thermistor.c

#include <stdio.h>
#include <adc.h>
#include <device.h>
#include <math.h>
#include <sensor.h>
#include <zephyr.h>
#include <gpio.h>

#include <nrf_saadc.h>

#define LOG_LEVEL LOG_LEVEL_DBG
#include <logging/log.h>
LOG_MODULE_REGISTER(thermistor_drv);

struct thermistor_config {
        const char *adc_name;
        uint32_t adc_input;

        const char *gpio_name;
        uint32_t gpio_pin;
        uint32_t gpio_flags;

        uint32_t b_value_k;
        uint32_t res_25c_ohms;
        uint32_t top_res_ohms;
};

struct thermistor_data {
	struct device *adc;
        struct device *gpio;
	struct adc_channel_cfg adc_cfg;
	struct adc_sequence adc_seq;
	struct adc_sequence_options adc_seq_opts;
	int16_t adc_buf;
};

static int thermistor_sample_fetch(struct device *dev, enum sensor_channel chan)
{
	struct thermistor_data *data = dev->driver_data;
        const struct thermistor_config *config = dev->config->config_info;
        int ret;

        /* Apply power */
        ret = gpio_pin_write(data->gpio, config->gpio_pin, 1);
        if (ret != 0) {
                LOG_ERR("failed to enable (%d)", ret);
                return -EIO;
        }

        /* (re)-configure channel */
        ret = adc_channel_setup(data->adc, &data->adc_cfg);
        if (ret != 0) {
                LOG_ERR("failed to set up channel (%d)", ret);
                return -EIO;
        }

        /* Sample ADC */
        ret = adc_read(data->adc, &data->adc_seq);
        if (ret != 0) {
                LOG_ERR("adc read failed (%d)", ret);
                return -EIO;
        }

        /* Disable power */
        ret = gpio_pin_write(data->gpio, config->gpio_pin, 0);
        if (ret != 0) {
                LOG_ERR("failed to enable (%d)", ret);
                return -EIO;
        }

        return 0;
}

static int thermistor_channel_get(struct device *dev,
                                  enum sensor_channel chan,
                                  struct sensor_value *val)
{
	struct thermistor_data *data = dev->driver_data;
        const struct thermistor_config *config = dev->config->config_info;

        /* ADC measurement uses VDD as reference, so it is in units of
           VDD / 16384 */
        uint16_t meas = data->adc_buf;
        if (meas > 16383)
                meas = 16383;

        /* Get resistance of thermistor in ohms;
             ratio = meas / 16384
             ratio = r_th / (r_th + r_top)
             r_th = ratio / (1 - ratio) * r_top
                     = meas * r_top / (16384 - meas)
        */
        uint32_t r_th  = meas * config->top_res_ohms / (16384 - meas);

        /* Calculate temperature:
           1/T = 1/T0 + 1/B * ln(R / R0)
        */
        float t_inv = (1.0f / 298.15f) +
                logf(r_th / (float)config->res_25c_ohms) / config->b_value_k;
        if (t_inv < 0.0001f)
                t_inv = 0.0001f;

        /* Convert to C and return */
        float t = 1.0f / t_inv - 273.15;
	val->val1 = (s32_t)t;
	val->val2 = ((s32_t)(t * 1000000)) % 1000000;

	return 0;
}

static const struct sensor_driver_api thermistor_api = {
	.sample_fetch = &thermistor_sample_fetch,
	.channel_get = &thermistor_channel_get,
};

static int thermistor_init(struct device *dev)
{
	struct thermistor_data *data = dev->driver_data;
        const struct thermistor_config *config = dev->config->config_info;

        /* Configure ADC */
	data->adc = device_get_binding(config->adc_name);
	if (data->adc == NULL) {
		LOG_ERR("can't bind ADC");
		return -ENXIO;
	}

        data->adc_cfg = (struct adc_channel_cfg) {
                .gain             = ADC_GAIN_1_4,
                .reference        = ADC_REF_VDD_1_4,
                .acquisition_time = ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 10),
                .channel_id       = 0,
                .input_positive   = NRF_SAADC_INPUT_AIN0 + config->adc_input,
        };

        data->adc_seq_opts = (struct adc_sequence_options) {
                .extra_samplings = 0,
        };

        data->adc_seq = (struct adc_sequence) {
                .channels     = BIT(0),
                .buffer       = &data->adc_buf,
                .buffer_size  = sizeof(data->adc_buf),
                .resolution   = 14,
                .oversampling = 0,
                .options      = &data->adc_seq_opts,
        };

        /* Configure GPIO */
        data->gpio = device_get_binding(config->gpio_name);
        if (data->gpio == NULL)
        {
                LOG_ERR("can't bind GPIO");
                return -ENXIO;
        }

        return gpio_pin_configure(data->gpio,
                                  config->gpio_pin, config->gpio_flags);
}

#define THERMISTOR_INIT(n)                                              \
        static struct thermistor_data thermistor_##n##_data;            \
        static struct thermistor_config thermistor_##n##_cfg = {        \
                .adc_name     = DT_THERMISTOR_##n##_INPUT_ADCS_CONTROLLER, \
                .adc_input    = DT_THERMISTOR_##n##_INPUT_ADCS_INPUT,   \
                .gpio_name    = DT_THERMISTOR_##n##_ENABLE_GPIOS_CONTROLLER, \
                .gpio_pin     = DT_THERMISTOR_##n##_ENABLE_GPIOS_PIN,   \
                .gpio_flags   = DT_THERMISTOR_##n##_ENABLE_GPIOS_FLAGS, \
                .b_value_k    = DT_THERMISTOR_##n##_B_VALUE_K,          \
                .res_25c_ohms = DT_THERMISTOR_##n##_RESISTANCE_25C_OHMS, \
                .top_res_ohms = DT_THERMISTOR_##n##_TOP_RESISTOR_OHMS,  \
        };                                                              \
        DEVICE_AND_API_INIT(thermistor_##n##_dev,                       \
                            "THERMISTOR_" DT_THERMISTOR_##n##_LABEL,    \
                            &thermistor_init, &thermistor_##n##_data,   \
                            &thermistor_##n##_cfg, POST_KERNEL,         \
                            CONFIG_SENSOR_INIT_PRIORITY, &thermistor_api)

#ifdef DT_THERMISTOR_0
THERMISTOR_INIT(0);
#endif
#ifdef DT_THERMISTOR_1
THERMISTOR_INIT(1);
#endif
#ifdef DT_THERMISTOR_2
THERMISTOR_INIT(2);
#endif
#ifdef DT_THERMISTOR_3
THERMISTOR_INIT(3);
#endif
#ifdef DT_THERMISTOR_4
#error Add more thermistors here...
#endif