6/19

EEPROM Emulation with STR71x

2.2 PROGRAM/ERASE CYCLE

The requirement number of program/erase cycles is computed by dividing the needed number

of erase cycles by the total number of Data-sets in the Flash banks (for example in Figure 1:

n+m). When this number is still higher than the Flash write/erase endurance characteristics, a

closer analysis is needed to understand when the Data-sets are updated:

– When Data-sets need to be updated during operation, it is proposed to use a buffer in RAM

and to save the data before shutting-down the microcontroller.

– When Data-sets are updated only before a power-down sequence, it is proposed to increase

the size of the Flash bank or to use a 3rd bank (see additional information in Section 3).

2.3 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 imple-

mented. Read-While-Write, when present, allows other memory blocks to be accessed during

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

RAM during programming/erasing. Read-While-Write does not prevent having a RAM buffer if

access 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 STR71xF variants support program and erase suspend com-

mands.

2.4 FLASH ORGANIZATION

The concept described above shows the Flash bank split into 2 parts:

– Data-set storage: which keeps all variable information,

– Status bit storage: which keeps 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.

2.5 DATA-SET STATUS BITS

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

table.