STM8 SWIM Debug Protocol

[andryas_mariotto]

Hello,

I’ve been working on implementing a SWIM master using an STM8 device to interface with a STM8S105K6 target. This is part of a tester software project that was previously using the ST7 debug protocol and is now being ported to the STM8.

I’ve followed the entry sequence described in UM0470, and I believe the waveforms match what's expected. Attached below is a screenshot from the oscilloscope:

• Yellow trace = RESET pin

• Green trace = SWIM pin

As you can see:

1. A long low pulse initiates the SWIM entry sequence.

2. Followed by 4 x 1kHz pulses and 4 x 2kHz pulses.

3. Then I set the SWIM line high and switch it to input mode to await the slave response (Step 4 per UM0470).

However, no response is received from the target. The line stays high and never toggles.

Below is the relevant code section (i use raisonance ride7 dev environment):

unsigned char swim_entry(void){
unsigned long a;
unsigned char m = 1;
char buf[1];

swim_config(0);

ADC_disable;
RES(1);
SWIM(1);
delay_ms(10);

RES(0);
delay_ms(10);
SWIM(0);
delay(1000);
//delay (30); // 16us+ delay to get into SWIM mode

// start 1khz sequence:
for(a = 0; a < 4; a++){
SWIM(1);
delay(FIRST_PULSE);
SWIM(0);
delay(FIRST_PULSE);
}

// start 2khz sequence:
for(a = 0; a < 4; a++){
SWIM(1);
delay(SECOND_PULSE);
SWIM(0);
delay(SECOND_PULSE);
}
SWIM(1);
swim_config(1); // swim as input mode

for (a=0; a<10000; a++)
{
delay(125);
if (!IN_SWIM)
{
m = 0;
}

if (m == 0 && IN_SWIM)
{
m = 2;
break;
}
}

if(m != 2){
RES(1);
return FALSE;
}

swim_config(0);
SWIM(1);
delay(100);
// write 0x0A0 to the SWIM_CSR
buf[0] &= 0b000;
buf[0] |= SWIM_BITMASK;
if(!swim_write(3, 0, 0x7F, 0x90, buf)) {
RES(1);
return FALSE;
}

// End
SWIM(1);
RES(1);
delay_ms(20);
return TRUE;
}

Any ideas of what could be happening?

I appreciate any insight from others who’ve implemented custom SWIM masters, especially from STM8 to STM8 communication.

Thanks in advance!

[ahsrabrifat · Accepted solution]

If the target has an enabled watchdog timer and no main firmware, it could be resetting before responding. Either disable the watchdog or flash a basic dummy firmware to keep it alive.