Modbus CRC using an STM32 controller

The Modbus RTU protocol calls for the calculation of the CRC for each message transmitted. 

From the perspective of an STM32 controller the CRC calculation properties are  : 

  hcrc.Init.DefaultPolynomialUse = DEFAULT_POLYNOMIAL_DISABLE;
  hcrc.Init.DefaultInitValueUse = DEFAULT_INIT_VALUE_ENABLE;
  hcrc.Init.GeneratingPolynomial = 32773;
  hcrc.Init.CRCLength = CRC_POLYLENGTH_16B;
  hcrc.Init.InputDataInversionMode = CRC_INPUTDATA_INVERSION_BYTE;
  hcrc.Init.OutputDataInversionMode = CRC_OUTPUTDATA_INVERSION_ENABLE;
  hcrc.InputDataFormat = CRC_INPUTDATA_FORMAT_BYTES;

 In hexadecimal notation the polynomial value is 0x8005. Equivalent to using  this polynomial in the MX configurator : X15+X2+X0. The CRC can be calculated by the hardware CRC block by using this function:

uint16_t hwCalcCrc(uint8_t *d, int bytes) 
{
    int i;
    hcrc.Instance->CR |= 1;// Reset the calculator
    for (i=0; i<bytes; i++)
     *(__IO uint8_t *)(__IO void *)(&hcrc.Instance->DR) = d[i];
    return hcrc.Instance->DR;
}

 The C pre-processor  replaces volatile for the __IO macro. So, this is a trick to keep  the compiler from optimizing out repeated writes to the same static register.

// WORKS:
*(__IO uint8_t *)(__IO void *)(&hcrc.Instance->DR) = d[i];

// DOESN'T WORK
hcrc.Instance->DR = d[i]; 

Test message: 

The CRC of 

[ 0x01, 0x10, 0x00, 0x00, 0x00, 0x01, 0x02, 0xAA, 0xAA]

 is 0x8f58.