AN886 Datasheet, PDF (2/9 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
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 customer’s 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
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EPROM
COST
OTP
_
ROM
_
FLEXIBILITY
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