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EMULATED DATA EEPROM WITH ST7 HDFLASH MEMORY

1 PRINCIPLE

Two different implementations can be distinguished:

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Assuming that it is possible to limit the data EEPROM byte values to 00h..FEh (as FFh is the

default HDFlash erased value), the principle of this emulation is to reserve n bytes in the

HDFlash for each emulated data EEPROM byte which has to be cycled n times. With this

solution FFh value can not be used.

As shown in Figure 1., a “ptr” pointer gives the access to the emulated data EEPROM byte.

– For a read operation, from “ptr” address, read the first data byte not equal to FFh to get the

current value.

– For a write operation, from “ptr” address, look for the last data byte equal to FFh and then

write the new value at this location using the HDFlash “Byte programming” Embedded

Command (same as IAP method).

Figure 1. HDFlash Emulated Data EEPROM (value in 00h..FEh)

s

Assuming no limitation for the data EEPROM byte values (00h..FFh), each byte value will

need 2 byte locations. The first one will determine if it is the current value (example: 00h =

current value, FFh = not yet used value) and the second one will contain the current byte

value. So the principle of this emulation is to reserve 2xn bytes in the HDFlash for each

emulated data EEPROM byte which has to be cycled n times.

As shown in Figure 2., a “ptr” pointer gives the access to the emulated data EEPROM byte.

– For a read operation, from “ptr” address, read the first data byte not equal to FFh (equal to

00h), the current value is the next adjacent byte.

– For a write operation, from “ptr” address, look for the last data byte equal to FFh and then

write the new value at this location and 00h in the previous location using the HDFlash

“Byte programming” Embedded Command (same as IAP method).

In both methods, the programming is address decreasing, which means that when cu.0rrent

value is found, the next byte to be programmed is the previous one (refer to diagrams).

In the case of the ST72F521R9 (biggest product) for instance, that means that if sectors 2 and

1 are empty (FFh), the first programmed byte will be at the last address of the sector 1

(EFFFh), because F000h is the first address of the sector 0 containing the user program (this

sector is write protected in user mode).

FFh

FFh

...

FFh

FFh

...

FFh

val1

initial value

FFh

FFh

...

FFh valm

...

val2

val1

ptr

ptr

current value

current value

INITIAL STATE

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