AN886 Datasheet, PDF (3/9 Pages) STMicroelectronics – A customer who develops an MCU-based application needs various levels of flexibility in order

SELECTING BETWEEN ROM, FASTROM AND OTP FOR A MICROCONTROLLER
1 TYPICAL APPLICATION DEVELOPMENT FLOW
ST
Solution
Code
Updates
Number
of Units
Design
Phase
EPROM
••••
•
Validation
Phase
OTP
•••
••
Pre-production
Phase
OTP
••
•••
Production
Phase
OTP
ROM
•
None
••••
•••••
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.
ROM
OTP
EPROM
Cheaper than OTP (simpler
process and testing)
Lower failure rate
Lower cost compared to win-
dowed EPROM
(use of cheaper plastic packag- ^{High flexibility}
es)
(Programming, Code check
+ (less handling, no program-
ming)
Ability to be programmed di-
rectly by the final user
and Erasure cycle in less than
60 minutes)
Limited flexibility
(customer code implemented _{Higher failure rate compared to}
- ^{at masking stage)}
ROM due to customer handling
and programming
Expensive ceramic packages
Higher inventory risks
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