Pololu Qik 2s9v1 Manuel d'utilisateur télécharger pdf (Page 21)

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 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 <stdio.h>

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);

}

6. Cyclic Redundancy Check (CRC) Error Detection Page 21 of 24

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Pololu Qik 2s9v1 Manuel d'utilisateur Page 21

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