterations - The first filter iterations field selects the number of samples taken as input to the first level of filtering. 00 Encoding 0 - Sets samples taken to 6 01 Encoding 1 - Sets samples taken to 10 10 Encoding 2 - Sets samples taken to 18 11 Encoding 3 - Sets samples taken to 34 Charge Discharge Current - The Charge Discharge Current field selects the supply current to be used when charging and discharging an electrode. 000000 Encoding 0 - Disables Electrode Charging 000001 Encoding 1 - Sets the current to 1A ~ 111111 Encoding 63 - Sets the current to 63 A
5:0 CDC
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FILTER CONFIGURATION REGISTER
7 R W Reset: 0
6 CDT 0
5
4 SFI
3
2
1 ESI
0
0
0
0
0
0
0
= Unimplemented
Figure 9. Filter Configuration Register Table 10. Filter Configuration Register Field Descriptions Field 7:5 CDT Description Charge Discharge Time - The Charge Discharge Time field selects the amount of time an electrode charges and discharges. 000 Encoding 0 - Invalid 001 Encoding 1 - Time is set to 0.5 s 010 Encoding 2 - Time is set to 1 s ~ 111 Encoding 7 - Time is set to 32 s. Second Filter Iterations - The Second Filter Iterations field selects the number of samples taken for the second level filter. 00 Encoding 0 - Number of samples is set to 4 01 Encoding 1 - Number of samples is set to 6 10 Encoding 2 - Number of samples is set to 10 11 Encoding 3 - Number of samples is set to 18 Electrode Sample Interval - The Electrode Sample Interval field selects the period between samples used for the second level of filtering. 000 Encoding 0 - Period set to 1 ms 001 Encoding 1 - Period set to 2 ms ~ 111 Encoding 7 - Period set to 128 ms
4:3 SFI
2:0 ESI
The SFI, ESI and FFI are described in AN3890. In addition to these global (same for all electrodes) settings, the MPR121 electrodes can also be independently configured. ELECTRODE CHARGE CURRENT REGISTER
7 R W Reset: 0 0
6 0 0
5
4
3 CDC
2
1
0
0
0
0
0
0
0
= Unimplemented
Figure 10. Electrode Charge Current Register Table 11. Electrode Charge Current Register Field Descriptions Field 5:0 CDC Description Electrode # Charge Discharge Current - The Charge Discharge Current field selects the supply current to be used when charging and discharging an electrode. 000000 Encoding 0 - Disables Electrode Charging 000001 Encoding 1 - Sets the current to 1A ~ 111111 Encoding 63 - Sets the current to 63 A
MPR121 Sensors Freescale Semiconductor 15
ELECTRODE CHARGE TIME
7 R W Reset: 0 0
6
5 CDT#
4
3 0
2
1 CDT#
0
0
0
0
0
0
0
0
= Unimplemented
Figure 11. Electric Charge Time Register Table 12. Electrode Charge Time Register Field Descriptions Field 6:4 CDT# Description Electrode # Charge Discharge Time - The Charge Discharge Time field selects the amount of time an electrode charges and discharges. 000 Encoding 0 - Global value is used. 001 Encoding 1 - Time is set to 0.5 s 010 Encoding 2 - Time is set to 1 s ~ 11 Encoding 7 - Time is set to 32 s. Electrode # Charge Discharge Time - The Charge Discharge Time field selects the amount of time an electrode charges and discharges. 000 Encoding 0 - Global value is used. 001 Encoding 1 - Time is set to 0.5 s 010 Encoding 2 - Time is set to 1 s ~ 11 Encoding 7 - Time is set to 32 s.
2:0 CDT#
AUTO-CONFIGURATION One of the new features added in the MPR121 that was not included in the MPR03X is the ability to automatically configure the Charge Current the Charge Time. This eliminates much of the guess involved with touch sensors and allows the same settings to properly configure the device for a wide range of application and electrodes. As show earlier in this document, the sensitivity of the sensor is maximized by having the baseline be as high as possible for a specific baseline capacitance. The restriction on the high side is that a system should not charge above VDD - 0.7 V due to this being a non-linear region. Thus the target voltage used is approximately VDD - 0.7 V. Table 13. Voltage (VDD) 1.8 V VDD VDD - 0.7 V 1.1 V 2.3 V ADC 625 785 Baseline 156 196
This implies that the automatic configuration system should target approximately 156 when VDD is 1.8 V and 196 when VDDis 3.0 V. The following three registers should be set based on the VDD in the system. If the voltage is unregulated, set the values assuming the lowest voltage necessary for the battery. If the final voltage supply in the system is not known, just use the 1.8 V values as they represent the worst case. This lower setting will not dramatically affect the performance, thus the 1.8 V could be considered default and be used in all cases where fine tuning is not required.
MPR121 16 Sensors Freescale Semiconductor
AUTO-CONFIG USL REGISTER
7 R W Reset: 0
6
5
4 USL
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 12. AUTO-CONFIG USL Register Table 14. AUTO-CONFIG USL Register Field Descriptions Field 7:0 USL Description Upper Limit - The Upper Limit for the auto-configuration baseline search is set to this value. 00000000 - Upper Limit set to 0 00000001 - Upper Limit set to 1 ~ 11111111 - Upper Limit set to 255
As this register represents the upper limit for the auto-configuration the value can be calculated by:
V DD - 0.7 VSL = ------------------------- 256 V DD
For the 1.8 V system, this value is 156 or 0x9C. AUTO-CONFIG TARGET LEVEL REGISTER
Equation 8
7 R W Reset: 0
6
5
4 T_L
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 13. AUTO-CONFIG Target Level Register
MPR121 Sensors Freescale Semiconductor 17
Table 15. AUTO-CONFIG Target Level Register Field Descriptions Field 7:0 TL Description Target Level - The Target Level for the auto-configuration baseline search is set to this value. 00000000 - Target Level set to 0 00000001 - Target Level set to 1 ~ 11111111 - Target Level set to 255
This register represents the target level for the auto-configuration. The value can be calculated by: V DD - 0.7 T arg et = ------------------------- 256 0.9 V DD For a 1.8 V system, this value is 140 or 0x8C. AUTO-CONFIG LSL REGISTER
90% of USL
7 R W Reset: 0
6
5
4 LSL
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 14. AUTO-CONFIG LSL Register Table 16. AUTO-CONFIG LSL Register Field Descriptions Field 7:0 LSL Description Lower Limit - The Lower Limit for the auto-configuration baseline search is set to this value. 00000000 - Lower Limit set to 0 00000001 - Lower Limit set to 1 ~ 11111111 - Lower Limit set to 255
This register represents the lower limit for the auto-configuration. The value can be calculated by: V DD - 0.7 T arg et = ------------------------- 256 0.65 V DD For a 1.8 V system, this value is 101 or 0x65. The last setting required to set up the auto-configuration system is the AUTO e Register. AUTO-CONFIG CONTROL REGISTER
65% of USL
7 R W Reset: 0 AFES
6
5 RETRY
4
3 BVA
2
1 ARE
0 ACE 0
0
0
0
0
0
0
= Unimplemented
Figure 15. AUTO-CONFIG Control Register
MPR121 18 Sensors Freescale Semiconductor
Table 17. AUTO-CONFIG Control Register Field Descriptions Field 7:6 AFES Description First Filter Iterations - The first filter iterations field selects the number of samples taken as input to the first level of filtering. This value must match the FFI set in the AFE Configuration register for proper AUTO-CONFIG functionality. 00 Encoding 0 - Sets samples taken to 6 01 Encoding 1 - Sets samples taken to 10 10 Encoding 2 - Sets samples taken to 18 11 Encoding 3 - Sets samples taken to 34 Retry - The Retry value determines under what circumstances the auto-configuration system will retry. 00 - Retry disabled 01 - Retry enabled 10 - Retry enabled 11 - Retry enabled Baseline Value Adjust - The baseline value adjust determines the initial value of the baseline registers after auto-configuration completes. 00 - Baseline is not changed 01 - Baseline is cleared 10 - Baseline is set to the AUTO-CONFIG baseline with the lower 3 bits cleared 11 - Baseline is set to the AUTO-CONFIG baseline Automatic Reconfiguration Enable - The automatic reconfiguration enable, enables or disables automatic reconfiguration. 0 - ARE is disabled 1 - ARE is enabled Automatic Configuration Enable - The automatic configuration enable, enables or disables automatic configuration. 0 - ACE is disabled 1 - ACE is enabled
5:4 RETRY
3:2 BVA
1 ARE
0 ACE
