A. crc computation in c – Pololu Qik 2s12v10 User Manual

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Steps 3, 4, & 5:

_______________________________________________

1 0 0 0 1 0 0 1 ) 1 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

XOR 1 0 0 0 1 0 0 1 | | | | | | | | | | | | | | |

--------------- | | | | | | | | | | | | | | |

1 0 0 1 0 0 0 1 | | | | | | | | | | | | | |

_______________ | | | | | | | | | | | | | |

1 1 0 0 0 0 0 0 | | | | | | | | | | |

1 0 0 0 1 0 0 1 | | | | | | | | | | |

_______________ | | | | | | | | | | |

1 0 0 1 0 0 1 0 | | | | | | | | | |

1 0 0 0 1 0 0 1 | | | | | | | | | |

_______________ | | | | | | | | | |

1 1 0 1 1 0 0 0 | | | | | | |

1 0 0 0 1 0 0 1 | | | | | | |

_______________ | | | | | | |

1 0 1 0 0 0 1 0 | | | | | |

1 0 0 0 1 0 0 1 | | | | | |

_______________ | | | | | |

1 0 1 0 1 1 0 0 | | | |

1 0 0 0 1 0 0 1 | | | |

_______________ | | | |

1 0 0 1 0 1 0 0 | |

1 0 0 0 1 0 0 1 | |

_______________ | |

1 1 1 0 1 0 0 = 0x17

So the full command packet we would send to retrieve the PWM configuration parameter with CRC enabled is:
0x83, 0x01, 0x17

There are some tricks you can use in your programs to make the CRC calculation much faster. You can find an
example of this

Section 6.a

6.a. CRC Computation in C

The following example program shows how to compute a CRC in the C programming language. The idea is that
the CRC of every possible byte is stored in a lookup table, so that it can be quickly loaded when computing the
CRC of a longer message. The individual CRC bytes are XORed together with the C operator ^ to get the final
CRC of the message. In the example main() routine, this is applied to generate the CRC byte in the message 0x83,
0x01, that was used in

Section 6

#include

const unsigned char CRC7_POLY = 0x91;

unsigned char CRCTable[256];

unsigned char GetCRC(unsigned char val)

{

unsigned char j;

for (j = 0; j < 8; j++)

{

if (val & 1)

val ^= CRC7_POLY;

val >>= 1;

}

return val;

}

void GenerateCRCTable()

{

int i, j;

// generate a table value for all 256 possible byte values

for (i = 0; i < 256; i++)

{

CRCTable[i] = GetCRC(i);

Qik 2s12v10 User's Guide

6. Cyclic Redundancy Check (CRC) Error Detection

Page 30 of 33