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 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 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 Application Note discusses the first group of families. For information on the second group, refer to Application Note AN1068.

Rev. 1.0

AN886/0903

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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 typically 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 leadtime.

ROM (Read Only Memory): ROM devices are programmed at the fabrication step using 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 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

+	EPROM
COST	OTP
_	ROM
	_	+
	FLEXIBILITY
		VR02100A

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SELECTING BETWEEN ROM, FASTROM AND OTP FOR A MICROCONTROLLER

1 TYPICAL APPLICATION DEVELOPMENT FLOW

		Design	Validation	Pre-production	Production
		Phase	Phase	Phase		Phase
ST		EPROM	OTP	OTP	OTP		ROM
Solution
Code		∙∙∙∙	∙∙∙	∙∙	∙		None
Updates
Number		∙	∙∙	∙∙∙	∙∙∙∙		∙∙∙∙∙
of Units

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 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 use OTP memory. Finally, when the mass production 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 most effective solution is to continue using STMicroelectronics’ competitively 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	Lower cost compared to win-
			dowed EPROM
		process and testing)		High flexibility
			(use of cheaper plastic packag-
		Lower failure rate
			es)	(Programming, Code check
	+	(less handling, no program-		and Erasure cycle in less than
			Ability to be programmed di-
		ming)		60 minutes)
			rectly by the final user
		Limited flexibility
		(customer code implemented	Higher failure rate compared to
	-	at masking stage)	ROM due to customer handling	Expensive ceramic packages
			and programming
		Higher inventory risks

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