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ST7 CHECKSUM SELFCHECKING CAPABILITY

3 DESCRIPTION OF “SAFE” ROUTINE OPERATION

At each reset, the Safe routine calculates the checksum from E003h to FFFFh and compares

it to CS0, CS1 and CS2. If the comparison is OK, the user application (the main routine) will be

run and in our example, the PBDR register is set to FFh (all LEDS set). If the comparison isn’t

OK, the microcontroller is halted (only a reset can make it exit from this state and this means

the user application won’t be run). The halt state (please refer to AN980 for more details) puts

the MCU in its lowest power consumption mode. The internal oscillator is turned off, causing

all internal processing to be stopped, including the operation of the on-chip peripherals.

Note: During the execution of the Safe routine, the I/O lines will be in their reset state (gener-

ally floating input) because they won’t have been modified.

If the checksum comparison is not OK, the I/O lines remain unchanged in their reset state.

The main program in our application just switches all LEDs on and then waits in an infinite

loop. You can replace this by your own application.

4 FLOWCHARTS

Figure 1. Safe routine (for ST72264G2)

with:

s

CS means checksum stored in CSO, CS1 and CS2.

s

ch_sum is the three byte variable created to store the checksum calculated by the Safe

routine.

main

calculate the checksum

from E003 to FFFF

stored into ch_sum

ch_sum=CS?

no

HALT

yes

(in our application PBDR=FF

and a delay loop)