sensor-patterns

sensor-patterns

Knowledge Base

Sensor driver, calibration, and filtering patterns for embedded C.

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Pattern 1: SPI Sensor Register Protocol

Most SPI sensors use: CS assert → [address byte with R/W bit] → [data bytes] → CS deassert.

/* Generic SPI sensor: 8-bit register address, bit7 = R/W (1=read) */
/* Example: ICM-42688 IMU, MPU-9250, LIS3DH */

uint8_t spi_sensor_read_reg(uint8_t reg)
{
    uint8_t tx[2] = { reg | 0x80U, 0x00U };  /* Bit7=1: read */
    uint8_t rx[2] = { 0 };

    spi_cs_assert();
    spi_transfer_bytes(tx, rx, 2);
    spi_cs_deassert();

    return rx[1];   /* Byte 0 = dummy (address phase), byte 1 = data */
}

void spi_sensor_write_reg(uint8_t reg, uint8_t val)
{
    uint8_t tx[2] = { reg & 0x7FU, val };  /* Bit7=0: write */

    spi_cs_assert();
    spi_transfer_bytes(tx, NULL, 2);
    spi_cs_deassert();
}

/* Burst read: read N registers starting at addr */
void spi_sensor_read_burst(uint8_t start_reg, uint8_t *buf, uint8_t len)
{
    uint8_t addr = start_reg | 0x80U;

    spi_cs_assert();
    spi_transfer_bytes(&addr, NULL, 1);     /* Send address */
    spi_transfer_bytes(NULL, buf, len);     /* Receive data */
    spi_cs_deassert();
}

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Pattern 2: Temperature Sensor with NTC Thermistor

NTC (Negative Temperature Coefficient) resistance decreases with temperature. Steinhart-Hart equation.

#include <math.h>

#define R_SERIES   10000.0f   /* 10kΩ series resistor */
#define R_NOM      10000.0f   /* Nominal resistance at T_NOM */
#define T_NOM      298.15f    /* 25°C in Kelvin */
#define B_COEFF    3950.0f    /* Beta coefficient from datasheet */

/* Returns temperature in Celsius */
float ntc_adc_to_celsius(uint16_t adc_raw, uint16_t adc_max)
{
    /* Voltage divider: R_NTC = R_SERIES * ADC / (ADC_MAX - ADC) */
    float resistance = R_SERIES * (float)adc_raw / (float)(adc_max - adc_raw);

    /* Steinhart-Hart simplified (B-parameter equation) */
    float temp_K = 1.0f / (1.0f/T_NOM + (1.0f/B_COEFF) * logf(resistance / R_NOM));

    return temp_K - 273.15f;
}