CRC Calculator Routine

[timmers]

Having just busted CRC calculations, could we have a neat built in polynomial calculator. With the increasing need for CRC calculations it would save an awful lot of work for someone embarking into data comms or storage projects.

We have written some code for calculating a 16 bit poly and at 4Mhz it takes 2 milliseconds to execute! I am sure this could be halved with some natty code.

Tim.

[Darrel Taylor]

We have written some code for calculating a 16 bit poly and at 4Mhz it takes 2 milliseconds to execute! I am sure this could be halved with some natty code.

Is that 2 mS per byte?

Just timed my CRC routines at 4Mhz and it's ~825uS per byte for 16-bit Algorithm's, and ~710uS for 8-bit ones. Of course they all vary a little depending on the polynomial, initial value and whether there's a final XOR or bit reversal or not.

For the most part ... A little less than half.
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[Darrel Taylor]

Oh, and I forgot about this ...

This was my first version.
It was for CRC-16, but I later found that there are many versions of CRC-16.
Even if they have the same "Polynomial".

Might give you some idea's ....

CRC       VAR WORD
CRC_IN    VAR BYTE
Idx       VAR BYTE
CRC_Len   CON 16
CRC_Poly  CON $1021  ; x16 + x12 + x5 + 1

;-----CRC-16 -----------------------------------------------------
CRC16:
  CRC.HighByte = CRC.HighByte ^ CRC_IN
  FOR Idx = 0 to 7
    IF CRC.0(CRC_Len-1) THEN
      CRC = (CRC << 1) ^ CRC_Poly
    ELSE
      CRC = CRC << 1
    ENDIF
  NEXT Idx
RETURN

That's actually faster than my previous measurements.
But the other program can do any polynomial version, which is probably more than you need.
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[timmers]

Hi Darrel,
Its 2mS for the whole CRC routine. I simply toggle a pin at the start of the subroutine and again at the exit. It is working through 8 or so bytes to calculate the CRC value.

Out of interest, could you expand upon the line :-
IF CRC.0(CRC_Len-1) THEN
I don't understand the purpose or method of the code in parenthesis.

Thanks,
Tim.

[Darrel Taylor]

In the context of that program ...

IF CRC.0(CRC_Len-1) THEN

compiles to the exact same thing as ...

IF CRC.15 THEN

And at the assembly level, it's 2 instructions ...

&nbsp; btfss _CRC + 1, 7
&nbsp; goto TestFailed

This allows the CRC length to be changed via a constant, with the least amount of code.
And gets around the desire to use a variable or constant as the bit number.

IF CRC.CRC_Len THEN ; This notation doesn't work.
<hr>
The CRC.0(x) notation tells PBP to treat the variable as a BIT array, and is discussed in these threads.

Bits, Bytes Words and Arrays (Melanie)
http://www.picbasic.co.uk/forum/showthread.php?t=544

Indexing Port Pins (Bruce)
http://www.picbasic.co.uk/forum/showthread.php?t=3753

hth,

[Darrel Taylor]

Oh shoot, and I just went back and looked at how I timed my CRC routines, and the ~825uS was for a 9-byte array with the chars "123456789".

That would have sounded so much better in my first post.
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