Implementing the EEPROM emulation

AN2540 - Application note

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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 STR91xFxx 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

Valid0

Erased1

Valid0

Receive1

Transfer_

complete0

Valid1

Erased0

Valid1

Receive0

Valid1

Transfer_

complete1

Valid0

Sector0 Full

Transfer data from sector0 to sector1 complete

Erase0

Sector1 Full

Write data in sector0

Copy data from sector0

Erase1

to sector1

Copy data from sector1

to sector0

Write data in sector1

Transfer data from sector1 to sector0 complete

States related to sector1

States related to sector0

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