bmi270 PWR_CONF and PWR_CTRL value

[XudanMorris-Wei]

Hello everyone:

New to the community. Running into an interesting behavior with BMI270, wondering if any one has an answer there. 

Using BMI270, able to read the CHIP-ID correctly. After soft-reset and wait for 1000ms, reading the value of registers, PWR_CONF and PWR_CTRL, both comes back as 0x00, while it is supporsed to be 0x03 for PWR_CONF. Intersting is if read PWR_CTRL first then read PWR_CONF, it comes back as0x03 and 0x00. Any idea why?

Attaching some source code here for reference.

void device_imu_bmi270_reset()
{
uint8_t value = 0xFF;

mcu_spi_io_pkg bmi270_spi_io_pkg;
bmi270_spi_io_pkg.bytesToSendReceive = 2;
bmi270_spi_io_pkg.p_tx_buffer = tx_buffer;
bmi270_spi_io_pkg.p_rx_buffer = rx_buffer;
bmi270_spi_io_pkg.devnum = MCU_SPI_DEV_BMI270;

tx_buffer[0] = BMI270_COMMAND_WRITE | BMI270_REG_CMD;
tx_buffer[1] = BMI270_COMMAND_RESET;
mcu_spi_io(&bmi270_spi_io_pkg);

printLog("BMI270 resetting\r\n");
sl_sleeptimer_delay_millisecond(1000);

//device_imu_bmi270_read_register_onebyte(BMI270_REG_PWRCONF); //0x03
device_imu_bmi270_read_register_onebyte(BMI270_REG_PWRCTRL); //0x00
device_imu_bmi270_read_register_onebyte(BMI270_REG_PWRCONF); //0x03

//simply switch these two above lines, the output gets correct  

}

The read function is defined as 

uint8_t device_imu_bmi270_read_register_onebyte(uint8_t addr)
{
mcu_spi_io_pkg bmi270_spi_io_pkg;
bmi270_spi_io_pkg.bytesToSendReceive = 3;
bmi270_spi_io_pkg.p_tx_buffer = tx_buffer;
bmi270_spi_io_pkg.p_rx_buffer = rx_buffer;
bmi270_spi_io_pkg.devnum = MCU_SPI_DEV_BMI270;

tx_buffer[0] = BMI270_COMMAND_READ | addr;
tx_buffer[1] = BMI270_DUMMY_DATA; // 16 bit protocol. dummy byte needed before read actual data
tx_buffer[2] = BMI270_DUMMY_DATA; // dummy
mcu_spi_io(&bmi270_spi_io_pkg); // send data to BMI270

printLog("Read byte is 0x%02x\r\n",rx_buffer[2]);
return rx_buffer[2];
}

Thanks for your inputs

[etracer · Accepted solution]

After the soft reset the BMI270 will be back in I2C mode and you'll have to get it to switch back to SPI mode before attempting to read the registers. I ususlly just toggle the CS low for 10ms but you can also perform a "dummy" register read to get it to switch. I also add a small delay after switching before reading the first register to give it time to switch but this isn't documented in the datasheet so I don't know if it's required (but probably a good idea).