Sensors

The previous four pages described, validated, and compiled the BME280 driver into your firmware — through the overlay, the binding, and Kconfig. This page reads from it.

Zephyr's sensor API provides a uniform interface for every sensor, regardless of interface (I2C, SPI) or manufacturer. If a driver exists in the Zephyr tree, reading a sensor is always the same three steps:

1. Get the device
2. Call sensor_sample_fetch()
3. Call sensor_channel_get() for each measurement

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The sensor API — reading the BME280

#include <zephyr/drivers/sensor.h>

/* Step 1: get the device (resolved at compile time from DTS) */
static const struct device *bme280 = DEVICE_DT_GET(DT_NODELABEL(bme280));

int main(void)
{
    struct sensor_value temp, press, hum;

    if (!device_is_ready(bme280)) {
        LOG_ERR("BME280 not ready");
        return -ENODEV;
    }

    while (1) {
        /* Step 2: trigger a measurement */
        sensor_sample_fetch(bme280);

        /* Step 3: read the measured values */
        sensor_channel_get(bme280, SENSOR_CHAN_AMBIENT_TEMP, &temp);
        sensor_channel_get(bme280, SENSOR_CHAN_PRESS,        &press);
        sensor_channel_get(bme280, SENSOR_CHAN_HUMIDITY,     &hum);

        LOG_INF("T: %d.%06d C  P: %d.%06d kPa  RH: %d.%06d %%",
                temp.val1, temp.val2,
                press.val1, press.val2,
                hum.val1, hum.val2);

        k_sleep(K_SECONDS(10));
    }
}

That's the whole app. The overlay says "BME280 is on i2c0 at 0x76," the Kconfig pulled in the driver, the binding validated the node — and now sensor_sample_fetch knows how to talk to it.

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struct sensor_value format

Sensor values use a fixed-point format to avoid floating point:

struct sensor_value {
    int32_t val1;   /* Integer part */
    int32_t val2;   /* Fractional part (millionths) */
};

A reading of 24.319000 °C is stored as {.val1 = 24, .val2 = 319000}.

To get a double:

double temp_c = sensor_value_to_double(&temp);

info

sensor_value_to_double() is provided by Zephyr. Don't compute val1 + val2 / 1e6 manually — the sign handling is subtle when values are negative.

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Supported sensor channels

The BME280 supports temperature, pressure, and humidity. Other sensors fill in different channels from the same standard list:

Channel constant	Meaning
SENSOR_CHAN_AMBIENT_TEMP	Temperature (°C)
SENSOR_CHAN_HUMIDITY	Relative humidity (%RH)
SENSOR_CHAN_PRESS	Atmospheric pressure (kPa)
SENSOR_CHAN_ACCEL_XYZ	3-axis acceleration (m/s²)
SENSOR_CHAN_GYRO_XYZ	3-axis angular velocity (rad/s)
SENSOR_CHAN_LIGHT	Ambient light (lux)

Not every sensor supports every channel. If a channel isn't supported, sensor_channel_get() returns -ENOTSUP.

The power of this design: swap the BME280 for an SHT31 or an LIS2DH in the overlay, change CONFIG_BME280=y to CONFIG_SHT3XD=y, and the rest of the application code is unchanged.

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Next: keep the BME280 powered only when you need it

Reading the sensor once every 10 seconds and leaving it powered the other 9.99 seconds wastes ~50× more energy than necessary. The next page — Power Management — wraps these fetches with pm_device_action_run() so the BME280 sleeps between measurements.