max31855 v0.1.0

MAX31855 thermocouple amplifier over SPI.

Application scenarios

• A reflow oven or hot plate.
• A kiln or furnace monitor.
• A thermocouple probe on a test bench.

Contract

typedef struct {
    int32_t thermocouple_milli_c;
    int32_t internal_milli_c;
    uint8_t fault;
    uint8_t open_circuit;
    uint8_t short_to_gnd;
    uint8_t short_to_vcc;
} max31855_reading_t;

uint8_t max31855_read(max31855_port_t port, max31855_reading_t *reading);

• Reads one 32-bit frame.
• Thermocouple resolution is 0.25 degrees C; internal is 0.0625 degrees C.
• Returns 1 when a frame was read, including a fault frame; inspect fault.
• Returns 0 on a bus error.

Parameters

• The thermocouple type (K, J, N, T, S, R, E) is fixed by the chip variant.
• Keep the cold-junction side away from the hot zone.
• Use a 0.1 uF bypass capacitor near the chip.

Memory and timing

• 8 bytes; one 4-byte SPI transfer.
• Conversion time is about 100 ms.

Integration

max31855_reading_t temperature;
if (max31855_read(board_max31855_port(), &temperature)) {
    if (temperature.fault) {
        report_fault(temperature.open_circuit, temperature.short_to_gnd);
    } else {
        show(temperature.thermocouple_milli_c / 1000);
    }
}

Adapters

• The adapter drives CS low, reads four bytes, and drives CS high.
• Reference: LibDriver max31855 (MIT) and the Maxim datasheet.
• This implementation is original.

When not to use

• A thermistor or RTD is sufficient; use a simpler front end.
• An isolated thermocouple; add an isolator.

Verification

make test covers positive and negative temperatures, the internal junction,

and the three fault flags.

void *context;
void (*select)(void *context);
uint8_t (*transfer)(void *context, uint8_t value);
void (*deselect)(void *context);

int32_t thermocouple_milli_c;
int32_t internal_milli_c;
uint8_t fault;
uint8_t open_circuit;
uint8_t short_to_gnd;
uint8_t short_to_vcc;