Python and CRC8 verification

[Pilous Droip]

Posted on July 03, 2018 at 10:11

Helo friends,

I try calculate CRC8 in python and compare with CRC in STM And I have a problem.

My python CRC is return wrong value.

Python script:

def AddToCRC(b, crc):
 b2 = b
 for i in xrange(8):
 odd = ((b2^crc) & 1) == 1
 crc >>= 1
 b2 >>= 1
 if (odd):
 crc ^= 0x8C
 return crc�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?

And I call this function here:

TestValue = [0, 0xAB, 0xFF]
check = 0
for i in TestValue:
 check = AddToCRC(i, check)
print(hex(check))�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?

And I get CRC8 in STM32F: 0x8B

And CRC from python script: 0xF8

Any idea, how to calculate CRC8 in python for STM32?

#crc8 #crc

[Pilous Droip]

Posted on July 04, 2018 at 07:26

I modificated the function, and It works.

uint32_t CRC__Calculate8(uint8_t* Buffer, uint32_t BufferSize, const uint8_t reset, uint8_t *returnCRC)
{
 uint32_t index;
 /* Reset CRC data register if necessary */
 if (reset) {
 /* Reset generator */
 LL_CRC_ResetCRCCalculationUnit(CRC);
 }
 /* Compute the CRC of Data Buffer array*/
 for (index = 0; index < (BufferSize / 4); index++)
 {
 uint32_t d32 = (uint32_t)((Buffer[4 * index] << 24) | (Buffer[4 * index + 1] << 16) | (Buffer[4 * index + 2] << 8) | Buffer[4 * index + 3]);
 LL_CRC_FeedData32(CRC, d32);
 }
 /* Last bytes specific handling */
 if ((BufferSize % 4) != 0)
 {
 if (BufferSize % 4 == 1)
 {
 uint8_t d8 = Buffer[4 * index];
 LL_CRC_FeedData8(CRC, d8);
 }
 if (BufferSize % 4 == 2)
 {
 uint16_t d16 = (uint16_t)((Buffer[4 * index ]<<8) | Buffer[4 * index + 1]);
 LL_CRC_FeedData16(CRC, d16);
 }
 if (BufferSize % 4 == 3)
 {
 uint16_t d16 = (uint16_t)((Buffer[4 * index]<<8) | Buffer[4 * index + 1]);
 uint8_t d8 = Buffer[4 * index + 2];
 LL_CRC_FeedData16(CRC, d16);
 LL_CRC_FeedData8(CRC, d8);
 }
 }
 /* Return computed CRC value */
 *returnCRC = (LL_CRC_ReadData8(CRC));
 return ERR_CRC_OK;
}�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?

And call function:

uint8_t Test[] = {0x00, 0xAB, 0xFF, 0xFE, 0x01};
CRC__Calculate8(Test, 5, 1, &returnCRC);�?�?�?�?

Now It return: 0xF4

But if I want tocalc the CRC of 32bit value? Would I replace LL_CRC_ReadData8 with LL_CRC_ReadData32, and modife init as:

LL_CRC_SetPolynomialCoef(CRC, polynomial_left); 
LL_CRC_SetPolynomialSize(CRC, LL_CRC_POLYLENGTH_32B);
LL_CRC_SetInitialData(CRC, 0);
LL_CRC_SetInputDataReverseMode(CRC, LL_CRC_INDATA_REVERSE_BYTE); // BYTE
LL_CRC_SetOutputDataReverseMode(CRC, LL_CRC_OUTDATA_REVERSE_BIT); // STM32 backassward�?�?�?�?�?

[Tesla DeLorean]

Posted on July 04, 2018 at 13:53

>>But if I want to calc the CRC of 32bit value?

You're going to have to be a lot more precise in your description.

The code supplied will provide an 8-bit CRC across a 4-byte, 32-bit word as provided. It should work consistently for any size buffer.

For a 32-bit polynomial CRC you're going to have to consider multiple factors related to how it shifts and how the data is presented to the feedback terms.

https://community.st.com/0D50X00009XkgXtSAJ