max31855 Raspberry Pi Pico lib

max31855.c

#include "pico/stdlib.h"
#include "hardware/spi.h"

#include "max31855.h"

static inline void max31855_cs_select(uint pin_cs) {
    asm volatile("nop \n nop \n nop");
    gpio_put(pin_cs, 0);  // Active low
    asm volatile("nop \n nop \n nop");
}

static inline void max31855_cs_deselect(uint pin_cs) {
    asm volatile("nop \n nop \n nop");
    gpio_put(pin_cs, 1);
    asm volatile("nop \n nop \n nop");
}

void max31855_init(const max31855_config *config) {
    spi_init(config->spi, 5000 * 1000); // 5 MHz
    gpio_set_function(config->pin_data, GPIO_FUNC_SPI);
    gpio_set_function(config->pin_sck, GPIO_FUNC_SPI);

    // Chip select is active-low, so we'll initialise it to a driven-high state
    gpio_init(config->pin_cs);
    gpio_set_dir(config->pin_cs, GPIO_OUT);
    gpio_put(config->pin_cs, 1);
}

void max31855_read(const max31855_config *config, struct max31855_result *result) {
    uint8_t buffer[4];
    
    max31855_cs_select(config->pin_cs);
    sleep_ms(1);
    spi_read_blocking(config->spi, 0, buffer, 4);
    max31855_cs_deselect(config->pin_cs);
    sleep_ms(1);

    int16_t thermocouple = MAX31855_SIGN_EXTEND((buffer[0] << 6) | (buffer[1] >> 2), 14);
    int16_t internal = MAX31855_SIGN_EXTEND((buffer[2] << 4) | (buffer[3] >> 4), 12);

    result->thermocouple = (1.0f * thermocouple) / 4;
    result->internal = (1.0f * internal) / 16;
    result->faults = ((buffer[1] & 0x01) << 8) | (buffer[3] & 0x07);
}

max31855.h

#ifndef _MAX31855_H
#define _MAX31855_H

#include <stdint.h>

#define MAX31855_FAULT     0x100    // 1 when any of the SCV, SCG, or OC faults are active
#define MAX31855_SCV_FAULT  0x04    // short-circuited to VCC
#define MAX31855_SCG_FAULT  0x02    // short-circuited to GND
#define MAX31855_OC_FAULT   0x01    // open (no connections)

#define MAX31855_SIGN_EXTEND(value, bit) (((value) ^ (1u << ((bit) - 1))) - (1u << ((bit) - 1)))

typedef struct {
    spi_inst_t *spi;

    uint pin_data;
    uint pin_cs;
    uint pin_sck;
} max31855_config;

struct max31855_result
{
    float thermocouple;
    float internal;
    uint16_t faults;
};

void max31855_init(const max31855_config *config);

void max31855_read(const max31855_config *config, struct max31855_result *result);

#endif