ST AN2594 Application note

AN2594

Application note

EEPROM emulation in STM32F10x microcontrollers

Introduction

Many applications require EEPROM (electrically erasable programmable read-only memory) for non-volatile data storage. For low-cost purposes, the STM32F10x devices do not use EEPROM. Instead, they implement EEPROM emulation using the embedded Flash memory.

This application note explains the differences between external EEPROM and embedded Flash memory, and it describes a software method for emulating EEPROM using the onchip Flash memory of the STM32F10x devices.

This document also focuses on some embedded aspects in emulated EEPROM data storage, that the reader is assumed to know.

Glossary

Low-density devices are STM32F101xx, STM32F102xx and STM32F103xx microcontrollers where the Flash memory density ranges between 16 and 32 Kbytes.

Medium-density devices are STM32F10x and STM32F103xx microcontrollers where the Flash memory density ranges between 32 and 128 Kbytes.

High-density devices are STM32F10x and STM32F103xx microcontrollers where the Flash memory density ranges between 256 and 512 Kbytes.

Connectivity line devices are STM32F105xx and STM32F107xx microcontrollers.

August 2009

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Contents	AN2594

Contents

1	Embedded Flash memory versus EEPROM: main differences . . . . . . .		5
	1.1	Difference in write access time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.2	Difference in writing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.3	Difference in erase time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6

2	Implementing EEPROM emulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	2.1 Principle	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.1.1	Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	2.1.2	EEPROM software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8

3	Embedded application aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			11
	3.1	Data granularity management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
		3.1.1	Programming on a word-by-word basis . . . . . . . . . . . . . . . . . . . . . . . . .	11
		3.1.2	Programming on a byte-by-byte basis . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	3.2	Wear leveling: Flash memory endurance improvement . . . . . . . . . . . . . .		11

3.2.1 Wear-leveling implementation example . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 Page header recovery in case of power loss . . . . . . . . . . . . . . . . . . . . . . 12 3.4 Cycling capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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AN2594	List of tables

List of tables

Table 1. Differences between embedded Flash memory and EEPROM . . . . . . . . . . . . . . . . . . . . . . 5 Table 2. Status combinations and actions to be taken . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Table 3. Maximum number of variables stored in emulated EEPROM (with 10 000 cycles) . . . . . . 14 Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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List of figures	AN2594

List of figures

Figure 1. Header status switching between page0 and page1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 2. EEPROM variable format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 3. Data update flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. WriteVariable flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 5. Page swap scheme with four pages (wear leveling). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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AN2594	Embedded Flash memory versus EEPROM: main differences

1Embedded Flash memory versus EEPROM: main differences

Electrically erasable programmable read-only memory (EEPROM) is a key component of many embedded applications that require non-volatile storage of data updated with a byte or word granularity during run time.

On the other hand, the microcontrollers used in those systems are each time more often based on embedded Flash memory. To eliminate components, save silicon space and reduce system cost, the STM32F10xxx Flash memory may be used instead of EEPROM for simultaneous code and data storage.

Unlike Flash memory, however, external EEPROM does not require an erase operation to free up space before data can be rewritten. Hence a special software management is required to store data into embedded Flash memory.

Obviously the emulation software scheme depends on many factors, including the EEPROM reliability, the architecture of the Flash memory used, and the product requirements.

The main differences between embedded Flash memory and external serial EEPROM are generic to any microcontroller that use the same Flash memory technology (it is not specific to the STM32F10xxx family products). The major differences are summarized in Table 1.

Table 1.	Differences between embedded Flash memory and EEPROM
Feature		External EEPROM	Emulated EEPROM using on-chip Flash
			memory
		– a few ms
Write time		– random byte: 5 to 10 ms	Word program time: 20 µs
		– page: a hundred µs per word (5 to
		10 ms per page)
Erase time		N/A	Page/Mass Erase time: 20 ms
Write method		– once started, is not CPU-dependent	once started, is CPU-dependent: a CPU
		– only needs proper supply.	reset will stop the write process even if the
			supplied power stays within specifications.
		– serial: a hundred µs	– parallel: a hundred ns
Read access		– random word: 92 µs	– very few CPU cycles per word.
		– page: 22.5 µs per byte	– Access time: 35 ns
			– from 10 kilocycles to 100 kilocycles (the
Write/Erase		– from 10 kilocycles to 1 000	use of many on-chip Flash memory pages
			is equivalent to increasing the number of
cycles		kilocycles
			write cycles) see Section 3.4: Cycling
			capability

1.1Difference in write access time

As Flash memories have a shorter write access time, critical parameters can be stored faster in the emulated EEPROM than in an external serial EEPROM, thereby improving data storage.

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