The normal setup of the system is to set this register to 0x0B or 0b00001011. This means that the FFI is 00, but if the FFI in the AFE Configuration Register is different, it must be changed to match. For a description of this register, please refer to AN3890. The RETRY is disabled because in production systems, this will not be required. The BVA is set to 10 which allows the baseline to be updated. 10 is used instead of 11 because this guarantees that the baseline will be lower than the data. This is preferable as it protects against false touches. If somehow the baseline started higher than the data, a touch would be triggered and the detection system would have to be reset to work correctly. Last, both the automatic configuration and automatic reconfiguration are enable. Reconfiguration will trigger any time the baseline drifts outside the range set by the USL and the LSL. There is also a set of flags which show when the automatic configuration has failed. For normal sized touch electrodes, this cannot occur without the USL, LSL and TSL being incorrectly set. The most likely configuration error is to set the USL (upper limit) at a lower value than the LSL (lower limit). Thus, as the algorithm searches for settings that work, it would always result in a fail throwing the OOR (Out Of Range) status flag. The ARFF and ACFF also tell the user which type of configuration cycle caused the error. If it was triggered during an initial calibration, the ACFF will trigger. If the fail occurs during a reconfiguration, the ARFF will trigger. ELE0-7 OUT OF RANGE STATUS REGISTER
7 R W Reset: 0 E7S
6 E6S 0
5 E5S 0
4 E4S 0
3 E3S 0
2 E2S 0
1 E1S 0
0 E0S 0
= Unimplemented
Figure 16. ELE0-7 Out Of Range Status Register
MPR121 Sensors Freescale Semiconductor 19
Table 18. ELE0-7 Out Of Range Status Register Field Descriptions Field 7 E7S 6 E6S 5 E5S 4 E4S 3 E3S 2 E2S 1 E1S 0 E0S Description Electrode 7 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 6 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 5 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 4 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 3 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 2 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 1 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 0 OOR Status - The Electrode 7 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed
MPR121 20 Sensors Freescale Semiconductor
ELE8-11, ELEPROX OUT OF RANGE STATUS REGISTER
7 R W Reset: 0 ARFF
6 ACFF 0
5 0 0
4
ELEPROXS
3 E11S 0
2 E10S 0
1 E9S 0
0 E8S 0
0
= Unimplemented
Figure 17. ELE8-11, ELEPROX Out Of Range Status Register Table 19. ELE8-11, ELEPROX Out Of Range Status Register Field Descriptions
Field Description
7 ARFF
Automatic Reconfiguration Fail Flag - The Automatic Reconfiguration Fail Flag shows is the OOR was triggered during a reconfiguration cycle. 0 - Auto-reconfiguration did not cause the OOR flag 1 - Auto-reconfiguration did cause the OOR flag Automatic Configuration Fail Flag - The Automatic Configuration Fail Flag shows is the OOR was triggered during an initial configuration cycle. 0 - Auto-configuration did not cause the OOR flag 1 - Auto-configuration did cause the OOR flag Electrode PROX OOR Status - The Electrode PROX OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 11 OOR Status - The Electrode 11 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 10 OOR Status - The Electrode 10 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 9 OOR Status - The Electrode 9 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed Electrode 8 OOR Status - The Electrode 8 OOR Status shows if the AUTO-CONFIG has failed. 0 - Auto-configuration Successful 1 - Auto-configuration Failed
6 ACFF
4 ELEPROXS
3 E11S
2 E10S 1 E9S 0 E8S
MPR121 Sensors Freescale Semiconductor 21
AN3890: MPR121 Capacitance Sensing -- Filtering and Timing
INTRODUCTION The capacitance sensing front end of the MPR121 produces data at extremely high rates, which significantly improves the capabilities of a filtering system. The capacitance engine described in AN3889 act on a 1 s - 32 s per sample data rate. THis application note will discuss the first and second level filters in the MPR121 and how they impact timing and power consumption.
AFE AQUISITION
RAW DATA 1 - 32 s
1st FILTER 1 - 128 s
2nd FILTER 4 - 2048 s
BASELINE FILTER
TOUCH
STATUS REGISTER
IRQ
Figure 18. Data Flow in the MPR121 The first level filter is configured through the use of the First Filter Iterations (FFI) and the Charge Discharge Time (CDT). AFE CONFIGURATION REGISTER The AFE Configuration Register is used to set both the CDC and the number of samples taken in the lowest level filter. The address of the AFE Configuration Register is 0x5C.
7 R W Reset: 0 FFI
6
5
4
3 CDC
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 19. AFE Configuration Register
MPR121 22 Sensors Freescale Semiconductor
Table 20. AFE Configuration Register Field Descriptions Field 7:6 FFI Description First Filter Iterations - The first filter iterations field selects the number of samples taken as input to the first level of filtering. 00 Encoding 0 - Sets samples taken to 6 01 Encoding 1 - Sets samples taken to 10 10 Encoding 2 - Sets samples taken to 18 11 Encoding 3 - Sets samples taken to 34 Charge Discharge Current - The Charge Discharge Current field selects the supply current to be used when charging and discharging an electrode. 000000 Encoding 0 - Disables Electrode Charging 000001 Encoding 1 - Sets the current to 1A ~ 111111 Encoding 63 - Sets the current to 63 A
5:0 CDC
The properties of the filter are determined by these two settings, but the CDT is determined by the capacitance being measured, as discussed in AN3889. The FFI sets the number of samples being measured. The result of an FFI setting of 6 or 0x00 would be to take 6 samples, toss the maximum and minimum, then average the remaining 4 samples. The results of an oscilloscope output on an electrode with the setting of FFI = 0x00 and CDT is shown in Figure 20.
FFI VDD - 0.7 V
1 2 3 4 5 6
VOLTAGE
CDT 1 s
2 s
4 s
6 s TIME
8 s
10 s
12 s
Figure 20. The first level of filtering delivers data to a second filter stage. The second filter stage averages samples over more time, in this example anywhere from 1 ms to 128 ms. Then a value can be selected for how many samples should be averaged. FILTER CONFIGURATION REGISTER
7 R W Reset: 0
6 CDT 0
5
4 SFI
3
2
1 ESI
0
0
0
0
0
0
0
= Unimplemented
Figure 21. Filter Configuration Register
MPR121 Sensors Freescale Semiconductor 23
Table 21. Filter Configuration Register Field Descriptions Field 7:5 CDT Description Charge Discharge Time - The Charge Discharge Time field selects the amount of time an electrode charges and discharges. 000 Encoding 0 - Invalid 001 Encoding 1 - Time is set to 0.5 s 010 Encoding 2 - Time is set to 1 s ~ 111 Encoding 7 - Time is set to 32 s. Second Filter Iterations - The Second Filter Iterations field selects the number of samples taken for the second level filter. 00 Encoding 0 - Number of samples is set to 4 01 Encoding 1 - Number of samples is set to 6 10 Encoding 2 - Number of samples is set to 10 11 Encoding 3 - Number of samples is set to 18 Electrode Sample Interval - The Electrode Sample Interval field selects the period between samples used for the second level of filtering. 000 Encoding 0 - Period set to 1 ms 001 Encoding 1 - Period set to 2 ms ~ 111 Encoding 7 - Period set to 128 ms
4:3 SFI
2:0 ESI
Note: In most cases the CDT in this register is not used. It will normally be auto-configured as described in AN3889 While the 1 ms to 128 ms does affect the filtering, the main purposed of adjusting the sample rate would be to change the average current consumption of the device. Figure 22 illustrates this adjustment.
VOLTAGE
First Filter
0
1 ms
2 ms TIME (ms)
3 ms
Figure 22. From this, it can be seen that the 12 s up time from the 1 ms samples results in a very low percent of duty cycle. This results in a very low average current consumption.
MPR121 24 Sensors Freescale Semiconductor
AVERAGE CURRENT CONSUMPTION (A)
400
393 A
300
200
199 A
100
102 A 54 A 29 A
10 1 2 4 8 16
17 A 32 64
11 A
8 A 128
SAMPLE RATE (ms)
Figure 23. Average Supply Current The output data is the Filtered Data High and Low is the data coming out of the second stage filter. This means that the response time of the output is the SFI times the ESI. This usually results in 16 ms and 4 iterations being used to get 64 ms response time while still optimizing the power consumption. At each 64 ms, a decision would be made regarding touch by comparing the Baseline with the filtered data output, resulting in a a worst case of the full 64 ms plus half the previous cycle, equalling 96 ms.
