Hi,
Scott Dattalo wrote:

>Yeah, it sure is...
>Okay, my turn:
>http://www.dattalo.com/technical/software/pic/crc_32.asm
>
>There are two implementations for the 18F. One is 94 instructions long
and
>takes between 35 and 85 cycles. The other is 76 instructions and 76
>cycles.

Hmm.. I lacked the insigtfullness to compact the tables into the smaller
one, but it's never to late to learn. My boolean exp. was so ugly that i
didn't see
the trees beacuse of the forest :). Anyway I haven't come up with
anything
faster than yours I only jotted down the table based version but with a
shorter
table. Goes around 93-168 instructions executed but will take less
program memory.
If the table is aligned something around 16 instr. can be shaved off in
best case.
That would be something around 77-168 instructions executed instead.

I think the full (128 bytes) table version should be possible to take
down to around
70'ish(fixed) instruction but as one still has the table I see no gain
in that, your 76 cycle
version seems to be the best sofar.


/Tony

*untested* and ignores upper table pointer (assumed cleared/not used).


CRC_32_W_Short
        GLOBAL  CRC_32_W_Short
        XORWF   CRC32+3,W       ; do xor with lowest byte
        MOVWF   Index    ; save
        ; swap bytes in the crc32 ( crc32 =3D3D crc32>>8 )
        MOVFF   CRC32+2,CRC32+3
        MOVFF   CRC32+1,CRC32+2
        MOVFF   CRC32,CRC32+1
        CLRF    CRC32
        ; setup table pointer
        MOVLW   HIGH(CRC32_NIBBLE_TABLE_SHORT);
        MOVWF   TBLPTRH         ; setup high adress
        MOVLW   LOW(CRC32_NIBBLE_TABLE_SHORT)   ;
        MOVWF   TBLPTRL ; set low adress
CRC_32_W_LOOP
        RRCF    Index,F ; rotate lowest bit into carry
        BC      CRC_32_W_LOOP_XOR       ; bit set=3Ddo xor

        ; skip xor as bit is not set
        ; advance TBLPTRL by 4 bytes
        MOVLW   D'4'
        ADDWF   TBLPTRL,F
        BTFSC   STATUS,C
        INCF    TBLPTRH,F
        BRA     CRC_32_W_LOOP_SKP   ; do next bit
CRC_32_W_LOOP_XOR
        TBLRD*+
        MOVF    TABLAT,W
        XORWF   CRC32,F
        TBLRD*+
        MOVF    TABLAT,W
        XORWF   CRC32+1,F
        TBLRD*+
        MOVF    TABLAT,W
        XORWF   CRC32+2,F
        TBLRD*+
        MOVF    TABLAT,W
        XORWF   CRC32+3,F
CRC_32_W_LOOP_SKP       =09
        MOVLW   LOW(CRC32_NIBBLE_TABLE_SHORT_END)
        XORWF   TBLPTRL,W
        BNZ     CRC_32_W_LOOP
        ; all done
        RETURN

CRC32_NIBBLE_TABLE_SHORT
        DB   0x77,0x07,0x30,0x96
        DB   0xee,0x0e,0x61,0x2c
        DB   0x07,0x6d,0xc4,0x19
        DB   0x0e,0xdb,0x88,0x32
        DB   0x1d,0xb7,0x10,0x64
        DB   0x3b,0x6e,0x20,0xc8
        DB   0x76,0xdc,0x41,0x90
        DB   0xed,0xb8,0x83,0x20
CRC32_NIBBLE_TABLE_SHORT_END

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads