ST AN886 Application note
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 fl exibility 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 several device types within two main groups of microcontroller product families:
– EPROM, OTP, FASTROM and ROM microcontroller families
– Flash, FASTROM and ROM microcontroller families
This Appli cation Note disc usses t he first g roup of familie s. Fo r infor mation o n the second
group, refer to Application Note AN1068.
Rev. 1.0
AN886/0903 1/9
1
SELECTING BETWEEN ROM, FASTROM AND OTP FOR A MICROCONTROLLER
DEFINITION OF TERMS
Windowed EPROM (Erasable Programmable Read Only Mem ory): this type of MC U device
can be program med th en erase d using UV lig ht. EPRO M MCU devices are s old in cera mic
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 programm ed. OT Ps ar e typi-
cally programmed by the customer.
FASTROM (Factory Advanced Service Technique Read Only Memory): this type of MCU is a
OTP devic e p re-programmed by ST Microe lectro nics with th e cus tom er’s code and s elec ted
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 o f a shorter leadtime.
ROM (Read Only Memory): ROM devices are programmed at the fabrication step usi ng a special 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 easil y erased or
programmed). ROM is the cheapest technology but provides little flexibility whereas OTP and
EPROM are m ore flex ible but the ir ma nuf actur ing c ost is high er. T he hig h co st of E PRO M
MCU devices is due to the price of ceramic packages.
Figure 1. Cost versus Flexibility for different MCU types
+
COST
_
ROM
_
OTP
FLEXIBILITY
EPROM
+
VR02100A
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SELECTING BETWEEN ROM, FASTROM AND OTP FOR A MICROCONTROLLER
1 TYPICAL APPLICATION DEVELOPMENT FLO W
ST
Solution
Code
Updates
Number
of Units
Design
Phase
EPROM OTP OTP OTP ROM
•••• ••• •• • None
• •• ••• •••• •••••
Validation
Phase
Pre-production
Phase
Production
Phase
When a new application is developed, different device versions will be used at each step of the
development, depending on the required programming flexibilit y .
During the design p hase , a hi gh flexi bility i s r equ ired a nd only a sm all nu mbe r of pa rts are
necessary, therefore the use of UV erasable EPROM is recommended. Then, fewer code corrections (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 us e OTP memor y. Finally, when the mass produc-
tion phase begins, there is no more need for corrections since the product has been fully optimized, so ROM is the most adapted if very high volumes are needed. Otherwise (low to medium volumes) the mos t effective soluti on is to continue usin g STMicroelec tronics’ comp etitively priced OTP.
The following table summarizes the main benefits and drawbacks of using ROM, OTP or windowed EPROM MCU devices.
ROM OTP EPROM
Cheaper than OTP (simpler
process and testing)
Lower failure rate
+
(less handling, no programming)
Limited flexibility
(customer code implemented
at masking stage)
-
Higher inventory risks
Lower cost compared to windowed EPROM
(use of cheaper plastic packages)
Ability to be programmed di-
rectly by the final user
Higher failure rate compared to
ROM due to customer handling
and programming
High flexibility
(Programming, Code check
and Erasure cycle in less than
60 minutes)
Expensive ceramic packages
3/9
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