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Integrated Mixed-Signal Solutions
STAC9766 and STAC9750 Crystal Elimination Circuit Use and Implementation for Motherboards
SigmaTel Application Note
Version 1.1
2-AN010-1.1-0801
Copyright (c) 2001 SigmaTel, Inc. All rights reserved. All contents of this document are protected by copyright law and may not be reproduced without the express written consent of SigmaTel, Inc. SigmaTel, the SigmaTel logo, and combinations thereof are trademarks of SigmaTel, Inc. Other product names used in this publication are for identification purposes only and may be trademarks or registered trademarks of their respective companies. The contents of this document are provided in connection with SigmaTel, Inc. products. SigmaTel, Inc. has made best efforts to ensure that the information contained herein is accurate and reliable. However, SigmaTel, Inc. makes no warranties, express or implied, as to the accuracy or completeness of the contents of this publication and is providing this publication "AS IS". SigmaTel, Inc. reserves the right to make changes to specifications and product descriptions at any time without notice, and to discontinue or make changes to its products at any time without notice. SigmaTel, Inc. does not assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential, or incidential damages.
STAC9766 and STAC9750 Crystal Elimination Circuit Use and Implementation for Motherboards
The STAC9766 and STAC9750 incorporate an internal crystal elimination circuit that allows the standard 24.576 MHz AC'97 external crystal to be eliminated from motherboard applications. This document describes the normal "crystal installed" mode of operation, as well as the "crystal eliminated" mode.
NORMAL CRYSTAL CONFIGURATION
The 24.576 MHz frequency is not typically available on motherboards, and is usually generated by an external crystal connected directly to the audio codec. The frequency was originally selected because of the simple relationship between 44.1 kHz audio and 48 kHz audio data streams. In the normal mode with the crystal installed, the crystal is connected to the XTALin and XTALout pins of the codec. Capacitors C1 and C2 are connected at either side of the crystal for stability and lower EMI emissions. The CID1 and CID0 pins (pins 45 and 46) of the codec are used to select the codec ID, which is 0 for primary codecs, and 1, 2, or 3 for secondary codecs. For motherboard codecs, the CID pins are normally left open and the codec is assigned the primary 0 ID automatically.
C2 2 24.576MHz Y1 C1 3 XTAL_OUT XTAL_IN
STAC9750 STAC9766 AC'97 Audio Codec
46
CID1
CID1,0 = OPEN For Primary Codec Operation
45
CID0
CRYSTAL ELIMINATION MODE
The crystal elimination mode is automatically entered when the XTALout pin is grounded. In this mode the codec will accept an external clock source from the motherboard. The expected frequencies are: 48 MHz that is related to the USB clocks, 27 MHz that is related to digital video systems, or 14.31818 MHz that is a former graphics card clock but still used on most motherboards today. A 24.576 MHz clock may also be used in both normal mode, and crystal elimination mode.
R2 Clock C3
To motherboard clock source.
2
XTAL_IN
3 R1
XTAL_OUT
STAC9750 STAC9766 AC'97 Audio Codec
R1 can use same pads as C1
46 45
CID1
CID0
* R3
* R4
2
2-AN010-1.1-0801
STAC9766 and STAC9750 Crystal Elimination Circuit Use and Implementation for Motherboards
The CID pull-down resistors (R3 and R4) should be installed according to the table below to indicate which frequency is used. The C1 surface mount pads can be used for the shorting resistor on XTALout, or another component can be added to the schematic as an assembly option. The clock ac-coupling capacitor can range from 0.1 uF to 0.01 uF and a series resistor can be added to reduce any EMI issues. If the resistor is added, it should generally be located closer to the clock source for best results.
Clock Source at XTALin 48 MHz 27 MHz 14.31818 MHz 24.576 MHz R3 for CID1 1K Open Open 1K R4 for CID0 Open 1K Open 1K Codec Status Primary, ID = 0 Primary, ID = 0 Primary, ID = 0 Primary, ID = 0
Note: XTALout must be shorted to ground in this mode Note: Clock source configurations are different for CA1 and CA2 revisions.
The STAC9766/67and STAC9750/51 datasheets, product briefs and additional information is available at www.sigmatel.com
2-AN010-1.1-0801
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