AN1336

PFI/PFO operation in a system (how does it

work?)

Figure 6: Power failure sequence

This safeguard window can be used for a number of purposes, depending on the

application:

Power save

The MCU can switch off, one by one, all non-critical peripheral components to conserve

energy for safeguard routines.

Data transfer

The MCU may transfer data from the scratch pad memory to the non-volatile memory. It

takes only a few MCU cycles if using NVRAM, but can take several milliseconds when this

data needs to be stored in an EEPROM or Flash memory.

Scratch pad RAM overwrite

Many applications are now required to run encode/decode algorithms (e.g. DES or RCA)

for higher security.

Therefore it is sometimes preferable to overwrite the working space before power-down to

prevent the contents of the RAM from being read illegitimately.

Safeguard Window

(t2) Reset and/or Write Protect

(whichever occurs first)

(t1) VCC begins to fall

(t0) Power-Fail Input detected -

Begin Safeguard Routine

VUNREG

PFI

VCC

VPFD

t0 t1 t2

t

V

AI04219

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