On Thu, 22 Aug 2002, Scott Dattalo wrote:

> On Wed, 21 Aug 2002, Tony K|bek wrote:
>
> > Ok, here we go again.
> > This poly (and table that goes with it) was a tad harder to optimize :)
> > However there are some small thing that can be done, however there are
> > some
>
> Tony,
>
> Hint:
>
> The MSB of the bytes in the high table is the same as the LSB of the bytes
> of the low table.
>
> Challenge:
>
> This algorithm can be implemented in 24 cycles.
>
>
> The hint and challenge aren't necessarily correlated.


Give up, Tony? Okay, it's a few more than 24 cycles. ( Doing it my head I
saw 16 plus a little overhead...)


CRC_Update:

        xorwf   CRC16_Low,w
        movwf   Temp

        movf    CRC16_High,W
        movwf   CRC16_Low

        clrw
        btfsc   Temp,0
         movlw  0xcc
        btfsc   Temp,1
         xorlw  0x8d
        btfsc   Temp,2
         xorlw  0x0f
        btfsc   Temp,3
         xorlw  0x0a
        movwf   CRC16_High
        andlw   0xf0
        xorwf   CRC16_High,f
        xorwf   CRC16_Low,f
        clrw
        btfsc   CRC16_High,3
         movlw  0xcc
        btfsc   Temp,4
         xorlw  0xcc
        btfsc   Temp,5
         xorlw  0xd8
        btfsc   Temp,6
         xorlw  0xf0
        btfsc   Temp,7
         xorlw  0xa0
        xorwf   CRC16_High,f
        movlw   0x01
        btfsc   CRC16_High,7
         xorwf  CRC16_Low,f

        return

It's probably clear from the comments how it works. :)

Scott

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