DocID5833 Rev 2

9/19

AN1015

Preventive techniques

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For STM8 users, the STM8 "TRAP" instruction is also very convenient (only one instruction

byte:83) for generating a software interrupt in order to recover from a jump to an unexpected

location in memory.

Another effective way for recovery to normal operation is filling memory by value of illegal

instruction opcode which fetch and execution generates reset on STM8 microcontrollers.

memory accesses and illegal program behavior which may occure if the system is exposed

to EMC disturbances. The undefined instruction opcode may be used to fill the unused

memory of STM32 microcontroller to rise the Usage Fault exception where the fail safe

routine recovers from errors in case of program counter run-away.

Another option is to use System serivce call with SVC instruciton to execute fail safe routine.

2.3

Input filtering and comparison

The routine which checks several times that that the state of input pin is stable before

validating the state and continuing the program execution is a good practice to avoid

unwanted reaction on spikes caused by external noise induced in input circuit.

This is a simple means of critical input filtering for no extra cost!

2.4

Management of unused interrupt vectors

To avoid problems caused by unexpected interrupt occurrences (whatever the source) it is

recommended to manage all the possible interrupt sources by putting a valid interrupt

routine address in the corresponding vector.

In the example below the unused interrupt vectors point to a fault management routine label

filled with a simple “return from interrupt” instruction.

2.5

Removing illegal and critical bytes from your code

2.5.1

Critical Bytes

A critical byte is an instruction switching MCU in low power modes which is decoded by the

microcontroller and forces it to stop executing any further instructions.

When the PC is corrupted it often becomes desynchronized (as most of the instructions

have several bytes), and as a result it may read and decode critical bytes.

To check and minimize the occurrence of these critical bytes you can check the program

".list" file.

Very often critical bytes are generated by the compiler as label address bytes. In this case, if

you simply insert one or several NOP instructions, all the label addresses will shift and this

will change the critical byte value to another value.

2.5.2

IIlegal Bytes

Illegal bytes are defined as any byte value which is not part of the instruction set. They will

either be executed as a NOP instruction or (on some MCUs) a reset is generated if an illegal