MPR121 Sensors Freescale Semiconductor 25
AN3891: MPR121 Baseline System
INTRODUCTION Touch acquisition takes a few different parts of the system in order to detect touch. The baseline filter and touch detection are tightly coupled. The purpose of the baseline filter is to "filter out touches" resulting in a system that is similar to a long term average but also takes into account that one specific signature. A touch must have different properties than noise and environmental change with respect to the filter response. This is accomplished through four register types that operate under different conditions. These are Max Half Delta (MHD), Noise Half Delta (NHD), Noise Count Limit (NCL) and Filter Delay Limit (FDL).
AFE AQUISITION
RAW DATA 1 - 32 s
1st FILTER 1 - 128 s
2nd FILTER 4 - 2048 s
BASELINE FILTER
TOUCH
STATUS REGISTER
IRQ
Figure 24. Data Flow in the MPR121 MAX HALF DELTA (NHD)
7 R W Reset: 0 0
6 0 0
5
4
3 MHD
2
1
0
0
0
0
0
0
0
= Unimplemented
Table 22. Max Half Delta Register Field Descriptions Field 5:0 MHD Description Max Half Delta - The Max Half Delta determines the largest magnitude of variation to pass through the third level filter. 000000 DO NOT USE THIS CODE 000001 Encoding 1 - Sets the Max Half Delta to 1 ~ 111111 Encoding 63 - Sets the Max Half Delta to 63
MPR121 26 Sensors Freescale Semiconductor
NOISE HALF DELTA (NHD)
7 R W Reset: 0 0
6 0 0
5
4
3 NHD
2
1
0
0
0
0
0
0
0
= Unimplemented
Figure 25. Noise Half Delta Register Table 23. Noise Half Delta Register Field Descriptions Field 5:0 NHD Description Noise Half Delta - The Noise Half Delta determines the incremental change when non-noise drift is detected. 000000 DO NOT USE THIS CODE 000001 Encoding 1 - Sets the Noise Half Delta to 1 ~ 111111 Encoding 63 - Sets the Noise Half Delta to 63
NOISE COUNT LIMIT (NCL)
7 R W Reset: 0
6
5
4 NCL
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 26. Noise Count Limit Register Table 24. Noise Count Limit Register Field Descriptions Field 7:0 NCL Description Noise Count Limit - The Noise Count Limit determines the number of samples consecutively greater than the Max Half Delta necessary before it can be determined that it is non-noise. 00000000 Encoding 0 - Sets the Noise Count Limit to 1 (every time over Max Half Delta) 00000001 Encoding 1 - Sets the Noise Count Limit to 2 consecutive samples over Max Half Delta ~ 11111111 Encoding 255 - Sets the Noise Count Limit to 255 consecutive samples over Max Half Delta
MPR121 Sensors Freescale Semiconductor 27
FILTER DELAY LIMIT (FDL)
7 R W Reset: 0
6
5
4 FDL
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 27. Filter Delay Limit Register Table 25. Filter Delay Limit Register Field Descriptions Field 7:0 FDL Description Filter Delay Limit - The Filter Delay Limit determines the rate of operation of the filter. A larger number makes it operate slower. 00000000 Encoding 0 - Sets the Filter Delay Limit to 1 00000001 Encoding 1 - Sets the Filter Delay Limit to 2 ~ 11111111 Encoding 255 - Sets the Filter Delay Limit to 255
Additionally there are different conditions in the system that affects how these registers operate. These are rising data, falling data or touched data. When the data changes between these conditions, the current filter process is cancelled and all filter counters return to zero. The operation of the filter is in the relationship between the 2nd filter data and the baseline filter value. The occurrence of a touch will also change the operation of the system. The touch generation process is described in the application note AN3892. The falling data system is enabled any time the 2nd filter data is less than the baseline filter data. The rising data system is enabled any time the 2nd filter data is greater than the baseline filter data. The following cases describe the baseline system when it is not changing between the three states mentioned above.
Case 1
Small incremental changes to the system represent long term slow (environmental) changes in the system. The MHD setting regulates this case by allowing any data that is less than two times the MHD to pass the filter. Thus, if the baseline is 700 and the data is 701 with a MHD of one, then the baseline filter would increase to equal the data for the next cycle.
MHD = 1
Baseline Data
Figure 28. Max Half Delta
MPR121 28 Sensors Freescale Semiconductor
Case 2
Changes that are larger than double the MHD are regarded as noise and accounted for by the values of the NHD and NCL. Any data outside the MHD is rejected by the filter however sequential values that fall into this category are counted and if enough sequential data exists then the baseline will be adjusted. In this case, the NCL regulates how many sequential data points must be seen before the data is changed. When the count is reached, the baseline is incremented by the NHD.
MHD = 1 NCL = 3 NHD = 1
Case 1 comes into effect
Baseline Data
NHD added to baseline 3 counts 3 counts
Figure 29.
Case 3
When the data is inconsistent but greater than double than MHD the baseline will not vary. Each time a transition takes place, the filter counters are reset, thus the fact that the data is oscillating around the baseline means that the noise is rejected and the baseline will not vary.
MHD = 1 NCL = 3 NHD = 1
2 < NCL
1 < NCL
Baseline Data
2 < NCL
1 < NCL
Figure 30.
Case 4
Low frequency changes to the data can trick the filter in some instances. The FDL is also available to slow down the overall system. This is done by taking an average of the specified number of values before running them through the baseline filter.
Data Averaged Data
Figure 31.
MPR121 Sensors Freescale Semiconductor 29
After this averaging the filter reacts to Cases 1, 2, and 3.
MHD = 1 NCL = 3 NHD = 1 FDL = 4
Case 1
Case 2
Case 1 Baseline Data Averaged Data
1
2
3
1
2
3
Figure 32.
ADVANCED CASES
With an understanding of the basic cases, more advanced cases can be discussed. In a touch sensor system, we can take advantage of some known properties to improve the functionality of the filter. These include direction of change, touch occurrence and the rate of touch. The first four cases are still utilized but more functionality is added. The following cases described how different settings are useful as opposed to what exactly the settings do, like cases 1-4.
Case 5
The direction of change for a touch in the system is always negative. Thus the system takes advantage of this by allowing for varying parameters for different directions of change. Since a touch can only be in the decreasing direction, it is usually best to set the decreasing filter to be slower that the increasing one. This allows for automatic recovery from a bad baseline reading.
B
C Baseline Data
A
A. As the touch occurs, the baseline is decreased slowly due to a non solid touch, but due to the slow reaction, a touch is still detected. B. The baseline quickly snaps back to the initial value by having fast filtering in the positive direction C. The repeated touch is easily handled since the baseline quickly adjusted; if it was slow, the second touch would have resulted in a possible false negative for a touch detection. Figure 33.
MPR121 30 Sensors Freescale Semiconductor
Case 6
The system needs the capability to handle environment changes that appear very similar to actual touches. In Case 5, the touch was a real touch, but slow enough that initially it is thought better for the baseline not to change at all.
A C Baseline Data B
A. The decrease is the interface being cleared with a wet rag, causing a relatively slow capacitance change. The baseline accurately tracks this slow change. B. The baseline begins to increase as the interface becomes dry. C. A delta from the new baseline allows a touch to be accurately detected. Figure 34.
Case 7
This case is when a touch is occurring. While the baseline system does not detect a touch, it is obviously an important part of the process. The baseline can be set to slowly calibrate a touch from the system preventing keys from becoming stuck. Only the NHD, NCL and FDL are necessary since the value can never be less than double the MHD.
A
C Baseline Data B
A. The touch is detected which disengages the increasing/decreasing baseline filter but leaves it enabled with very slow filtering B. Even though the touch has not been released it times out and is eventually rejected. C. Normal baseline filter is engaged. Figure 35.
MPR121 Sensors Freescale Semiconductor 31
Case 8
This case can also prevent keys from being stuck due to misuse. For example, if a metal pen touches a button, this may initially engage the button but the pen is calibrated out over time and, normal function resumes. The same applies to water, food humid environments and other instances that generate capacitance change.
