AN2077 View Datasheet(PDF) - STMicroelectronics
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MFG CO.
'AN2077' PDF : 19 Pages View PDF
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.
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