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3.3 Read-While-Write

Most currently available Flash technologies must complete a program or erase operation be-

fore code or data can be read from another memory block. There is a common misconception

that EEPROM emulation can only be done when Read-While-Write functionality is implement-

ed. Read-While-Write, when present, allows to access other memory blocks during erasing

and programming; this means that the CPU program does not need to be copied into RAM

during programing / erasing. Read-While-Write does not prevent to have a RAM buffer if ac-

cess into the emulated-EEPROM is needed during programming.

When Read-While-Write is not supported, program or erase suspend command can be used

to temporarily read code. All ST10F2xx variants support program and erase suspend com-

mands.

3.4 Flash organization

The concept described above is showing the Flash bank split in 2 parts:

– Data-set storage: keep all variable information,

– Status bit storage: keep status of the Flash bank and of the Data-sets.

Other organizations are possible (ex: to include the Data-set status bits inside each Data-set)

for which users may see advantages for their application.

3.5 Data-set status bits

Two status bits are proposed for each Data-set with the combinations shown in the following

table.

With those combinations, the status bits will change from “11”, combination after erase,

through “01” till “00”, configurations reached by incremental bit programming. Reserved con-

figuration should be used by user’s software to detect which Flash bank hold valid data.

User’s software should define rules on handling Data-set status bits so that in any possible sit-

uation in the application (ex: CPU reset), the software can retrieve which bank is active and

which Data-set has the valid information.

Ex: when the data have been successfully written into the new Data-set, the status bits of the

new and old Data-sets must be updated. A specific sequence to achieve these updates should

be defined.

3.6 Active Flash bank selection

The detection of the active Flash bank after power-up should rely on the analysis of the infor-

mation inside the banks. To support this analysis, specific status bits in each Flash bank can

Table 2. Status bit for Data-set

Status bit value

Meaning

11

Data-set without data (virgin)

01

Data-set with valid data (programmed)

00

Data-set with old or invalid data (dirty)

10

Reserved (invalid)