A
B Baseline Data
C
A. Valid normal touch B. False touch filtered out C. Touch from new adjusted baseline Figure 36.
MPR121 32 Sensors Freescale Semiconductor
AN3892: MPR121 Jitter and False Touch Detection
INTRODUCTION Touch acquisition takes a few different parts of the system in order to detect touch. The baseline filter and touch detection are tightly coupled. The purpose of the touch detection block is to use the baseline value and the 2nd level filter data to determine when a user has touched an electrode. The electrodes are independently configured using the Touch Threshold and Release Threshold registers. The global Debounce register also controls when a touch is detected by adding some minimal delay. The data is then output through a couple of registers: Filtered Data High, Filtered Data Low, Baseline Data and two touch output registers.
AFE AQUISITION
RAW DATA 1 - 32 s
1st FILTER 1 - 128 s
2nd FILTER 4 - 2048 s
BASELINE FILTER
TOUCH
STATUS REGISTER
IRQ
Figure 37. Data Flow in the MPR121 First, the MPR121 touch sensor detects touch by the methods in this application note, and the data is output through the first two registers in the map. The two touch status registers both trigger an interrupt on any change of the data. Thus, as a touch happens (bit is set) an interrupt will be triggered, and when a touch is released (bit is cleared) it will also trigger. To clear the interrupt all you must do is initiate a I2C communication, with the intent that you read register 0x00 and 0x01 to determine which electrodes are touched.
MPR121 Sensors Freescale Semiconductor 33
TOUCH STATUS REGISTER 0
.
7 R W Reset: 0 E7S
6 E6S 0
5 E5S 0
4 E4S 0
3 E3S 0
2 E2S 0
1 E1S 0
0 E0S 0
= Unimplemented
Figure 38. Touch Status Register 0 Table 26. Touch Status Register 0 Field Descriptions Field 7 E7S 6 E6S 5 E5S 4 E4S 3 E3S 2 E2S 1 E1S 0 E0S Description Electrode 7 Status - The Electrode 7 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 6 Status - The Electrode 6 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 5 Status - The Electrode 5 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 4 Status - The Electrode 4 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 3 Status - The Electrode 3 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 2 Status - The Electrode 2 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 1 Status - The Electrode 1 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 0 Status - The Electrode 0 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched
MPR121 34 Sensors Freescale Semiconductor
TOUCH STATUS REGISTER 1
7 R W Reset: 0 OVCF
6 0 0
5 0 0
4
EPROXS
3 E11S 0
2 E10S 0
1 E9S 0
0 E8S 0
0
= Unimplemented
Figure 39. Touch Status Register1 Table 27. Touch Status Register 1 Field Descriptions Field 7 OVCF Description Over Current Flag - The Over Current Flag will be set any time the wrong value of Rext is connected to the MPR121's Rext pin. This is to protect the part from high current that could result from an incorrect resistor value. 0 - Correct Rext resistor value 1 - Incorrect Rext resistor value Electrode PROX Status - The Electrode PROX Status bit shows touched nor not touched. 0 - Not Touched 1 - Touched Electrode 11 Status - The Electrode 11 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 10 Status - The Electrode 10 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 9 Status - The Electrode 9 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched Electrode 8 Status - The Electrode 8 Status bit shows touched or not touched. 0 - Not Touched 1 - Touched
4 EPROXS
3 E11S 2 E10S 1 E9S 0 E8S
FILTER DATA HIGH
.
7 R W Reset: 0 0
6 0 0
5 0 0
4 0 0
3 0 0
2 0 0
1 FDHB 0
0
0
= Unimplemented
Figure 40. Filtered Data High Register Table 28. Filtered Data High Register Field Descriptions Field 7:0 FDHB Description Filtered Data High Bits - The Filtered Data High Bits displays the higher 2 bits of the 10 bit filtered A/D reading. 00 Encoding 0 ~ 11 Encoding 3
MPR121 Sensors Freescale Semiconductor 35
FILTERED DATA LOW
7 R W Reset: 0
6
5
4 FDLB
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 41. Filtered Data Low Register Table 29. Filtered Data Low Register Field Descriptions Field 7:0 FDLB Description Filtered Data Low Byte - The Filtered Data Low Byte displays the lower 8 bits of the 10 bit filtered A/D reading. 00000000 Encoding 0 ~ 11111111 Encoding 255
BASELINE VALUE
7 R W Reset: 0
6
5
4 BV
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 42. Filtered Data High Register Table 30. Filtered Data High Register Field Descriptions Field 7:0 BV Description Baseline Value - The Baseline Value byte displays the higher 8 bits of the 10 bit baseline value. 00000000 Encoding 0 - The 10 bit baseline value is between 0 and 3. ~ 11111111 Encoding 255 - The 10 bit baseline value is between 1020 and 1023.
In this system, a touch is defined as any time the difference between the Filtered Data and the Baseline Value is greater than the threshold. Since this calculation is done totally internal to the part, it is unnecessary for the user to actually do this math in the software. If it were being done, the steps would be to first combine the Filtered Data Low and Filtered Data High values into a single 10-bit number. Thus,
Data = Filtered Data High * 256 + Filtered Data Low
The baseline is then shifted to the left to make it equal scale to the Data.
Baseline = Baseline Value * 4
Internally to the device, the full 10-bit value is stored, but as this level of precision is not necessary as the low two bits are disregarded for output. The Touch Threshold is a user defined value. There is both a touch and an un-touch threshold to provide hysteresis.
MPR121 36 Sensors Freescale Semiconductor
TOUCH THRESHOLD REGISTER
7 R W Reset: 0
6
5
4 TTH
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 43. Touch Threshold Register Table 31. Touch Threshold Register Field Descriptions Field 7:0 TTH Description Touch Threshold - The Touch Threshold Byte sets the trip point for detecting a touch. 00000000 Encoding 0 ~ 11111111 Encoding 255
RELEASE THRESHOLD REGISTER
7 R W Reset: 0
6
5
4 RTH
3
2
1
0
0
0
0
0
0
0
0
= Unimplemented
Figure 44. Release Threshold Register Table 32. Release Threshold Register Field Descriptions Field 7:0 RTH Description Release Threshold - The Release Threshold Byte sets the trip point for detecting a touch. 00000000 Encoding 0 ~ 11111111 Encoding 255
MPR121 Sensors Freescale Semiconductor 37
For the system to recognize a touch the delta must be greater than the Touch Threshold.
Delta = Baseline - Data Trigger Touch Delta > Touch Threshold
A release is triggered when the Delta falls below the Release Threshold. This can happen for both changes to the Baseline and actual Data changes. To understand how the Baseline can change, refer to AN3891.
Trigger Release Delta < Touch Threshold
DEBOUNCE TOUCH AND RELEASE REGISTER The last register available in this set is the Debounce register. The Debounce register maintains the accuracy of touch and releases by further improving the performance. The debounce allows two different settings to prevent bounce in the end system. If the value is set to 0x22, the requirement would be three sequential changes in status before the change would be recognized.
7 R W Reset: 0 0
6
5 DR
4
3 0
2
1 Dt
0
0
0
0
0
0
0
0
= Unimplemented
Figure 45. Debounce Touch and Release Register Table 33. Debounce Touch and Release Register Field Descriptions Field 6:4 DR Description Debounce Release - The Debounce Release determines the number of sequential release detections before an interrupt is triggered and a release is reported. 000 Encoding 0 - Consecutive releases detection before Status change is 1 001 Encoding 1 - Consecutive releases detection before Status change is 2 ~ 007 Encoding 7 - Consecutive releases detection before Status change is 8 Debounce Touch - The Debounce Touch determines the number of sequential touch detections before an interrupt is triggered and a touch is reported. 000 Encoding 0 - Consecutive touch detection before Status change is 1 001 Encoding 1 - Consecutive touch detection before Status change is 2 ~ 007 Encoding 7 - Consecutive touch detection before Status change is 8
2:0 DT
CONCLUSION The use of each of the features together can have a great effect of the jitter and false couch rejection. Jitter is prevented by utilizing the two threshold settings. Thus the provided hysteresis prevent Jitter on the data from going through the to the output Depending on environmental conditions, the Debounce can be used to eliminate the remainder of dramatic change of the signal that aren't really touches. Additional filtering can be done before the data gets to the touch detection system. Refer to Freescale Application Note AN3890.
