I'm re-posting a discussion on implementing the M109 from reprap forums:

Elias_Bakken · February 11, 2014, 1:23pm

I’m re-posting a discussion on implementing the M109 from reprap forums: http://forums.reprap.org/read.php?146,303976

James_Armstrong · February 11, 2014, 3:04pm

Look at the Repetier code, it may give some insight. Looks like they wait for the current buffer to drain (complete all moves) then set the temperature and wait in a loop for the temperature to be hit and report the current temperature every second.

github.com

repetier/Repetier-Firmware/blob/master/src/ArduinoAVR/Repetier/Commands.cpp#L928


    default:
        Com::printF(PSTR("?"));
        break;
    }
}

void tmc2130DiagnosticReadingSingleDriver(FSTRINGPARAM(text), TMC2130Stepper* tmc) {
    Com::printF(text);
    Com::println();
    Com::printF(Com::tSpace);
    Com::printHexNumber(tmc->DRV_STATUS());
    Com::println();
}

void tmc2130DiagnosticReadings() {
    struct struct_tmc2130_drivers* drv = tmc2130_drivers;
    Com::printFLN(Com::tTrinamic2130DiagnosticReadings);
    tmc2130DiagnosticReadingSingleDriverLineName(0);
    for (uint8_t idx = 0; tmc2130_drivers[idx].driver != NULL; idx++) {
        Com::printF(PSTR("\t"));
        Com::printF(tmc2130_drivers[idx].name);