Parallel Verilog CRC algorithm from C-like reference - c

Question

I have a set of c-like snippets provided that describe a CRC algorithm, and this article that explains how to transform a serial implementation to parallel that I need to implement in Verilog.
I tried using multiple online code generators, both serial and parallel (although serial would not work in final solution), and also tried working with the article, but got no similar results to what these snippets generate.
I should say I'm more or less exclusively hardware engineer and my understanding of C is rudimentary. I also never worked with CRC other than straightforward shift register implementation. I can see the polynomial and initial value from what I have, but that is more or less it.
Serial implementation uses augmented message. Should I also create parallel one for 6 bits wider message and append zeros to it?
I do not understand too well how the final value crc6 is generated. CrcValue is generated using the CalcCrc function for the final zeros of augmented message, then its top bit is written to its place in crc6 and removed before feeding it to the function again. Why is that? When working the algorithm to get the matrices for the parallel implementation, I should probably take crc6 as my final result, not last value of CrcValue?
Regardless of how crc6 is obtained, in the snippet for CRC check only runs through the function. How does that work?
Here are the code snippets:
const unsigned crc6Polynom =0x03; // x**6 + x + 1
unsigned CalcCrc(unsigned crcValue, unsigned thisbit) {
unsigned m = crcValue & crc6Polynom;
while (m > 0) {
thisbit ^= (m & 1);
m >>= 1;
return (((thisbit << 6) | crcValue) >> 1);
}
}
// obtain CRC6 for sending (6 bit)
unsigned GetCrc(unsigned crcValue) {
unsigned crc6 = 0;
for (i = 0; i < 6; i++) {
crcValue = CalcCrc(crcValue, 0);
crc6 |= (crcValue & 0x20) | (crc6 >> 1);
crcValue &= 0x1F; // remove output bit
}
return (crc6);
}
// Calculate CRC6
unsigned crcValue = 0x3F;
for (i = 1; i < nDataBits; i++) { // Startbit excluded
unsigned thisBit = (unsigned)((telegram >> i) & 0x1);
crcValue = CalcCrc(crcValue, thisBit);
}
/* now send telegram + GetCrc(crcValue) */
// Check CRC6
unsigned crcValue = 0x3F;
for (i = 1; i < nDataBits+6; i++) { // No startbit, but with CRC
unsigned thisBit = (unsigned)((telegram >> i) & 0x1);
crcValue = CalcCrc(crcValue, thisBit);
}
if (crcValue != 0) { /* put error handler here */ }
Thanks in advance for any advice, I'm really stuck there.

Answer 1

xoring bits of the data stream can be done in parallel because only the least signficant bit is used for feedback (in this case), and the order of the data stream bit xor operations doesn't affect the result.
Whether the hardware would need a parallel version depends on how a data stream is handled. The hardware could calculate the CRC one bit at a time during transmission or reception. If the hardware is staged to work with 6 bit characters, then a parallel version would make sense.
Since the snippets use a right shift for the CRC, it would seem that data for each 6 bit character is transmitted and received least significant bit first, to allow for hardware that could calculate CRC 1 bit at a time as it's transmitted or received. After all 6 bit data characters are transmitted, then the 6 bit CRC is transmitted (also least significant bit first).
The snippets seem wrong. My guess at what they should be:
/* calculate crc6 1 bit at a time */
const unsigned crc6Polynom =0x43; /* x**6 + x + 1 */
unsigned CalcCrc(unsigned crcValue, unsigned thisbit) {
crcValue ^= thisbit;
if(crcValue&1)
crcValue ^= crc6Polynom;
crcValue >>= 1;
return crcValue;
}
Example for passing 6 bits at a time. A 64 by 6 bit table lookup could be used to replace the for loop.
/* calculate 6 bits at a time */
unsigned CalcCrc6(unsigned crcValue, unsigned sixbits) {
int i;
crcValue ^= sixbits;
for(i = 0; i < 6; i++){
if(crcValue&1)
crcValue ^= crc6Polynom;
crcValue >>= 1;
}
return crcValue;
}
Assume that telegram contains 31 bits, 1 start bit + 30 data bits (five 6 bit characters):
/* code to calculate crc 6 bits at a time: */
unsigned crcValue = 0x3F;
int i;
telegram >>= 1; /* skip start bit */
for (i = 0; i < 5; i++) {
crcValue = CalcCrc6(unsigned crcValue, telegram & 0x3f);
telegram >>= 6;
}