AN1068/0101 1/8 application note selecting between rom, fastrom and flash for a microcontroller by microcontroller division applications introduction a customer who develops an mcu-based application needs various levels of flexibility in order to perform code modifications at different times in the life cycle of the product (these levels are explained on the next page). to satisfy these requirements, stmicroelectronics supports sev- eral device types within two main groups of microcontroller product families: C eprom, otp, fastrom and rom microcontroller families C flash, fastrom and rom microcontroller families this application note discusses the second group of families. for information on the first group, refer to application note an886. definition of terms flash: flash devices are electrically programmable and erasable. device programming is typ- ically performed by the customer, so changes to the program code can be made quickly and easily. flexibility is further enhanced for the customer by the use of in-circuit programming, in application programming, and in-circuit testing. n in-circuit programming (icp) allows the customer to program or erase the device after it has been soldered on the application board. n in application programming (iap) allows the user to program or erase part of the flash memory while the application is running. n ict allows the customer to program test routines in flash memory to be executed during the board manufacturing phase and subsequently replaced by the final application code. fastrom (factory advanced service technique read only memory): this type of mcu is a flash device pre-programmed by stmicroelectronics with the customers code and selected options. the advantage of fastrom, compared to flash, is improved programming effi- ciency for large quantities (10,000+) and compared to rom, it has the advantage of a shorter leadtime and the devices can be reprogrammed. rom (read only memory): rom devices are programmed by stmicroelectronics at the fab- rication step using a special mask containing the customer code. therefore, the code cannot be modified afterwards. costs are highly dependent on the flexibility given to the device (ability to be easily erased or programmed). rom is the cheapest technology but provides little flexibility whereas flash is more flexible but its manufacturing cost is higher. 1
selecting between rom, fastrom and flash for a microcontroller 2/8 1 typical application development flow when a new application is developed, different device versions will be used at each step of the development, depending on the required programming flexibility . during the design and validation phases , a high flexibility is required and only a small number of parts are necessary, therefore the use of flash is recommended. the next step is pre-production phase : only a few code updates are needed at a reasonable device cost. again, the best choice is to use flash memory. finally, when the mass produc- tion phase begins, there is no more need for corrections since the product has been fully op- timized, so rom is the most suitable if very high volumes are needed. otherwise (low to me- dium volumes) the most effective solution is to continue using stmicroelectronics' competi- tively priced flash. the following table summarizes the main benefits and drawbacks of using rom or flash mcu devices. design phase validation phase pre-production phase production phase st solution flash flash flash flash rom code updates none number of units rom flash + cheaper than flash (simpler process and testing) lower failure rate (less handling, no programming) ability to be programmed directly by the final user depending on the silicon technology used, 100 or 10,000 write/erase cycle endurance at 25c ambient temperature - limited flexibility (customer code implemented at masking stage) higher inventory risks higher failure rate compared to rom due to customer handling and programming expensive silicon manufacturing process
3/8 selecting between rom, fastrom and flash for a microcontroller 2 flash write/erase cycle endurance during the product and process qualification phase, stmicroelectronics performs three times the minimum number of write/erase cyclesat room temperature (300 cycles to qualify a min. spec of 100 cycles at 25c). in addition, 100 cycles are performed over the whole operating temperature range (-40 -125c) during pruduct qualification. due to process differences, some microcontrollers (all st9 mcus and st7 devices with more that 8 kbytes of flash memory, are specified with a min. endurance of 10,000 write/erase cy- cles.to reduce chip size and cost smaller st7 devices use a process that guarantees 100- cycle endurance for flash memory. data retention tests are performed at high temperature to guarantee 20 data retention.
selecting between rom, fastrom and flash for a microcontroller 4/8 3 comparison of rom and flash rom and flash devices have almost the same functional and electrical behaviour in an appli- cation because stmicroelectronics designs the two products with the same methodology. rom and flash devices are qualified using the same procedures. they are tested using the same test flows (except for the flash-specific programming tests) and with the same parameter limits . a good indication of device similarity is emc (electromagnetic noise immunity) meas- urements performed on both versions (rom and flash) for the same mcu device. st72334 (rom version) st72c334 (flash version) 100 khz. 1 ghz. 100 khz. 1 ghz.
5/8 selecting between rom, fastrom and flash for a microcontroller 4 typical manufacturing lead time for rom and flash the complexity of all the operations needed to manufacture a component implies a certain time period. understanding stmicroelectronics mcu manufacturing cycle is important in order to establish good relationships with customers. the numbers given here are typical and subject to change in the future figure 1. typical fabrication leadtime. in order to limit lead time on rom products, stmicroelectronics has introduced pre-diffusion technology on selected products. this allows a two-week reduction in total cycle time. also no- tice that mcu devices have to be ordered in specified minimum quantities for rom versions. figure 2. typical leadtime for standard products 5* 7 1 1 1 1 1 transit time** wafer fabrication standard wafers* *pre-diffused wafers (selected products) wafer probing assembly final test shipment **wafer and assembly sites are typically located in different countries. ty p i c a l leadtime for standard product 0 - 5,000 units > 5,000 units min. quantity 4 10 12 rom standard rom pre- diffused flash 2* (weeks) 12 4 rom fa st * gold standard products > 5,000/10000 units
selecting between rom, fastrom and flash for a microcontroller 6/8 5 minimum order quantities for rom and fastrom the following minimum order quantities apply to rom and fastrom microcontroller de- vices:. rom fastrom minimum quantity per year minimum order quantity minimum quantity per line item minimum quantity per year and per line item st6 family 100000 50000 10000 5000 st7 and st9 families 50000 25000 5000 5000
7/8 selecting between rom, fastrom and flash for a microcontroller 6 flash reliability why do rom devices have very low failure rates? for rom parts, the customer program is included at a specific mask level of the wafer fab- rication. therefore, the complete product functionality is present in both the die and the as- sembled product. this functionality can be fully evaluated at both wafer probing and final electrical test , thus ensuring a low reject rate at customer manufacturing stage. testing cycling endurance and data retention for flash devices to ensure optimal flash quality, stmicroelectronics tests two important device characteris- tics: write/erase cycling endurance and data retention . the program memory of a flash mcu should be seen as a number of cells that will be activated during programming and de- activated during erasing. during the various test phases, the dice are electrically tested and the memory is programmed and erased to verify cycling endurance . they are placed in high temperature bake to accel- erate any possible memory retention defects. the dice are then tested again to check data re- tention . description of flash cycling endurance test figure 3. comparison of the electrical tests performed on flash and rom although the write/erase cycling endurance of the flash die is verified as fully functional at probe test, the assembly process can affect this parameter in the finished product (for ex- ample by damaging some cells). it is therefore necessary to make a final test again to check cycling endurance . rom flash tested feature full electrical test memory check full electrical test memory check write/erase cycling flash and erasing programmability bake full electrical test memory check full electrical test memory check write/erase cycling flash and erasing programmability data retention full electrical test memory check die assembled product
selecting between rom, fastrom and flash for a microcontroller 8/8 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without the express written approval of stmicroele ctronics. the st logo is a registered trademark of stmicroelectronics ? 2001 stmicroelectronics - all rights reserved. purchase of i 2 c components by stmicroelectronics conveys a license under the philips i 2 c patent. rights to use these components in an i 2 c system is granted provided that the system conforms to the i 2 c standard specification as defined by philips. stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco - sin gapore - spain sweden - switzerland - united kingdom - u.s.a. http://www.st.com
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