FCS CALCULATION
FCS (Frame Check Sequence)is calculated over the entire PPP packet, not including
the start and stop flags (0x7E). For a transmitted packet the calculation is performed
prior to the addition of the 0x7D escape sequences to the octets lower then 0x20.
For a received packet the calculation is performed after eliminating the 0x7D escape
sequences and subtracting 0x20 from the octets marked by the escape sequences.
In the example code,
u16 buff[] is an array containing all octets in the packet with the
FCS octets made equal to zero.
u16 fcs_start is the position of the first octet in the packet
u16 fcs_stop is the position of the last octet in the packet
/*
**************************** fcs_calc *********************
Function: fcs_calc
Description: frame check sequence (fcs) calculation.
**************************************************************
*/
typedef unsigned short u16
u16 short fcs_calc(u16 fcs_start, u16 fcs_stop, u16 buff[])
{
#define P 0x8408
#define fcsinit 0xFFFF
static u16 fcstab[256];
u16 fcs;
u16 b, v;
u16 i;
u16 MSB_fcs, LSB_fcs;
for (b=0; ; ){
v = b;
for (i = 8; i--; )
v = v & 1 ? (v >> 1) ^ P : v >> 1;
fcstab[b] = v & 0xFFFF;
if (++b == 256)
break;
}
fcs = fcsinit;
for (i=fcs_start; i<fcs_stop; i++)
fcs = (fcs >> 8) ^ fcstab[(fcs ^ buff[i]) & 0xFF];
fcs ^= 0xFFFF;
LSB_fcs = ((fcs >> 8) & 0xFF);
MSB_fcs = fcs & 0xFF;
return(MSB_fcs, LSB_fcs);
}
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