Bluetooth Low Energy for Smart Plug
Dependencies: BLE_API mbed nRF51822
Diff: crc2.cpp
- Revision:
- 3:aaa92c61931a
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/crc2.cpp Mon Jul 13 13:14:34 2015 +0000 @@ -0,0 +1,219 @@ + + +#include "crc2.h" + + +/* + * Derive parameters from the standard-specific parameters in crc.h. + */ +#define WIDTH (8 * sizeof(crc)) +#define TOPBIT (1 << (WIDTH - 1)) + +#if (REFLECT_DATA == TRUE) +#undef REFLECT_DATA +#define REFLECT_DATA(X) ((unsigned char) reflect((X), 8)) +#else +#undef REFLECT_DATA +#define REFLECT_DATA(X) (X) +#endif + +#if (REFLECT_REMAINDER == TRUE) +#undef REFLECT_REMAINDER +#define REFLECT_REMAINDER(X) ((crc) reflect((X), WIDTH)) +#else +#undef REFLECT_REMAINDER +#define REFLECT_REMAINDER(X) (X) +#endif + + +/********************************************************************* + * + * Function: reflect() + * + * Description: Reorder the bits of a binary sequence, by reflecting + * them about the middle position. + * + * Notes: No checking is done that nBits <= 32. + * + * Returns: The reflection of the original data. + * + *********************************************************************/ +static unsigned long +reflect(unsigned long data, unsigned char nBits) +{ + unsigned long reflection = 0x00000000; + unsigned char bit; + + /* + * Reflect the data about the center bit. + */ + for (bit = 0; bit < nBits; ++bit) + { + /* + * If the LSB bit is set, set the reflection of it. + */ + if (data & 0x01) + { + reflection |= (1 << ((nBits - 1) - bit)); + } + + data = (data >> 1); + } + + return (reflection); + +} /* reflect() */ + + +/********************************************************************* + * + * Function: crcSlow() + * + * Description: Compute the CRC of a given message. + * + * Notes: + * + * Returns: The CRC of the message. + * + *********************************************************************/ +crc +crcSlow(unsigned char const message[], int nBytes) +{ + crc remainder = INITIAL_REMAINDER; + int byte; + unsigned char bit; + + + /* + * Perform modulo-2 division, a byte at a time. + */ + for (byte = 0; byte < nBytes; ++byte) + { + /* + * Bring the next byte into the remainder. + */ + remainder ^= (REFLECT_DATA(message[byte]) << (WIDTH - 8)); + + /* + * Perform modulo-2 division, a bit at a time. + */ + for (bit = 8; bit > 0; --bit) + { + /* + * Try to divide the current data bit. + */ + if (remainder & TOPBIT) + { + remainder = (remainder << 1) ^ POLYNOMIAL; + } + else + { + remainder = (remainder << 1); + } + } + } + + /* + * The final remainder is the CRC result. + */ + return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE); + +} /* crcSlow() */ + + +crc crcTable[256]; + + +/********************************************************************* + * + * Function: crcInit() + * + * Description: Populate the partial CRC lookup table. + * + * Notes: This function must be rerun any time the CRC standard + * is changed. If desired, it can be run "offline" and + * the table results stored in an embedded system's ROM. + * + * Returns: None defined. + * + *********************************************************************/ +void +crcInit() +{ + crc remainder; + int dividend; + unsigned char bit; + + + /* + * Compute the remainder of each possible dividend. + */ + for (dividend = 0; dividend < 256; ++dividend) + { + /* + * Start with the dividend followed by zeros. + */ + remainder = dividend << (WIDTH - 8); + + /* + * Perform modulo-2 division, a bit at a time. + */ + for (bit = 8; bit > 0; --bit) + { + /* + * Try to divide the current data bit. + */ + if (remainder & TOPBIT) + { + remainder = (remainder << 1) ^ POLYNOMIAL; + } + else + { + remainder = (remainder << 1); + } + } + + /* + * Store the result into the table. + */ + crcTable[dividend] = remainder; + } + +} /* crcInit() */ + + +/********************************************************************* + * + * Function: crcFast() + * + * Description: Compute the CRC of a given message. + * + * Notes: crcInit() must be called first. + * + * Returns: The CRC of the message. + * + *********************************************************************/ +crc +crcFast(unsigned char const message[], int nBytes) +{ + crc remainder = INITIAL_REMAINDER; + unsigned char data; + int byte; + + + /* + * Divide the message by the polynomial, a byte at a time. + */ + for (byte = 0; byte < nBytes; ++byte) + { + data = REFLECT_DATA(message[byte]) ^ (remainder >> (WIDTH - 8)); + remainder = crcTable[data] ^ (remainder << 8); + } + + /* + * The final remainder is the CRC. + */ + return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE); + +} /* crcFast() */ +