gefran BDI50 Manuel D'utilisation page 155

2.2 CRC CHECK:
CRC Check Code is calculated from SLAVE Address to end of the data. The calculation
method is illustrated as follow:
(1). Load a 16-bit register with FFFF hex (all's1).Call this the CRC register.
(2). Exclusive OR the first 8-bit byte of the message with the low-order byte of the 16-bit
CRC register, putting the result in the CRC register.
(3). Shift the CRC register one bit to the right (toward the LSB), Zero-filling the MSB,
Extract and examines the LSB.
(4). (If the LSB was 0): Repeat Steps(3)(another shift). (If the LSB was 1): Exclusive OR
the CRC register with the polynomial value A001 hex (1010 0000 0000 0001), putting
the result in the CRC register.
(5). Repeat Steps (3) and (4) until 8 shifts been performed. When this is done, a complete
8-bit byte will be processed .
(6). Repeat Steps (2) through (5) for next 8-bit byte of the message, Continue doing this
until all bytes have been processed. The final content of the CRC register is the CRC
value. Placing the CRC into the message: When the 16-bit CRC (2 8-bit bytes) is
transmitted in the message, the Low-order byte will be transmitted first, followed by
the high-order byte, For example, if the CRC value is 1241 hex, the CRC-16 Upper
put the 41h, the CRC-16 Lower put the 12h.

CRC calculation application program
UWORD ch_sum (UBYTE long , UBYTE *rxdbuff)
{
BYTE i = 0;
UWORD wkg = 0xFFFF;
while (long--)
{
wkg ^= rxdbuff++;
for (i = 0 ; i < 8; i++)
{
if (wkg & 0x0001)
{
wkg = (wkg >> 1) ^ 0xa001;
}
else
{
wkg = wkg >> 1;
}
}
}
return(wkg);
}
App 3-3