[OpenTRV-dev] CRC computation

[Bo Herrmannsen]

not sure at all but should it not be:

CRC-7: 0x48 = (x7+x4+1)‡

-----Oprindelig meddelelse----- 
From: Damon Hart-Davis
Sent: Sunday, April 20, 2014 9:44 PM
To: Closed list for developer discussions
Subject: [OpenTRV-dev] CRC computation

Duh!  My brain hurts!

Can anyone confirm/deny if this, just written by me, is a correct 
implementation of CRC7 (0x5B polynomial) as given in the paper specified?

// Update 7-bit CRC with next byte.
// Polynomial 0x5B = (x+1)(x^6 + x^5 + x^3 +x^2 + 1).
// See: 
http://users.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf
// For 2 or 3 byte payloads this should have a Hamming distance of 4 and be 
within a factor of 2 of optimum error detection.
uint8_t crc7_5B_update(uint8_t crc, const uint8_t datum)
  {
  crc ^= datum;
  for(uint8_t i = 0; ++i <= 8; )
    {
    if(crc & 0x80)
      { crc = (crc << 1) ^ 0x5B; }
    else
      { crc <<= 1; }
    }
  return(crc & 0x7f);
  }

I’m after something with good behaviour on a 2-byte payload and need a 7-bit 
CRC in this case for packing efficiency.

Also, I assume that it’s customary to start with 0xff for the CRC before 
feeding it real bytes, unless there is a good reason otherwise?

Rgds

Damon
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