ST AN2540 Application note

AN2540

Application note

EEPROM emulation in

STR91xF devices

Introduction

Replacing external EEPROM with emulated EEPROM from the embedded-Flash memory of
the microcontroller is a complex development. This application note is aimed at readers that
are already familiar with the techniques used to secure the content of evolutive information
in the external EEPROM of embedded applications.This application note explains the
differences between external/internal EEPROMs and embedded-Flash memory. It also
gives advice on how to replace external EEPROM with emulated-EEPROM using the on­chip Flash memory of STR91xF devices.

This document also focuses on some embedded aspects in emulated-EEPROM data
storage, that are assumed to be known by the reader.

Overview

Electrically erasable and programmable read-only memory (EEPROM) is a key component
in many embedded applications requiring non-volatile storage of data that are updated at a
byte, half-word or word granularity during run time.

On the other hand, the microcontrollers used in those systems are more and more based on
embedded-Flash memory. To eliminate components, save silicon area and reduce system
cost, the STR91xF Flash memory could eventually replace the external EEPROM for
simultaneous code and data storage.

However unlike Flash memory, external EEPROM does not require a block erase operation
to free up space before data can be rewritten. A special software management is required to
store data into Flash memory.

Obviously the emulation software scheme depends on many factors including the EEPROM
reliability, Flash memory architecture and product requirements. Two approaches to
implementation are described in detail in this application note using the on-chip Flash
memory of the STR91xF microcontrollers.

June 2007 Rev 1 1/25

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Contents AN2540 - Application note

Contents

1 Embedded Flash memory vs. EEPROM: main differences . . . . . . . . . . 5

1.1 Difference in write access time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Difference in writing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Difference in erase time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Appropriate solution for Emulated EEPROM in the STR91xF . . . . . . . . 7

2.1 STR91xF on-chip Flash memory features . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 STR91xF Flash memory library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Implementing the EEPROM emulation . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 1st method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.1 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.2 EEPROM software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 2nd method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3.1 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3.2 EEPROM software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.4 Program execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4 Embedded application aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1 Data granularity management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1.1 Programming on a word-by-word basis . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1.2 Programming on a byte-by-byte basis . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2 Wear-leveling: Flash endurance improvement . . . . . . . . . . . . . . . . . . . . . 19

4.2.1 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3 Sector header recovery in case of power loss . . . . . . . . . . . . . . . . . . . . . 20

4.4 Emulated EEPROM parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.4.1 Program/Erase parameter cycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.4.2 Program timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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AN2540 - Application note List of tables

List of tables

Table 1. Differences between Embedded Flash memory and EEPROM . . . . . . . . . . . . . . . . . . . . . . 5

Table 2. EepromFormat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 3. FindValidSector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 4. ReadVariable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 5. WriteVariable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 6. WriteVerifyVariableFull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 7. EepromSectorTransfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Table 8. Status combinations and actions to be taken . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table 9. Write time related to the current implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 10. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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List of figures AN2540 - Application note

List of figures

Figure 1. Header field status switching between sector0 and 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 2. Sector swap scheme: Sector1 erased . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 3. Sector swap scheme: Sector0 erased . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 4. 1st method: WriteVariable flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 5. Data storage procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 6. Data update flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 7. 2nd method: WriteVariable flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 8. WriteOnebyte function description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 9. Sector swap scheme with four sectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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AN2540 - Application note Embedded Flash memory vs. EEPROM: main differences

1 Embedded Flash memory vs. EEPROM: main

differences

Before describing the proposed concept for EEPROM emulation, it is important to
remember the main differences between the embedded Flash memory of a microcontroller
and serial external EEPROMs. These differences are the same for any microcontroller (that
is they are not specific to STR91xF products). They are summarized in Ta bl e 1 .

Table 1. Differences between Embedded Flash memory and EEPROM

Feature External EEPROM

A few ms

Write time

Erase time N/A seconds (e.g.: 1.5 s)

Write method

Read access

– random byte: 5 to 10 ms
– page: equivalent to a hundred µs per

word (5 to 10 ms per page)

Once started, is not CPU-dependent,
needs only proper supply.

Serial: a hundred µs
random word: 92 µs
page: 22.5 µs/byte

1.1 Difference in write access time

As the Flash memory has a shorter write access time, critical parameters can be stored
faster into the emulated EEPROM than into an external serial EEPROM. The use of the
Flash memory therefore improves the system robustness.

Emulated EEPROM using on-chip Flash

memory

A few µs
(e.g.: 20 µs per 16-bit word)

Once started, is CPU-dependent: a CPU
reset will stop the write process even if the
supply stays within specifications.

Parallel: a hundred ns
very few CPU cycles per 16-bit word.

1.2 Difference in writing method

One of the major differences between external and emulated EEPROM for embedded
applications is the writing method.

