AN2077 Datasheet, PDF (3/19 Pages) STMicroelectronics – EEPROM Emulation with STR71x

EEPROM Emulation with STR71x
1 EMBEDDED FLASH AND EEPROM
Before describing the proposed concept for EEPROM emulation, it is important to bear in mind
the main differences between the embedded-Flash memory of a microcontroller and serial ex-
ternal EEPROMs. These differences apply to any microcontroller (i.e. not specific to STR71xF
variants). They are summarized in the table below.
Table 1. Differences between Embedded Flash and EEPROM
Feature
Write time
Erase time
EEPROM
several ms
random byte: 5 to 10ms
page: equivalent to hundred µs / word (5 to
10ms per page)
N/A
Write method
once started, is not CPU-dependent;
needs only proper supply.
serial: hundred µs
Write access random word: 92µs
page: 22.5µs /byte
Emulated EEPROM from Embedded-
Flash
several µs
(ex: 16µs per word)
seconds
(ex: 1.5s)
once started, is CPU-dependent: a CPU
reset will stop the write process even if
supply stays within specification.
parallel: hundred ns
a small number of CPU cycles per word.
1.1 DIFFERENCE IN WRITE ACCESS TIME
As Flash has a shorter write access time, critical parameters can be stored faster in the emu-
lated EEPROM than in a serial external EEPROM, thereby improving the robustness of the
system if the same safety concept is kept.
1.2 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 inside 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)
on their applications and to define the proper handling method.
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