MPR121 38 Sensors Freescale Semiconductor
AN3893: MPR121 Proximity Detection
INTRODUCTION MPR121 is a feature rich, second generation touch sensor controller after Freescale's initial release of the MPR03x series device. Like MPR03x, MPR121 has a unique feature that all the electrode inputs can be internally connected together so that all the surface touch sensing area on the inputs are "summed" together to act as a single large electrode pad. This can effectively increase the total area of the sensing conductor for non-contact near proximity detection for hand approaching. OVERVIEW Capacitive proximity detection uses the same principle as capacitive touch sensing. Each MPR121 input sensing channel can be used as contactless proximity detection as well as finger touch detection if each sensing pad is designed properly and relevant register are set properly. Typically a smaller pad size is used for finger touch button detection; while a larger pad size is necessary for contactless near proximity detection. On the other side, it's true that in most portable application design there is no dedicated big surface area left for proximity detection as the touch sensing buttons occupy all the available surface area. To make proximity detection at the same time of touch detection without additional dedicated large sensing pad, MPR121 has an internal input multiplexor which can connect all input sensing channels together so that all the touch sensing surface areas on the input pads are "summed" together effectively acting as a single large sensing pad. Using this scheme in typical applications, the 12 channels can be used for 12 key buttons touch sensing, and the surface area of all the pads can also used for proximity detection (e.g., hand approaching). PROXIMITY DETECTION REGISTER SETTING Like each independent touch sensing detection, the 13th Proximity Detection electrode also has its own register configurations, other than that, all the concepts applied to the proximity detection are the same as touch sensing detection. 1.0 Enable Proximity Sensing Proximity detection (a/k/a area detection mode) is enabled by configuring the Electrode Configuration Register (0x5E), see Table 34. In MPR121, this adds an area detection step (the 13th pseudo Electrode) before all the independent electrodes touch sensing detect sequence. Once configured, we refer to this area detection as the 13th Proximity Detection electrode. Table 34. Electrode Configuration Register 0x5E (Reset Default: 0x00) Bit Read Write Reset AD1 0 1 1 AD0 1 0 1 Bit7 CL[1] 0 EC3 x x x Bit6 CL[0] 0 EC2 x x x EC1 x x x Bit5 AD[1] 0 EC0 x x x Bit4 AD[0] 0 Bit3 EC[3] 0 Bit2 EC[2] 0 Description Area Detection by connecting ELE0~1. Area Detection by connecting ELE0~3. Area Detection by connecting ELE0~11. Bit1 EC[1] 0 Bit0 EC[0] 0
2.0 Proximity Data Register and Baseline Register Eleprox Electrode Registers (0x1C, 0x2D) contain the 10-bit raw data of the capacitance-voltage measurement value for the 13th Proximity Detection electrode. Table 35. Eleprox Electrode Register 0x1C, 0x2D (Reset Default: 0x00, 0x00) 0x1C Bit7 D7 Bit7 -- Bit6 D6 Bit6 -- Bit5 D5 Bit5 -- Bit4 D4 Bit4 -- Bit3 D3 Bit3 -- Bit2 D2 Bit2 -- Bit1 D1 Bit1 D9 Bit0 D0 Bit0 D8
0x1D
MPR121 Sensors Freescale Semiconductor 39
Eleprox Baseline Value Register (0x2A) contains the 8 MSBs of the 10-bit baseline value for the 13th Proximity Detection electrode. Writing to Baseline Value Register updates the 8 MSBs of baseline value and clears the 2 LSBs to zero. The Baseline Value Registers can only be written when in Shutdown Mode, but the current values may be read at any time. Table 36. Eleprox Baseline Value Register 0x2A (Reset Default: 0x00) 0x2A Bit7 D9 Bit6 D8 Bit5 D7 Bit4 D6 Bit3 D5 Bit2 D4 Bit1 D3 Bit0 D2
3.0
Proximity Sensing Status Indication
MPR121 provides a single proximity sensing status bit (ELE[12] in table below) in the Touch Status Register. This status bit changes as a result of internal detection algorithm using the proximity raw data with the proximity baseline value and proximity touch/release threshold setting. When ELE[12] is set, the proximity is deemed as detected, and undetected when ELE[12] is 0. Table 37. Status Register 0x00, 0x01 (Reset Default: 0x00) 0x00 Bit7 ELE[7] Bit7 OVCF Bit6 ELE[6] Bit6 0 Bit5 ELE[5] Bit5 0 Bit4 ELE[4] Bit4 ELE[12] Bit3 ELE[3] Bit3 ELE[11] Bit2 ELE[2] Bit2 ELE[10] Bit1 ELE[1] Bit1 ELE[9] Bit0 ELE[0] Bit0 ELE[8]
0x01
The update rate of this status bit will be determined by sampling rate and detection debounce setting. The status bit will not immediately change if the Debounce Register is non zero. This Debounce Register is globally effective to prevent possible flick noise for both touch and proximity sensing. The value in the Debounce Register determines how many numbers of sample intervals are needed to pass at the touch/release threshold before the status bit is finally changed. Table 38. Debounce Register 0x5B (Reset Default: 0x00) 0x5B Bit7 X Bit6 DR[2] Bit5 DR[1] Bit4 DR[0] Bit3 X Bit2 DT[2] Bit1 DT[1] Bit0 DT[0]
On ELEPROX status bit change, the interrupt pin will be asserted. 4.0 Proximity Detection Touch/Release Threshold Similar to the touch/release threshold for touch detection, the proximity detection also has a pair of touch/release threshold setting registers. The programmable threshold setting range is 0~63 count, representing the delta change below the baseline value when touched or released. The Threshold should be set according to the system SNR requirement and also provide adequate headroom for mass production variation. For normal application, set Touch Threshold slightly larger than Release Threshold so that there is no flick detection. Example: Touch Threshold = 0x08, Release Threshold = 0x05. Table 39. Eleprox Touch Threshold Register 0x59 (Reset Default: 0x00) 0x59 Bit7 D7 Bit6 D6 Bit5 D5 Bit4 D4 Bit3 D3 Bit2 D2 Bit1 D1 Bit0 D0
Table 40. Eleprox Release Threshold Register 0x5A (Reset Default: 0x00) 0x5A Bit7 D7 Bit6 D6 Bit5 D5 Bit4 D4 Bit3 D3 Bit2 D2 Bit1 D1 Bit0 D0
MPR121 40 Sensors Freescale Semiconductor
5.0
Proximity Baseline Filter Setting
As with the touch detection, the proximity detection also dedicates register sets for baseline filter control. These include the maximum half delta for rising/falling, the noise half delta for rising/falling/touched, the noise count limit for rising/falling/touched, and filter delay for rising/falling/touched. Table 41 shows an example setting for proximity sensing, the concept is to have quickest response on baseline rising (when hand takes away) and slowest response on baseline falling (when hand approaching). Refer to Freescale application note AN3891 for detailed description on baseline system. Table 41. Proximity Baseline Filter Registers 0x36~0x40 (Reset Default: all 0x00) Register Name ELEPROX Max Half Delta Rising ELEPROX Noise Half Delta Amount Rising ELEPROX Noise Count Limit Rising ELEPROX Filter Delay Limit Rising ELEPROX Max Half Delta Falling ELEPROX Noise Half Delta Amount Falling ELEPROX Noise Count Limit Falling ELEPROX Filter Delay Limit Falling ELEPROX Noise Half Delta Amount Touched ELEPROX Noise Count Limit Touched ELEPROX Filter Delay Limit Touched Register Address 0x36 0x37 0x38 0x39 0x3A 0x3B 0x3C 0x3D 0x3E 0x3F 0x40 Example Setting 0xFF 0xFF 0x00 0x00 0x01 0x01 0xFF 0xFF 0x00 0x00 0x00
6.0
Electrode Configuration for Proximity Sensing
Same as touch sensing, the proximity sensing requires that the charging current and time for the 13th Proximity Detection electrode to be properly set. This can be done in 3 ways: 1. Globally setting the AFE Configuration Register (0x5B) and Filter Configuration Register if recent current setting and time setting is zero. 2. Set by using Eleprox Electrode Current Register (0x6B) and Charge Time Register (0x72). 3. Using Auto-Configuration function to automatically set charge current and charge time for this 13th Proximity Detection electrode. It's recommended that Auto-Configuration is used for design efficiency if proximity sensing works properly in this way. Refer to Freescale application note AN3889 for details of the Auto-Configuration function.
