max31855.tc

MAX31855 Thermocouple Sensor Driver (SPI)

Source on GitHub

// MAX31855 Thermocouple Sensor Driver (SPI)
// Reads 14-bit thermocouple temperature via SPI
// Demonstrates: spiInit, spiSetCS, spiTransfer, WebCall, JsonCall

#define CS_PIN   5
#define SPI_MHZ  4

float tc_temp = 0.0;
int tc_ok = 0;
int tc_fault = 0;
char tc_lbl[32];

void EverySecond() {
    // Read 4 bytes from MAX31855 (read-only device, MOSI not needed)
    char buf[4];
    buf[0] = 0;
    buf[1] = 0;
    buf[2] = 0;
    buf[3] = 0;
    int n = spiTransfer(1, buf, 4, 1);
    if (n != 4) {
        tc_ok = 0;
        return;
    }

    // Check fault bit (bit 16 = buf[1] bit 0)
    if (buf[1] & 0x01) {
        tc_fault = buf[3] & 0x07;  // fault code in lowest 3 bits of byte 3
        tc_ok = 0;
        return;
    }

    // Thermocouple temp: bits 31..18 = 14-bit signed value in buf[0..1]
    // buf[0] = bits 31..24, buf[1] = bits 23..16
    int raw = ((buf[0] << 8) | buf[1]) >> 2;  // shift out bits 17..16
    if (raw & 0x2000) {
        raw = raw - 16384;  // sign extend 14-bit to int
    }
    tc_temp = (float)raw * 0.25;
    tc_ok = 1;
    tc_fault = 0;
}

void WebCall() {
    char out[64];
    if (tc_ok) {
        LGetString(0, tc_lbl);
        strcpy(out, "{s}MAX31855 ");
        strcat(out, tc_lbl);
        strcat(out, "{m}");
        webSend(out);
        sprintf(out, "%.1f &deg;C{e}", tc_temp);
        webSend(out);
    } else if (tc_fault) {
        sprintf(out, "{s}MAX31855{m}Fault 0x%02X{e}", tc_fault);
        webSend(out);
    } else {
        webSend("{s}MAX31855{m}no data{e}");
    }
}

void JsonCall() {
    if (!tc_ok) return;
    char out[64];
    sprintf(out, ",\"MAX31855\":{\"Temperature\":%.1f}", tc_temp);
    responseAppend(out);
}

int main() {
    // Init hardware SPI at 4 MHz (read-only, no MOSI needed)
    spiInit(-1, -1, -1, SPI_MHZ);
    spiSetCS(1, CS_PIN);

    tc_ok = 0;
    tc_fault = 0;
    tc_temp = 0.0;
    addLog("MAX31855 thermocouple driver ready");
    return 0;
}