AN2860

Software recommendations

Doc ID 15267 Rev 5

13/17

5

Software recommendations

The standard software techniques for improving EMC performance described in the AN1015

application note (“Software techniques for improving microcontroller EMC performance”)

also apply to the STM8 product family.

This section lists the STM8 specificities related to the software techniques referred in the

AN1015. It gives the values dedicated to STM8 products but it is strongly recommended to

read carefully the AN1015 in order to implement safely the software recommendations.

Some other techniques used to test the MCU correct state and to prevent from an

unexpected MCU behavior are described in details in the AN3181 (Guidelines for obtaining

IEC60335 Class B certification on STM8 family). These techniques are used in the

associated firmware.

5.1

Critical bytes and opcode technique

It is recommended to avoid occurrences of the following critical bytes in your code:

●

0x8E: opcode for HALT (instruction used to enter low power modes)

●

0x8F: opcode for WFI (instruction “wait for interrupt”)

●

0x82: opcode for INT (instruction used to jump in interrupt routine only)

●

0x8B: opcode for SWBRK (software breakpoint instruction used to stall the CPU in

debug mode)

These critical bytes can be generated by the linker when computing the address destination

in a branch instruction (JUMP or CALL) in absolute or relative addressing modes. To remove

such occurrences you can simply insert a NOP instruction before the target address. For

further explanation, refer to the AN1015 application note.

To keep a better control of unexpected jumps, it is recommended to fill unused memory

areas with the TRAP opcode (0x83).

Besides, if you want to generate an illegal opcode reset, illegal opcodes can be used, like for

example the following simple ones: 0x05, 0x0B, 0x71 and 0x75.