CRC Calculator Routine

[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.
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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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[aratti]

Darell is it too much asking what CRC stand for?

Al.

[Darrel Taylor]

Darell is it too much asking what CRC stand for?

Al.

Too much ... nope.
Too little ... maybe.

Cyclic Redundancy Check
http://en.wikipedia.org/wiki/Cyclic_redundancy_check
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