MPR121 Sensors Freescale Semiconductor 41
7.0 AFE and Filter Configuration Register The last two registers relevant to proximity detection are the AFE Configuration Register and Filter Configuration Register. These two registers set the numbers of samples for the 2 level filters and the sampling interval for the second level filter. Table 42. Filter Configuration Registers 0x5D (Reset Default: 0x24) 0x5D Bit7 Bit6 CDT[2:0] Bit5 Bit4 SFI[1:0] Bit3 Bit2 Bit1 ESI[2:0] Bit0
Table 43. AFE Configuration Registers 0x5C (Reset Default: 0x10) 0x5C Bit7 FFI[1:0] Bit6 Bit5 Bit4 Bit3 CDC[4:0] Bit2 Bit1 Bit0
The FFI[1:0], SFI[1:0] and ESI[2:0] bits in the registers are those related to the first filter, second filter and sample interval respectively. These two registers are powered up with default setting of 0x24 and 0x10 respectively. The default setting is already workable for proximity sensing, but since ESI[2:0] is 100, the sampling interval is at 16 ms. If lower power consumption is desired, the user can adjust it to the value to find a balance between the proximity detection response time current consumption. For a detailed explanation on these registers, please refer to Freescale application note AN3890. OTHER DESIGN CONSIDERATIONS 1. Remember the paralleled plate capacitor model when considering the proximity detection. Larger sensing area (the effective sensing area formed by the sensing pad and material under detection, e.g. the surface area of hand projected to the sensing pad) gives longer proximity sensing distance. 2. The electric energy store in the capacitance (thus the strength of the sensing field) is proportional to the square of the voltage potential applied. Setting the auto-configure target level as high as possible will help extend the proximity sensing range. 3. Since increasing the sensing pad area also has the problem of making it easier to receive the electric noise. It's possible that the original solid sensing pad can be replaced by a series of circles or x hatch patterns. Refer to Freescale application note AN3863 for more detailed discussion on electrode and layout design considerations.
MPR121 42 Sensors Freescale Semiconductor
AN3894: MPR121 GPIO and LED Driver Function
INTRODUCTION MPR121 is a feature rich second generation touch sensor controller after Freescale's initial release of the MPR03x series device. MPR121 not only has priority unique features like independent electrode auto configuration (refer to AN3889), 13th simulated pseudo electrode for proximity detection (refer to AN3893), it also has 8 GPIO ports with LED driver capability. The GPIO and LED driver function can be used when not all the 12 input sensing channels are occupied for touch sensing detection, which is made possible by internal multiplexed pin structure. This increaseS the cost efficiency of the system and makes the MPR121 fit for even wider application. MULTIFUNCTION PINS MPR121 has 12 input sensing channels ELE0~ELE11, which occupies pin 8 to pin 19. Among these, pin 12 to pin 19 are multifunction pins. When these multifunction pins are not configured as electrodes, they may be used to drive LED or for general GPIO purpose.
PIN # ELECTRODE GPIO 8 ELE0 -- 9 ELE1 -- 10 ELE2 -- 11 ELE3 -- 12 ELE4 GPIO0 13 ELE5 GPIO1 14 ELE6 GPIO2 15 ELE7 GPIO3 16 ELE8 GPIO4 17 ELE9 GPIO5 18 ELE10 GPIO6 19 ELE11 GPIO7
VDD 1.71 V to 2.75 V
VDD 1.71 V to 2.75 V 0.1 F
20 6 5 1 2 3 4 7
VDD VSS VREG IRQ SCL SDA ADDR REXT
ELE11/LED7 ELE10/LED6 ELE9/LED5 ELE8/LED4 ELE7/LED3 ELE6/LED2 ELE5/LED1 ELE4/LED0 ELE3
19 18 17 16 15 14 13 12 11 10 9 8
75 k 1% GND GND
ELE2 ELE1 ELE0 MPR121Q TOUCH SENSOR
Figure 46. Configuration 1: MPR121 runs from a 1.71 V to 2.75 V supply.
MPR121 Sensors Freescale Semiconductor 43
VDD 2.5 V to 3.6 V
VDD 2.5 V to 3.6 V 0.1 F
20 6 5 1 2 3 4 7
VDD VSS VREG IRQ SCL SDA ADDR REXT
ELE11/LED7 ELE10/LED6 ELE9/LED5 ELE8/LED4 ELE7/LED3 ELE6/LED2 ELE5/LED1 ELE4/LED0 ELE3
19 18 17 16 15 14 13 12 11 10 9 8
0.1 F 75 k 1% GND GND GND
ELE2 ELE1 ELE0 MPR121Q TOUCH SENSOR
Figure 47. Configuration 2: MPR121 runs from a 2.5 V to 3.6 V supply. These registers control GPIO function. D7~D0 bits corresponds GPIO7~GPIO0 pins respectively. The GPIO control registers can write always regardless Shutdown and Run mode. Table 44. GPIO Control Registers
Name GPIO Control 0 GPIO Control 1 GPIO Data GPIO Direction GPIO Enable GPIO Data Set GPIO Data Clear GPIO Data Toggle Address 0x73 0x74 0x75 0x76 0x77 0x78 0x79 0x7A D7 CTL0[7] CTL1[7] DAT[7] DIR[7] EN[7] SET[7] CLR[7] TOG[7] D6 CTL0[6] CTL1[6] DAT[6] DIR[6] EN[6] SET[6] CLR[6] TOG[6] D5 CTL0[5] CTL1[5] DAT[5] DIR[5] EN[5] SET[5] CLR[5] TOG[5] D4 CTL0[4] CTL1[4] DAT[4] DIR[4] EN[4] SET[4] CLR[4] TOG[4] D3 CTL0[3] CTL1[3] DAT[3] DIR[3] EN[3] SET[3] CLR[3] TOG[3] D2 CTL0[2] CTL1[2] DAT[2] DIR[2] EN[2] SET[2] CLR[2] TOG[2] D1 CTL0[1] CTL1[1] DAT[1] DIR[1] EN[1] SET[1] CLR[1] TOG[1] D0 CTL0[0] CTL1[0] DAT[0] DIR[0] EN[0] SET[0] CLR[0] TOG[0]
EN[7:0], DIR[7:0], CTL0[7:0], CTL1[7:0]: Configuration Register The number of touch sensing electrodes (and therefore the number of GPIO ports available) is configured by the Electrode Configuration register (0x5E) and GPIO Enable Register (0x77), but electrode configuration has higher priority than GPIO feature. When a pin is enabled as GPIO but is also selected as electrode by Electrode Configuration Register, the GPIO function is disabled immediately and it becomes an electrode during Run mode. But all 8 ports automatically become GPIO ports in Shutdown mode because none of the ports are being enabled as touch electrodes in Shutdown mode. During the shutdown mode just after power on reset, all 8 GPIO ports are in high impedance as all the GPIO ports are default disabled. Take care to program unused ports which are not going to be used as either touch electrodes or GPIO to avoid floating inputs or outputs shorted to a rail. One approach is to enable unused ports to be GPIO inputs with internal pull-up or pull-down.
MPR121 44 Sensors Freescale Semiconductor
The GPIO system allows the GPIO pins to be set as input or output. When an EN bit sets, the corresponding GPIO pin is enabled and the function is configured by CTL0, CTL1 and DIR bits. When the port is used as input, it can be configured as normal input or with additional internal pull-down or pull-up for input port. For output configuration, it can be push/pull or open drain.
EN 0 1 1 1 1 1 1 1 1 DIR X 0 0 0 0 1 1 1 1 CTL[0:1] XX 00 10 11 01 00 11 10 01 DESCRIPTION GPIO function is disabled. Port is high-z state. GPIO port becomes input port. GPIO port becomes input port with internal pull-down. GPIO port becomes input port with internal pull-up. Not defined yet (as same as CTL = 00). GPIO port becomes CMOS output port. GPIO port becomes high side only open drain output port for LED driver. GPIO port becomes low side only open drain output port. Not defined yet (as same as CTL = 00).
