Pololu DMC01, User Manual

message = [1 0 0 0 0 1 1 0] [0 0 0 1 1 1 1 1] 0 0 0 0 0 0 0

Steps 3, 4, & 5:

_______________________________________________

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

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

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

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

shift  ----> 1 0 0 0 1 0 0 1 | | | | | | | | | | |

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

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

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

_______________ | | | | | | | | | |

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

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

_______________ | | | | | | | | |

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

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

_______________ | | | | | | | |

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

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

_______________ | | | | | | |

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

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

_______________ | | | | | |

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

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

_______________ | | | | |

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

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

_______________ | | | |

1 1 1 1 1 1 1 0 | | |

1 0 0 0 1 0 0 1 | | |

_______________ | | |

1 1 1 0 1 1 1 0 | |

1 0 0 0 1 0 0 1 | |

_______________ | |

1 1 0 0 1 1 1 0 |

1 0 0 0 1 0 0 1 |

_______________ |

1 0 0 0 1 1 1 0

1 0 0 0 1 0 0 1

_______________

0 0 0 0 1 1 1 = 0x07

So the full command packet we would send to retrieve the raw channel inputs for all five channels with CRC
enabled is:

0x86, 0x1F, 0x07

There are some tricks you can use in your programs to make the CRC calculation much more efficient from a speed
perspective. You can find an example of this

Section 5.f

.

5.f. CRC-Generation Algorithm in C

The CRC algorithm is typically defined as a bit-wise operation, however it is possible to rewrite the algorithm to
work at the byte level if we do some calculations ahead of time. We can store CRC computations for all 256
possible byte values when our program first runs, and then retrieve those values as needed when generating a CRC
for a given message. This requires we set aside 256 bytes of RAM/EEPROM/flash, but it makes the CRC
calculation for an arbitrary message much faster than if we work through the message bit by bit.

The following sample code stores a CRC lookup table in RAM using the function

GenerateCRCTable()

, which

only needs to be called once (probably somewhere during the initialization phase of your program). You could
easily change this function to store the lookup table to EEPROM or flash. To process a message, initialize your CRC
variable to zero and use the

CRCAdd()

function to “add” each message byte to your CRC, as is done in the

getCRC()

function.

unsigned char CRCPoly = 0x89;  // the value of our CRC-7 polynomial

unsigned char CRCTable[256];

Pololu TReX User's Guide

© 2001–2009 Pololu Corporation

5. The Serial Interface

Page 18 of 22