Scott Dattalo <scott@dattalo.com> wrote:
> Since you're religiously opposed to assembly, this C source will save
> your soul:
>
> http://www.dattalo.com/technical/software/pic/crc_8bit.c
>
> A good C-compiler should turn the crc_bits() function into about 19
> instructions.
> Scott

I've tried four CRC8 routines in Keil C51 for a 8051 core:

1.- Atmel (i-button):
    machine cycles = 152
    prog. mem. =   0B
2.- Scott, Table routine:
    machine cycles = 20
    prog. mem. =   256B
3.- Scott, nibble routine:
    machine cycles = 65
    prog. mem. =   32B
4.- Scott, bits routine:
    machine cycles = 51
    prog. mem. =   0B
    (*)asm instrucctions= 30

For more speed, the use of table can make a difference, but the bit
routine... is the winner for little program memory spaces!

Thanks, Scott.

Dennis Crawley
Argentina
www.geocities.com/proyectosenpic
(spanish)


-----------------
(*)
    78: unsigned char crc_bits(int dato)
    79: {
    80:   int i = (dato ^ crc) & 0xff;
    81:
C:0x05C8    E50F     MOV      A,crc(0x0F)
C:0x05CA    6F       XRL      A,R7
C:0x05CB    FF       MOV      R7,A
    82:   crc = 0;
C:0x05CC    E4       CLR      A
C:0x05CD    F50F     MOV      crc(0x0F),A
    84:   if(i & 1)
C:0x05CF    EF       MOV      A,R7
C:0x05D0    30E003   JNB      0xE0.0,C:05D6
    85:     crc ^= 0x5e;
C:0x05D3    630F5E   XRL      crc(0x0F),#0x5E
    86:   if(i & 2)
C:0x05D6    EF       MOV      A,R7
C:0x05D7    30E103   JNB      0xE0.1,C:05DD
    87:     crc ^= 0xbc;
C:0x05DA    630FBC   XRL      crc(0x0F),#0xBC
    88:   if(i & 4)
C:0x05DD    EF       MOV      A,R7
C:0x05DE    30E203   JNB      0xE0.2,C:05E4
    89:     crc ^= 0x61;
C:0x05E1    630F61   XRL      crc(0x0F),#0x61
    90:   if(i & 8)
C:0x05E4    EF       MOV      A,R7
C:0x05E5    30E303   JNB      0xE0.3,C:05EB
    91:     crc ^= 0xc2;
C:0x05E8    630FC2   XRL      crc(0x0F),#0xC2
    92:   if(i & 0x10)
C:0x05EB    EF       MOV      A,R7
C:0x05EC    30E403   JNB      0xE0.4,C:05F2
    93:     crc ^= 0x9d;
C:0x05EF    630F9D   XRL      crc(0x0F),#0x9D
    94:   if(i & 0x20)
C:0x05F2    EF       MOV      A,R7
C:0x05F3    30E503   JNB      0xE0.5,C:05F9
    95:     crc ^= 0x23;
C:0x05F6    630F23   XRL      crc(0x0F),#0x23
    96:   if(i & 0x40)
C:0x05F9    EF       MOV      A,R7
C:0x05FA    30E603   JNB      0xE0.6,C:0600
    97:     crc ^= 0x46;
C:0x05FD    630F46   XRL      crc(0x0F),#0x46
    98:   if(i & 0x80)
C:0x0600    EF       MOV      A,R7
C:0x0601    30E703   JNB      0xE0.7,C:0607
    99:     crc ^= 0x8c;
   100:
C:0x0604    630F8C   XRL      crc(0x0F),#TH0(0x8C)
   101:   return crc;
C:0x0607    AF0F     MOV      R7,crc(0x0F)
}



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