DAT[7:0]: Data Register When a GPIO is as output, the GPIO port outputs the bit level of this register. The output level toggle holds on any electrode charging and AD conversion and the level transition will be occurred after the AD conversion. Reading this register returns the content of the DAT register (not a level of the port). When a GPIO is as input, reading this register returns latched input level of the corresponding port (not contents of the DAT register). A write changes content of the register, but not affect to the input function. SET[7:0]: Set Data Register Writing a "1" to bits in this register will set them in the Data Register. CLR[7:0]: Clear Data Register Writing a "1" to bits in this register will clear them in the Data Register. TOG[7:0]: Toggle Data Register Write a bit with "1" to the GPIO Data Set Register, GPIO Data Clear Register, and GPIO Toggle Register set/clear/toggle contents of the corresponding DAT bit. Write "0" has no meaning. Using of those registers allows any individual port(s) to be able individually set, cleared, or toggled without affecting other ports. Reading those register returns as same as DAT register reading. LED DRIVER Each GPIO pin has LED driver capability which can source up to 12 mA. When GPIO is used to driver LED, connect the GPIO output to an LED forward biased with its cathode to GND so that GPIO output high lights the LED. Place a current limiting resistor is series with LED to limit the current below 12 mA (refer to the typical application circuit). When LED dimming control is needed, the PWM control register can be set to get the desired dimming control. Alternatively, the PWM can also be used to drive the beeper. Table 45. PWM_[3:0}: PWM Duty Control Registers
Name PWM 0 PWM 1 PWM 2 PWM 3 Address 0x81 0x82 0x83 0x84 D7 PWM1[3] PWM3[3] PWM5[3] PWM7[3] D6 PWM1[2] PWM3[2] PWM5[2] PWM7[2] D5 PWM1[1] PWM3[1] PWM5[1] PWM7[1] D4 PWM1[0] PWM3[0] PWM5[0] PWM7[0] D3 PWM0[3] PWM2[3] PWM4[3] PWM6[3] D2 PWM0[2] PWM2[2] PWM4[2] PWM6[2] D1 PWM0[1] PWM2[1] PWM4[1] PWM6[1] D0 PWM0[0] PWM2[0] PWM4[0] PWM6[0]
MPR121 Sensors Freescale Semiconductor 45
PWM0[3:0] ~ PWM7[3:0] is used to set the PWM duty of GPIO0 ~ GPIO7 respectively. The power up reset default setting for these four register is 0x00. When a GPIO is programmed as output and the DAT register is "1" and if the corresponding PWM_ [3:0] register is not zero, the GPIO pin outputs PWM waveform. The PWM period is fixed 8ms (1/256 of 32 KHz OSC) and PWM_ [3:0] register decides duty of the waveform.
PWM_ [3:0] 0 1 2 -- 15 Description (_ is 0~7) PWM is off, GPIO outputs stable high when DAT register is "1" GPIO output duty is 1:15 (mostly output low) GPIO output duty is 2:14 (mostly output low) -- GPIO output duty is 15:1 (mostly output high)
These register can be read/write any time, even if MPR121 is in Run Mode. When the register changes during PWM enables, a mixed duty cycle would be possible to occur. The PWM duty is not so much accurate, because GPIO output transition (include PWM) inhibits during measurement state. Therefore, when interval time (=Touch Detection Sample Interval) is close to measurement time (depends on charge time, AFE Samples and number of measurement electrodes), the PWM operation is disturbed and the waveform couldn't keep programmed duty.
MPR121 46 Sensors Freescale Semiconductor
AN3895: MPR121 Serial Communication
INTRODUCTION The MPR121 uses an I2C Serial Interface. The I2C protocol implementation and the specifics of communicating with the Touch Sensor Controller are detailed in this application note. SERIAL-ADDRESSING The MPR121 operates as a slave that sends and receives data through an I2C 2-wire interface. The interface uses a Serial Data Line (SDA) and a Serial Clock Line (SCL) to achieve bi-directional communication between master(s) and slave(s). A master (typically a microcontroller) initiates all data transfers to and from the MPR121, and it generates the SCL clock that synchronizes the data transfer. The MPR121 SDA line operates as both an input and an open-drain output. A pull-up resistor, typically 4.7 k, is required on SDA. The MPR121 SCL line operates only as an input. A pull-up resistor, typically 4.7 k, is required on SCL if there are multiple masters on the 2-wire interface, or if the master in a single-master system has an open-drain SCL output. Each transmission consists of a START condition (Figure 48) sent by a master, followed by the MPR121's 7-bit slave address plus R/W bit, a register address byte, one or more data bytes, and finally a STOP condition.
SDA
tBUF tSU DAT tLOW tHD DAT tHIGH tR tF REPEAT ED ST ART CONDIT ION ST OP CONDIT ION ST ART CONDIT ION tSU STA tHD STA tSU STO
SCL
tHD STA ST ART CONDIT ION
Figure 48. Wire Serial Interface Timing Details START AND STOP CONDITIONS Both SCL and SDA remain high when the interface is not busy. A master signals the beginning of a transmission with a START (S) condition by transitioning SDA from high to low while SCL is high. When the master has finished communicating with the slave, it issues a STOP (P) condition by transitioning SDA from low to high while SCL is high. The bus is then free for another transmission.
SDA SCL
DATA LINE STABLE DATA VALID
CHANGE OF DATA ALLOWED
Figure 49. Start and Stop Conditions
MPR121 Sensors Freescale Semiconductor 47
BIT TRANSFER One data bit is transferred during each clock pulse (Figure 50). The data on SDA must remain stable while SCL is high.
SDA SCL
S
START CONDITION
Figure 50. Bit Transfer
P
STOP CONDITION
ACKNOWLEDGE The acknowledge bit is a clocked 9th bit (Figure 51) which the recipient uses to handshake receipt of each byte of data. Thus each byte transferred effectively requires 9 bits. The master generates the 9th clock pulse, and the recipient pulls down SDA during the acknowledge clock pulse, such that the SDA line is stable low during the high period of the clock pulse. When the master is transmitting to the MPR121, the MPR121 generates the acknowledge bit, since the MPR121 is the recipient. When the MPR121 is transmitting to the master, the master generates the acknowledge bit, since the master is the recipient.
START CONDITION
CLOCK PULSE FOR ACKNOWLEDGEMENT 1 2 8 9
SCL SDA
BY TRANSMITTER
SDA
BY RECEIVER
S
Figure 51. Acknowledge
THE SLAVE ADDRESS The MPR121 has a 7-bit long slave address (Figure 52). The bit following the 7-bit slave address (bit eight) is the R/W bit, which is low for a write command and high for a read command.
SDA
1 MSB
0
0
1
0
1
0
R/W
ACK
SCL
Figure 52. Slave Address The MPR121 monitors the bus continuously, waiting for a START condition followed by its slave address. When a MPR121 recognizes its slave address, it acknowledges and is then ready for continued communication. The MPR121 slave addresses are show in Table 46. Table 46. ADDR Pin Connection VDD VSS SDA SCL MPR121 48 Sensors Freescale Semiconductor I2C Address 0x4C 0x4D 0x4E 0x4F
MESSAGE FORMAT FOR WRITING THE MPR121 A write to the MPR121 comprises the transmission of the MPR121's keyscan slave address with the R/W bit set to 0, followed by at least one byte of information. The first byte of information is the command byte. The command byte determines which register of the MPR121 is to be written by the next byte, if received. If a STOP condition is detected after the command byte is received, the MPR121 takes no further action (Figure 53) beyond storing the command byte. Any bytes received after the command byte are data bytes.
Command byte is stored on receipt ofSTOP condition Acknowledge from MPR121
D15
D14
D13
D12
D11
D10
D9
D8
S
SLAVE ADDRESS R/W
0
A
COMMAND BYTE Acknowledge from MPR121
A
P
Figure 53. Command Byte Received Any bytes received after the command byte are data bytes. The first data byte goes into the internal register of the MPR121 selected by the command byte (Figure 54).
Acknowledge from MPR121 How command byte and data byte map into MPR121's registers Acknowledge from MPR121 S SLAVE ADDRESS R/W 0 A COMMAND BYTE A DATA BYTE 1 byte Auto-increment memory word address A P D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 Acknowledge from MPR121 D3 D2 D1 D0
Figure 54. Command and Single Data Byte Received If multiple data bytes are transmitted before a STOP condition is detected, these bytes are generally stored in subsequent MPR121 internal registers because the command byte address generally auto-increments. MESSAGE FORMAT FOR READING THE MPR121 MPR121 is read using MPR121's internally stored register address as address pointer, the same way the stored register address is used as address pointer for a write. The pointer generally auto-increments after each data byte is read using the same rules as for a write. Thus, a read is initiated by first configuring MPR121's register address by performing a write (Figure 53) followed by a repeated start. The master can now read 'n' consecutive bytes from MPR121, with first data byte being read from the register addressed by the initialized register address.
