There's no painless way to do this, but it needs to be done --

[proxmark3-svn] / common / crc16.c

unsigned short update_crc16( unsigned short crc, unsigned char c ) {\r
	unsigned short i, v, tcrc = 0;\r
\r
	v = (crc ^ c) & 0xff;\r
  for (i = 0; i < 8; i++) {\r
      tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;\r
      v >>= 1;\r
  }\r
\r
  return ((crc >> 8) ^ tcrc)&0xffff;\r
}\r