RL-GNSS-001 · Navigation · KAMAL NAV GM1
Magnetometer
MMC5983MA compass integration notes and a single-shot I2C read example.
Published May 4, 2026Updated May 9, 2026Reading time ~15 min
01Compass behavior#
The MMC5983MA provides 3-axis magnetic-field readings over I2C. Use SET/RESET at startup and periodically during operation to reduce residual magnetization effects. Calibrate hard-iron and soft-iron offsets in the final airframe, not on the bench.
02Single-shot read example#
#define MMC5983MA_ADDR 0x30
#define REG_XOUT_0 0x00
#define REG_STATUS 0x08
#define REG_CTRL0 0x09
#define BIT_TM_M 0x01
#define BIT_MEAS_DONE 0x01
void mmc5983ma_read_xyz(int32_t *x, int32_t *y, int32_t *z) {
i2c_write_byte(MMC5983MA_ADDR, REG_CTRL0, BIT_TM_M);
uint8_t status = 0;
while (!(status & BIT_MEAS_DONE)) {
status = i2c_read_byte(MMC5983MA_ADDR, REG_STATUS);
}
uint8_t buf[7];
i2c_read_bytes(MMC5983MA_ADDR, REG_XOUT_0, buf, 7);
uint32_t rx = ((uint32_t)buf[0] << 10) | ((uint32_t)buf[1] << 2) | ((buf[6] >> 6) & 0x03);
uint32_t ry = ((uint32_t)buf[2] << 10) | ((uint32_t)buf[3] << 2) | ((buf[6] >> 4) & 0x03);
uint32_t rz = ((uint32_t)buf[4] << 10) | ((uint32_t)buf[5] << 2) | ((buf[6] >> 2) & 0x03);
*x = (int32_t)rx - 131072;
*y = (int32_t)ry - 131072;
*z = (int32_t)rz - 131072;
}03Calibration#
Perform compass calibration after the module is mounted in the final airframe. Moving the board, cable, battery, or power wiring can change hard-iron and soft-iron offsets.