Here is another one, based on code from this page:

<http://www.digitalnemesis.com/ash/projects/EmbeddedCRC16/default.htm>

This is a CRC16 CCITT, not sure if it the same as dallas use though...

///// CRC16 functions

const byte CRC16TableHigh[]=3D{
        0x00, 0x10, 0x20, 0x30, 0x40, 0x50, 0x60, 0x70,
        0x81, 0x91, 0xA1, 0xB1, 0xC1, 0xD1, 0xE1, 0xF1
};

const byte CRC16TableLow[]=3D{
        0x00, 0x21, 0x42, 0x63, 0x84, 0xA5, 0xC6, 0xE7,
        0x08, 0x29, 0x4A, 0x6B, 0x8C, 0xAD, 0xCE, 0xEF
};

TWREG CRC16Work;                // 16 bit.

void    CRC16_Update4bits(byte val )
{
        byte    t;

        // Step one, extract the Most significant 4 bits of the CRC register
        t =3D CRC16Work.b.high >> 4;

        // XOR in the Message Data into the extracted bits
        t =3D t ^ val;

        // Shift the CRC Register left 4 bits
        CRC16Work.w<<=3D4;

        // Do the table lookups and XOR the result into the CRC Tables
        CRC16Work.b.high=3DCRC16Work.b.high ^ CRC16TableHigh[t & 0x0f];
        CRC16Work.b.low=3DCRC16Work.b.low ^ CRC16TableLow[t & 0x0f];
}

word    CRC16(void *adr,byte count)
{
        CRC16Work.w=3D0xffff;   // Initialize running CRC.
        for(;count>0;count--){
                CRC16_Update4bits(*((byte*)adr)>>4);
                CRC16_Update4bits(*((byte*)adr));
                ((byte*)adr)++;
        }
        return CRC16Work.w;
}

Regards / Ruben



> Perhaps the following code may be useful to someone either now or in th=
e
> future (at least it's nice to have in your engineering toolbox).
>=20
> I've adapted the fast look-up table CRC-16 from the Dallas Semiconducto=
r
> Application Note 27 to the PIC18F architecture:
>=20
> (I apologize to all if the text formatting comes across badly trashed.)
>=20

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

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