AN2061

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2 EMBEDDED-FLASH AND EEPROM

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 ex-

ternal EEPROMs. Those differences are generic to any microcontroller (i.e.: not specific to

ST10F2xx variants). They are summarized in the table below.

2.1 Difference in write access time

As Flash has shorter write access time, critical parameters can be stored faster in the emulat-

ed EEPROM than in a serial external EEPROM, thereby improving the robustness of the sys-

tem if the same safety concept is kept.

2.2 Difference in writing method

One of the important differences between external EEPROM and emulated EEPROM for em-

bedded applications is the writing method.

– Stand-alone external EEPROM: once started by the CPU, the writing of a word cannot

be interrupted by a CPU reset. Only supply failure will interrupt the writing process; so

properly sizing the decoupling capacitors can secure the complete writing process in-

side a stand-alone EEPROM.

– Emulated EEPROM from an embedded-Flash: once started by the CPU, the writing

can be interrupted by a power failure and by a CPU reset.

This difference should be analysed by system designers to understand the possible impact(s)

in their applications and to define the proper handling method.

2.3 Difference in erase time

The difference in erase time is the other important difference between stand-alone EEPROMs

and emulated EEPROM with embedded-Flash. Unlike Flash, EEPROM does not require a

block erase operation to free-up space before write. This means that some form of software

management is required to store data in Flash. Moreover, as the erase process of a block in

the Flash takes few seconds, power shut-down and other spurious events that may interrupt

Table 1. Differences between Embedded Flash and EEPROM

Feature

EEPROM

Emulated EEPROM from embedded-

Flash

Write time

some ms

random byte : 5 to 10ms

page: equivalent to hundred us / word (5

to 10ms per page)

some us

(ex : 16us per word)

Erase time

N/A

seconds

(ex : 1.5s)

Write method

once started, is not CPU dependent;

needs only proper supply.

once started, is CPU dependent: a CPU

reset will stop the write process even if

supply stays inside specification.

Write access

serial : hundred us

random word : 92us

page : 22.5us /byte

parallel : hundred ns

very few CPU cycles per word.