SPI behaviour differs between debug and release modes

[Ronnie Tango]

I have a custom board using NRF52832 which communicates with a BMI160 accelerometer/gyroscope chip by SPI.

I have been developing in Segger Embedded Studio (v5.10d) in debug mode, and I can configure the BMI160 and read data from the sensor without problems. 

However I have come across a strange issue: when I change from debug mode to release mode in the IDE, something goes wrong with the spi_read function.  In main() I have a simple check that the BMI160 is correctly initialised, which suddenly breaks in release mode:

  uint8_t tmpData = 0;

  uint8_t rslt = bmi160_get_regs(BMI160_CHIP_ID_ADDR, &tmpData, 1, &bmi160);
  // rslt = bmi160_get_regs(BMI160_ACCEL_RANGE_ADDR, &tmpData, 1, &bmi160);
  
  if(tmpData == BMI160_CHIP_ID)   // BMI160_CHIP_ID  BMI160_ACCEL_RANGE_4G  BMI160_ACCEL_RANGE_2G
  {
    ledOn(GREEN_LED_MASK);
  }
  else
  {
    ledOn(RED_LED_MASK);
  }

In debug mode, this works as expected (with line 4 commented out) - tmpData contains the correct BMI160_CHIP_ID value, and the green led goes on.

However in release mode, this doesn't work, and a second call to bmi160_get_regs() (i.e. line 4 uncommented) is required before tmpData == BMI160_CHIP_ID.  It doesn't matter what register the second get_regs reads - the result in tmpData after the two calls will be as it should be after the first. (In this example, if you were to add a third call of bmi160_get_regs() to any register, tmpData will hold the correct value for BMI160_ACCEL_RANGE_ADDR, and so on.)

I'm not sure whether it's a problem with my spi_read function as it seems to be fine otherwise, but here is that code:

int8_t bmi160_spi_read(uint8_t dev_id, uint8_t reg_addr, uint8_t *reg_data, uint16_t length) 
{
  static uint8_t rx_data[SPI_BUFFER_LEN];

  ret_code_t ret = nrf_drv_spi_transfer(&spi, &reg_addr, length, rx_data, length + 1);
  
  while (!spi_xfer_done) 
  {
    __WFE();
  }

  if (ret == NRF_SUCCESS) 
  {
    for (int i = 0; i < length; i++) 
    {
      *(reg_data + i) = rx_data[i + 1];
    }
  }
 
  return (int8_t)ret;
}

Could there be some difference in spi clock speeds between debug and release modes causing this? (I haven't had a chance to put it on an oscilloscope yet but will have access to workshop tomorrow..)  Or something else I can't think of..?

[Simonr]

Hi

Sorry about the delayed response, but Edvin is out of office for the time being, and we've only now gotten to his backlog. Does this spi_xfer_done event return true even though you are receiving the wrong values in release mode? Is it reached/trigged at all when your application is running?

I also think that what Edvin said about the speed of the application in Release mode is what's causing this. You can try confirming this by setting the Optimization Level in SEGGER's project options to NONE for instance?

Best regards,

Simon