Why isn't nrf_drv_twi_xfer() working configured like this?

[ndarkness]

Hello,

I am using NRF52 TWI master to read data from my sensor, I have read the twi_sensor example but it doesn't make use of nrf_drv_twi_xfer().

My question is quite straightforward, I have noticed that my settings to usenrf_drv_twi_xfer() like on Case A does not work, in that the register value written on the device is different from the one retrieved after reading the same register

Case A

int8_t twi_write_transfer(unsigned char slaveDeviceId, unsigned char reg_addr, unsigned char *txData, uint16_t bytesNumberTx)
  nrf_drv_twi_xfer_desc_t twi_WriteTransferData;

  twi_WriteTransferData.address          = slaveDeviceId;
  twi_WriteTransferData.p_primary_buf    = &reg_addr;
  twi_WriteTransferData.primary_length   = sizeof(reg_addr);
  twi_WriteTransferData.p_secondary_buf  = txData;
  twi_WriteTransferData.secondary_length = bytesNumberTx;
  twi_WriteTransferData.type             = NRF_DRV_TWI_XFER_TXTX;
  uint32_t flags = NRF_DRV_TWI_FLAG_NO_XFER_EVT_HANDLER;
  

  err_code = nrf_drv_twi_xfer(&twi, &twi_WriteTransferData, flags);
 APP_ERROR_CHECK(err_code);
 return err_code;

}

whereas this settings does work, Case B

Case B

int8_t twi1_write_transfer(unsigned char slaveDeviceId, unsigned char reg_addr, unsigned char *txData, uint16_t bytesNumberTx) {
  
  ret_code_t err_code;

  
  unsigned char aux[bytesNumberTx + 1];
  
  aux[0] = reg_addr;
  
  memcpy(&aux[1],txData,bytesNumberTx);

  nrf_drv_twi_xfer_desc_t twi_WriteTransferData;

  twi_WriteTransferData.address          = slaveDeviceId;
  twi_WriteTransferData.p_primary_buf    = aux;
  twi_WriteTransferData.primary_length   = sizeof(reg_addr)+bytesNumberTx;
  twi_WriteTransferData.p_secondary_buf  = NULL;
  twi_WriteTransferData.secondary_length = 0;
  twi_WriteTransferData.type             = NRF_DRV_TWI_XFER_TX;

  uint32_t flags = NRF_DRV_TWI_FLAG_NO_XFER_EVT_HANDLER;
  
err_code = nrf_drv_twi_xfer(&twi, &twi_WriteTransferData, flags);

  APP_ERROR_CHECK(err_code);
  
  return ((int8_t)err_code);
}

I am aware that the main difference is that on Case A more than one I2C transfer happens, however, I have check that with the first case the write into the device register failes, since reading the setting I tried to write returns the default value of that register on the device.

So, what am I setting wrong in case A?

[Jørgen Holmefjord]

Hi,

You are using different transfer types in the two cases. In case A, you set the type to NRF_DRV_TWI_XFER_TXTX, which by default will generate a stop condition and a repeated start between the two transfers. If your slave device expect both register address and data to be received before the stop condition, the first case will not write the data to the register. If case B works, this is the way to do your transfers. Please check the datasheet of the slave for more details on how to setup the correct TWI/I2C transfer.

Best regards,

Jørgen