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/*****************************************************************************

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 *

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 * $Id$

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 *

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 ****************************************************************************/

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#include "sii_crc.h"

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/*****************************************************************************/

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/** Calculates the SII checksum field.

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 *

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 * The checksum is generated with the polynom x^8+x^2+x+1 (0x07) and an

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 * initial value of 0xff (see IEC 61158-6-12 ch. 5.4).

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 *

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 * The below code was originally generated with PYCRC

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 * http://www.tty1.net/pycrc

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 *

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 * ./pycrc.py --width=8 --poly=0x07 --reflect-in=0 --xor-in=0xff

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 *   --reflect-out=0 --xor-out=0 --generate c --algorithm=bit-by-bit

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 *

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 * \return CRC8

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 */

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uint8_t calcSiiCrc(

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        const uint8_t *data, /**< pointer to data */

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        size_t length /**< number of bytes in \a data */

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        )

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{

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    unsigned int i;

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    uint8_t bit, byte, crc = 0x48;

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    while (length--) {

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        byte = *data++;

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        for (i = 0; i < 8; i++) {

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            bit = crc & 0x80;

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            crc = (crc << 1) | ((byte >> (7 - i)) & 0x01);

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            if (bit) crc ^= 0x07;

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        }

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    }

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    for (i = 0; i < 8; i++) {

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        bit = crc & 0x80;

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        crc <<= 1;

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        if (bit) crc ^= 0x07;

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    }

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    return crc;

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}

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/*****************************************************************************/