AN886/0903 1/9 rev. 1.0 AN886 application note selecting between rom, fastrom and otp 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 first group of families. for information on the second group, refer to application note an1068. 1
2/9 selecting between rom, fastrom and otp for a microcontroller definition of terms windowed eprom (erasable programmable read only memory): this type of mcu device can be programmed then erased using uv light. eprom mcu devices are sold in ceramic packaging with a quartz window. they are reprogrammable outside systems. otp (one time programmable eprom): otp is a type of eprom sold in plastic packaging. unlike uv eproms that have a quartz window in the package above the chip to allow erasure by uv light, otp memory cannot be erased once it has been programmed. otps are typi- cally programmed by the customer. fastrom (factory advanced service technique read only memory): this type of mcu is a otp device pre-programmed by stmicroelectronics with the customers code and selected options. the advantage of fastrom, compared to otp, is improved programming efficiency for large quantities (10,000+) and compared to rom, it has the advantage of a shorter lead- time. rom (read only memory): rom devices are programmed at the fabrication step using a spe- cial mask containing the customer code. therefore, the code can't be modified after that step. costs are highly depending on the flexibility given to the device (ability to be easily erased or programmed). rom is the cheapest technology but provides little flexibility whereas otp and eprom are more flexible but their manufacturing cost is higher. the high cost of eprom mcu devices is due to the price of ceramic packages. figure 1. cost versus flexibility for different mcu types rom otp eprom cost flexibility + + _ _ vr02100a 2
3/9 selecting between rom, fastrom and otp for a microcontroller 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 phase , a high flexibility is required and only a small number of parts are necessary, therefore the use of uv erasable eprom is recommended. then, fewer code cor- rections (and a significant number of parts) are needed during validation phase : at this point otp is the best solution. 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 otp 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 adapted if very high volumes are needed. otherwise (low to me- dium volumes) the most effective solution is to continue using stmicroelectronics competi- tively priced otp. the following table summarizes the main benefits and drawbacks of using rom, otp or win- dowed eprom mcu devices. design phase validation phase pre-production phase production phase st solution eprom otp otp otp rom code updates none number of units rom otp eprom + cheaper than otp (simpler process and testing) lower failure rate ( less handling, no program- ming) lower cost compared to win- dowed eprom (use of cheaper plastic packag- es) ability to be programmed di- rectly by the final user high flexibility (programming, code check and erasure cycle in less than 60 minutes) - limited flexibility (customer code implemented at masking stage) higher inventory risks higher failure rate compared to rom due to customer handling and programming expensive ceramic packages
4/9 selecting between rom, fastrom and otp for a microcontroller 2 comparison of production flows for rom and otp rom and otp devices have exactly the same functional and electrical behaviour in an appli- cation because stmicroelectronics designs the two products with the same methodology. rom and otp devices are qualified using the same procedures. they are tested using the same test flows (see further) and with the same parameter limits . a good indication of device similarity is emc (electromagnetic noise immunity) measurements performed on both versions (rom and otp) for the same mcu device. st6220 (rom version) st62t20 (otp version)
5/9 selecting between rom, fastrom and otp for a microcontroller 3 typical manufacturing lead time for rom and otp 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 2. typical fabrication leadtime in weeks. in order to limit lead time on rom products, stmicroelectronics has introduced pre-diffusion technology on selected products. this allows a two weeks reduction in total cycle time. also notice that mcu devices have to be ordered in specified minimum quantities for rom version. figure 3. 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 fast * gold standard products > 5,000/10000 units
6/9 selecting between rom, fastrom and otp for a microcontroller 4 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
7/9 selecting between rom, fastrom and otp for a microcontroller 5 otp 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 programmability and data retention for otp devices to ensure optimal otp quality, stmicroelectronics tests two important device characteristics: programmability and data retention . the program memory of an otp should be seen as a number of cells that will be activated during programming. these cells can be deactivated by lighting the die with uv light. this recovery is no longer possible once the otp die has been encapsulated in an opaque plastic package. therefore programmability and data retention can only be fully tested at wafer probing . at this step the dice are electrically tested and the memory is programmed to verify program- mability . then the wafers are placed in high temperature bake to accelerate any possible memory retention defects. the dice are then tested again to check data retention . after this test, uv light is used to deactivate the cells and the good dice are assembled. description of otp programmability test figure 4. details about the electrical tests performed on otp and rom although the programmability of the otp die is verified as fully functional at probe test, the as- sembly process can affect this parameter in the finished product (for example by damaging some cells). it is therefore necessary to make a final test to check programmability . but since uv light cannot be used to restore otp's programmed bytes once the die has been en- full electrical memory check full electrical otp programming memory check inking memory check bake programmability data retention rom otp wafer probing final test tested feature test test full electrical memory check test full electrical test full electrical memory check test (reserved number of bytes) vr02100e
8/9 selecting between rom, fastrom and otp for a microcontroller capsulated, it is not possible to test 100% of the user program area. for this reason the final test is limited to a reserved number of bytes which are programmed and then verified. otp rejects rates due to programmability failures as the programmability of the otp cannot be fully tested once the die has been encapsulated, a customer is likely to find a programming reject rate below 1% when programming is per- formed using qualified programming tools . this rate should be considered as normal for otps and should not generate customer quality returns. if higher failure rates are observed, it is recommended to check the programming equipment. recommended screening conditions for otp (data retention failures) programmability failures can be fully screened because gang programmers check the con- tent of otp devices after programming. but gang programmers cannot perform data reten- tion tests. high temperature aging is recommended for screening microcontrollers that use otp devices and performing data retention tests. the well-known retention failure mechanism corresponds to charge losses during the component life time. the customer is likely to find a data retention failure rate below 0.5% when programming is performed using qualified programming tools. therefore, it is strongly recommended to screen these potential failures as described hereafter: figure 5. recommended screening conditions for otp screening devices that are subject to data retention failures will allow customers to obtain otp devices with a similar quality level compared to rom devices. in order to further lower the reject levels for programmability and data retention, stmicroelec- tronics is continuously pursuing technology improvements in areas as soft die mounting, low stress mold compounds and passivation layer enhancements. vr02100f program verify high aging program mount nitrogen atmosphere normal atmosphere temperature verify 150 ? c / 24 h 125 ? c / 48 h program programmability failures data retention failures
9/9 selecting between rom, fastrom and otp for a microcontroller the present note which is for guidance only aims at providing customers with information regarding their products in order for them to save time. as a result, stmicroelectronics shall not be held liable for any direct, indirect or consequential damages with respect to any claims arising from the content of such a note and/or the use made by customers of the information contained herein in connection with their products. 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 express written approval of stmicroelectro nics. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners ? 2003 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia C belgium - brazil - canada - china C czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states www.st.com
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