● Standalone external EEPROM: once started by the CPU, the writing of a word cannot

be interrupted by a CPU reset. Only a power supply failure can interrupt the writing
process, so properly sizing the decoupling capacitors can secure the write process to a
standalone EEPROM.

● Emulated EEPROM from an embedded Flash memory: once started by the CPU, the

writing can be interrupted by a power failure and by a CPU reset. This difference should
be analyzed by system designers to understand the possible impact(s) in their
applications, and to determine a proper method to handle them.

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Embedded Flash memory vs. EEPROM: main differences AN2540 - Application note

1.3 Difference in erase time

The difference in erase time is the other major difference between standalone EEPROM and
emulated EEPROM with embedded Flash memory. Unlike Flash memory, EEPROM does
not require a block erase operation to free up space before writing. Moreover, as the erase
process of a block in the Flash memory takes a few seconds, power shut-down and other
spurious events that may interrupt the erase process (e.g.: reset) should be considered
when designing the Flash memory management software. This means that to design a
robust Flash memory management software it is necessary to have a good understanding of
the Flash memory erase process.

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AN2540 - Application note Appropriate solution for Emulated EEPROM in the STR91xF

2 Appropriate solution for Emulated EEPROM in the

STR91xF

The STR91xF microcontrollers support the hardware and software architecture necessary
to emulate EEPROM memory using the on-chip Flash memory.

2.1 STR91xF on-chip Flash memory features

■ The STR91x internal Flash memory consists of two banks: Main Flash memory (Bank 0)

and Secondary Flash memory (Bank 1). The Main Flash memory is up to 512 Kbytes in
size and includes up to eight 64-Kbyte sectors. The Secondary Flash memory is 32
Kbytes in size and consists of four 8-Kbyte sectors, it can be useful for the wear-leveling
feature (refer to Section 4.2).

■ One of the STR9 embedded Flash memory features is Read-while-Write (RWW) Dual

Bank operations. This means that the Main Flash memory (Bank0) can be used for code
storage and the smaller Secondary Flash memory, for data storage (EEPROM
emulation).

■ The Flash memory can be erased on a sector or bank basis, and programmed on a 16-

bit half-word basis.

■ Each bank can be programmed and erased over 100 000 cycles.

■ 20-year data retention.

■ Each sector can be individually protected and unprotected against program and erase

operations.

■ As the Flash memory has a shorter write access time, critical parameters can be stored

faster in the emulated EEPROM than in an external serial EEPROM.

■ Interrupt servicing during program/erase is possible.

■ CPU program does not need to be copied into RAM during program/erase: RAM less used

to perform EEPROM emulation.

■ Program/Erase Suspend and Resume commands supported. That is, Flash memory

sector erase may be suspended while data is read from other sectors in the same Flash
memory bank, and then resumed after reading.

2.2 STR91xF Flash memory library

The Flash memory programming library is a set of optimized C routines. It contains all that is
needed to program the Flash memory embedded in STR9 devices.

The Flash memory library contains the following source files:

■ 91x_fmi.c, that contains the function codes

■ 91x_fmi.h, that contains the function prototypes

To use the functions provided by the Flash memory library, these two files must be added to
the project. With the STR9 software (SW) library (FMI driver) it is easy to implement the
EEPROM emulation software.

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Implementing the EEPROM emulation AN2540 - Application note

3 Implementing the EEPROM emulation

3.1 Principle

This emulation is performed in various ways by considering the Flash memory limitations
and the product requirements. Two approaches are described below in detail. Both require a
minimum of two Flash memory sectors of identical size, that are allocated to non-volatile
data. One that is initially erased and can be programmed byte by byte, and the other that is
ready to take over when the first sector needs to be garbage-collected.

Since the STR91xF on-chip Flash memory can be programmed on a 16-bit half-word basis,
the data granularity in this implementation is 16 bits.

A header field that occupies the first 16-bit half word of each sector indicates the sector
status.

Each sector has four possible states:

● ERASED: The sector is empty.

● RECEIVE_DATA: The sector is receiving data from the full sector.

● VALID_SECTOR: The sector contains valid data and its state will not change until valid

data are completely transferred to the erased sector.

● TRANSFER_COMPLETE: Transfer of data to the other sector is finished and this

sector is no longer in use. The system can then erase it and prepare it for future data.

Figure 1. shows how to switch from one state to another for both sectors.

Figure 1. Header field status switching between sector0 and 1

Write data in sector0

Copy data from sector0

to sector1

Write data in sector1

Copy data from sector1

to sector0

States related to sector1

States related to sector0

Valid0 Erased1

Sector0 Full

Valid0 Receive1

Transfer data from sector0 to sector1 complete

Transfer_
complete0

Erased0 Valid1

Receive0 Valid1

Valid0

Valid1

Erase0

Sector1 Full

Transfer data from sector1 to sector0 complete

Transfer_
complete1

Erase1

ai14085

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