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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