Acknowledge from master Command byte is stored on receipt ofSTOP condition Acknowledge from MPR121 S SLAVE ADDRESS R/W 0 A COMMAND BYTE Acknowledge from MPR121 A S D15 D14 D13 D12 D11 D10 D9 D8 Repeated Start Acknowledge from MPR121 SLAVE ADDRESS R/W 1 A DATA BYTE n bytes auto-increment memory word address A P D7 D6 D5 D4 D3 D2 D1 D0 Stop
Figure 55. Reading MPR121 OPERATION WITH MULTIPLE MASTER The application should use repeated starts to address the MPR121 to avoid bus confusion between I2C masters.On a I2C bus, once a master issues a start/repeated start condition, that master owns the bus until a stop condition occurs. If a master that does not own the bus attempts to take control of that bus, then improper addressing may occur. An address may always be rewritten to fix this problem. Follow I2C protocol for multiple master configurations.
MPR121 Sensors Freescale Semiconductor 49
AN3944: MPR121 Quick Start Guide
INTRODUCTION The MPR121 is Freescale Semiconductor's top of the line touch sensor and can fit into a wide range of applications. These applications can all be accommodated by having a device a with a very large range of flexibility. While all of these added features can allow for a wide range of flexibility, they can also add an unnecessary layer of complication. For advanced users who want to do more than basic touch detection, additional information can be found in other application notes. To start, the device is configured through an I2C serial interface. The following table lists the registers that are initialized. The order they are written in is not significant except that register 0x05E, the Electrode Configuration Register must be written last.
Register Address 0x2B 0x2C 0x2D 0x2E 0x2F 0x30 0x31 0x32 0x41 0x42 0x43 0x44 0x45 0x46 0x47 0x48 0x49 0x4A 0x4B 0x4C 0x4D 0x4E 0x4F 0x50 0x51 0x52 0x53 0x54 0x55 0x56 0x57 0x58 0x5D 0x5E 0x7B MHD Rising NHD Amount Rising NCL Rising FDL Rising MHD Falling NHD Amount Falling NCL Falling FDL Falling
Register Name
Value 0x01 0x01 0x00 0x00 0x01 0x01 0xFF 0x02 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x04 0x0C 0x0B
Application Note AN3891 AN3891 AN3891 AN3891 AN3891 AN3891 AN3891 AN3891 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3890 AN3890 AN3889
Section A A A A B B B B C C C C C C C C C C C C C C C C C C C C C C C C D E F
ELE0 Touch Threshold ELE0 Release Threshold ELE1 Touch Threshold ELE1 Release Threshold ELE2 Touch Threshold ELE2 Release Threshold ELE3 Touch Threshold ELE3 Release Threshold ELE4 Touch Threshold ELE4 Release Threshold ELE5 Touch Threshold ELE5 Release Threshold ELE6 Touch Threshold ELE6 Release Threshold ELE7 Touch Threshold ELE7 Release Threshold ELE8 Touch Threshold ELE8 Release Threshold ELE9 Touch Threshold ELE9 Release Threshold ELE10 Touch Threshold ELE10 Release Threshold ELE11 Touch Threshold ELE11 Release Threshold Filter Configuration Electrode Configuration AUTO-CONFIG Control Register 0
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Register Address 0x7D 0x7E 0x7F
Register Name AUTO-CONFIG USL Register AUTO-CONFIG LSL Register AUTO-CONFIG Target Level Register
Value 0x9C 0x65 0x8C
Application Note AN3889 AN3889 AN3889
Section F F F
The following sections describe what each of the defaults do and recommendations for variations. Section A
Register Address 0x2B 0x2C 0x2D 0x2E MHD Rising NHD Amount Rising NCL Rising FDL Rising
Register Name
Value 0x01 0x01 0x00 0x00
Application Note AN3891 AN3891 AN3891 AN3891
Description: This group of setting controls the filtering of the system when the data is greater than the baseline. The setting used allow the filter to act quickly and adjust for environmental changes. Additionally, if calibration happens to take place while a touch occurs, the value will self adjust very quickly. This auto-recovery or snap back prevents repeated false negative for a touch detection. Variation: As the filter is sensitive to setting changes, it is recommended that users read AN3891 before changing the values. In most cases these default values will work
Section B
Register Address 0x2F 0x30 0x31 0x32 MHD Falling NHD Amount Falling NCL Falling FDL Falling
Register Name
Value 0x01 0x01 0xFF 0x02
Application Note AN3891 AN3891 AN3891 AN3891
Description: This group of setting controls the filtering of the system, when the data is less than the baseline. The settings slow down the filter as the negative charge is in the same direction as a touch. By slowing down the filter, touch signals are "rejected" by the baseline filter. While at the same time lon term environmental change that occur slower than at a touch are accepted. This low pass filter both allows for touches to be detected properly while preventing false positive by passing environmental change through the filter. Variation: As the filter is sensitive to setting changes, it is recommended that users read AN3891 before changing the values. In most cases these default values will work
MPR121 Sensors Freescale Semiconductor 51
Section C
Register Address 0x41 0x42 0x43 0x44 0x45 0x46 0x47 0x48 0x49 0x4A 0x4B 0x4C 0x4D 0x4E 0x4F 0x50 0x51 0x52 0x53 0x54 0x55 0x56 0x57 0x58
Register Name ELE0 Touch Threshold ELE0 Release Threshold ELE1 Touch Threshold ELE1 Release Threshold ELE2 Touch Threshold ELE2 Release Threshold ELE3 Touch Threshold ELE3 Release Threshold ELE4 Touch Threshold ELE4 Release Threshold ELE5 Touch Threshold ELE5 Release Threshold ELE6 Touch Threshold ELE6 Release Threshold ELE7 Touch Threshold ELE7 Release Threshold ELE8 Touch Threshold ELE8 Release Threshold ELE9 Touch Threshold ELE9 Release Threshold ELE10 Touch Threshold ELE10 Release Threshold ELE11 Touch Threshold ELE11 Release Threshold
Value 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A 0x0F 0x0A
Application Note AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892 AN3892
Description: The touch threshold registers set the minimum delta from the baseline when a touch is detected 0x0F or 15 in decimal is an estimate of the minimum value for touch. Most electrodes will work with this value even if they vary greatly in size and shape. The value of 0x0A or 10 is the release threshold register allowed for hysteresis in the touch detection. Variation: For very small electrodes, smaller values can be used and for very large electrodes the reverse is true. One easy method is to view the deltas actually seen in a system and set the touch at 80% and release at 70% of delta for good performance.
Section D
Register Address 0x5D Filter Configuration Register Name Value 0x04 Application Note AN3890
Description: There are three settings embedded in this register so it is only necessary to pay attention to one. The ESI controls the sample rate of the device. In the default, the setting used is 0x00 for 1 ms sample rate. Since the SFI is set to 00, resulting in 4 samples averaged, the response time will be 4 ms. Variation: To save power, the 1 ms can be increased to 128 ms by increasing the setting to 0x07. The values are base 2 exponential thus 0x01 = 2 ms; 0x02 = 4 ms; and so on to 0x07 = 128 ms. Most of the time, 0x04 results in the best compromise between power consumption and response time.
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Section E
Register Address 0x5E Register Name Electrode Configuration Value 0x0C Application Note AN3890
Description: This register controls the number of electrodes being enabled and the mode the device is in. There are only two modes, Standby (when the value is 0x00) and Run (when the value of the lower bit is non-zero). The default value shown enables all 12 electrodes by writing decimal 12 or hex 0x0C to the register. Typically other registers cannot be changed while the part is running so this register should always be written last. Variation: During debug of a system, this register will change between the number of electrodes and 0x00 every time a register needs to change. In a production system, this register will only need to be written when the mode is changed from Standby to Run or vise versa.
Section F
Register Address 0x7B 0x7D 0x7E 0x7F Register Name AUTO-CONFIG Control Register 0 AUTO-CONFIG USL Register AUTO-CONFIG LSL Register AUTO-CONFIG Target Level Register Value 0x0B 0x9C 0x65 0x8C Application Note AN3889 AN3889 AN3889 AN3889
Description: These are the settings used for the Auto Configuration. They enable AUTO-CONFIG and AUTO_RECONFIG. In addition they set the target range for the baseline. The upper limit is set to 190, the target is set to 180 and the lower limit is set to 140. Variation: In most cases these values will never need to be change, but if a case arises, a full description is found in application note AN3889.
CONCLUSION In many applications for the MPR121, the default settings presented in this document will be sufficient for both design time activities as well as in the production implementation.
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