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46 lines
995 B
C
46 lines
995 B
C
#define POLY 0x8408
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/*
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// 16 12 5
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// this is the CCITT CRC 16 polynomial X + X + X + 1.
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// This works out to be 0x1021, but the way the algorithm works
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// lets us use 0x8408 (the reverse of the bit pattern). The high
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// bit is always assumed to be set, thus we only use 16 bits to
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// represent the 17 bit value.
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*/
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#include "crc.h"
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WORD crc16(char *data_p, WORD length)
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{
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unsigned char i;
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unsigned int data;
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unsigned int crc = 0xffff;
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if (length == 0)
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return (~crc);
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do
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{
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for (i=0, data=(unsigned int)0xff & *data_p++;
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i < 8;
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i++, data >>= 1)
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{
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if ((crc & 0x0001) ^ (data & 0x0001))
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crc = (crc >> 1) ^ POLY;
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else crc >>= 1;
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}
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} while (--length);
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crc = ~crc;
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data = crc;
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crc = (crc << 8) | ((data >> 8) & 0xff);
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return (crc);
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}
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