Tag: c30
CRC With dsPIC
by Martyn on Mar.23, 2009, under News
CRC’s are probably the most time consuming thing you can do in a data stream, but they have their uses - and not just for checking that data is correct, but more on that in a later post.
I found that the microchip documentation extremely useful, and I suspect you will to: Section 36 PCRC. But the biggest shout out should go to billysdomain in the Microchip Forums who actually got it working, the bits which make it work are his code, lets begin:
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int CalculateCRC(DataPL *DataPacket)
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{
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unsigned int j, length = (DATAPL_LEN*2)-2;
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unsigned char *ptr, *data;
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ptr = (unsigned char *)&CRCDAT;
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CRCCON = 0×0F; // Configure the polynomial length (PLEN)
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_CRCIF = 0; // Clear the CRC flag
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_CRCIE = 1; // Enable the int
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CRCXOR = 0×1021; // Polynomial - crc-16
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CRCWDAT = 0×0000; // Initialize CRCWDAT with 0
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data = (*DataPacket).CHARS;
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length >>= 1;
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for(j = 0; j < length; j++)
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{
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//Must use pairs of bytes to get the correct result
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*ptr = *data++; //write data into FIFO
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*ptr = *data++; //write data into FIFO
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if (CRCCONbits.CRCFUL) //check if FIFO is full
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{
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CRCCONbits.CRCGO = 1; //start CRC engine
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while (!CRCCONbits.CRCMPT); //check if FIFO is empty
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CRCCONbits.CRCGO = 0; //stop CRC engine
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}
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}
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CRCDAT = 0×0000;
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CRCCONbits.CRCGO = 1; // (Re)Start the calculation
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comms_datapacket = DataPacket; // Save a reference
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// With a bit of luck CRC finishes before we send
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// the data via SPI…
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return 1;
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}
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//Use the software CRC calculator
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static const unsigned int CRC16Table[256] = {
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0×0000,0×1021,0×2042,0×3063,0×4084,0×50a5,0×60c6,0×70e7,
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0×8108,0×9129,0xa14a,0xb16b,0xc18c,0xd1ad,0xe1ce,0xf1ef,
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0×1231,0×0210,0×3273,0×2252,0×52b5,0×4294,0×72f7,0×62d6,
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0×9339,0×8318,0xb37b,0xa35a,0xd3bd,0xc39c,0xf3ff,0xe3de,
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0×2462,0×3443,0×0420,0×1401,0×64e6,0×74c7,0×44a4,0×5485,
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0xa56a,0xb54b,0×8528,0×9509,0xe5ee,0xf5cf,0xc5ac,0xd58d,
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0×3653,0×2672,0×1611,0×0630,0×76d7,0×66f6,0×5695,0×46b4,
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0xb75b,0xa77a,0×9719,0×8738,0xf7df,0xe7fe,0xd79d,0xc7bc,
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0×48c4,0×58e5,0×6886,0×78a7,0×0840,0×1861,0×2802,0×3823,
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0xc9cc,0xd9ed,0xe98e,0xf9af,0×8948,0×9969,0xa90a,0xb92b,
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0×5af5,0×4ad4,0×7ab7,0×6a96,0×1a71,0×0a50,0×3a33,0×2a12,
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0xdbfd,0xcbdc,0xfbbf,0xeb9e,0×9b79,0×8b58,0xbb3b,0xab1a,
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0×6ca6,0×7c87,0×4ce4,0×5cc5,0×2c22,0×3c03,0×0c60,0×1c41,
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0xedae,0xfd8f,0xcdec,0xddcd,0xad2a,0xbd0b,0×8d68,0×9d49,
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0×7e97,0×6eb6,0×5ed5,0×4ef4,0×3e13,0×2e32,0×1e51,0×0e70,
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0xff9f,0xefbe,0xdfdd,0xcffc,0xbf1b,0xaf3a,0×9f59,0×8f78,
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0×9188,0×81a9,0xb1ca,0xa1eb,0xd10c,0xc12d,0xf14e,0xe16f,
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0×1080,0×00a1,0×30c2,0×20e3,0×5004,0×4025,0×7046,0×6067,
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0×83b9,0×9398,0xa3fb,0xb3da,0xc33d,0xd31c,0xe37f,0xf35e,
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0×02b1,0×1290,0×22f3,0×32d2,0×4235,0×5214,0×6277,0×7256,
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0xb5ea,0xa5cb,0×95a8,0×8589,0xf56e,0xe54f,0xd52c,0xc50d,
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0×34e2,0×24c3,0×14a0,0×0481,0×7466,0×6447,0×5424,0×4405,
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0xa7db,0xb7fa,0×8799,0×97b8,0xe75f,0xf77e,0xc71d,0xd73c,
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0×26d3,0×36f2,0×0691,0×16b0,0×6657,0×7676,0×4615,0×5634,
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0xd94c,0xc96d,0xf90e,0xe92f,0×99c8,0×89e9,0xb98a,0xa9ab,
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0×5844,0×4865,0×7806,0×6827,0×18c0,0×08e1,0×3882,0×28a3,
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0xcb7d,0xdb5c,0xeb3f,0xfb1e,0×8bf9,0×9bd8,0xabbb,0xbb9a,
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0×4a75,0×5a54,0×6a37,0×7a16,0×0af1,0×1ad0,0×2ab3,0×3a92,
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0xfd2e,0xed0f,0xdd6c,0xcd4d,0xbdaa,0xad8b,0×9de8,0×8dc9,
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0×7c26,0×6c07,0×5c64,0×4c45,0×3ca2,0×2c83,0×1ce0,0×0cc1,
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0xef1f,0xff3e,0xcf5d,0xdf7c,0xaf9b,0xbfba,0×8fd9,0×9ff8,
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0×6e17,0×7e36,0×4e55,0×5e74,0×2e93,0×3eb2,0×0ed1,0×1ef0
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};
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unsigned int ChecksumCalcS(unsigned char *data, unsigned int len)
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{
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unsigned int crc = 0;
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unsigned int i;
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for(i = 0; i < len; i++)
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{
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crc = (crc << 8) ^ CRC16Table[((crc >> 8) ^ *data++)];
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}
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return crc;
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}
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void __attribute__((__interrupt__)) _NOPSV _CRCInterrupt(void)
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{
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unsigned short hardware, software;
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_CRCIF = 0; // Clear the interrupt
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while(!CRCCONbits.CRCMPT); // Wait for CRC shifter to clear FIFO
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(*comms_datapacket).CRC = CRCWDAT;
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hardware = CRCWDAT;
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software = ChecksumCalcS((*comms_datapacket).CHARS,
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(DATAPL_LEN*2)-2);
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CRCWDAT = 0;
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}
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Ok, a lot of things to note 
- This code will not work on its own
- Putting a breakpoint on the last line of code will allow a software / hardware comparison
- _NOPSV is a macro (google it)
- The DataPL datatype is a union of an int array, char array and a large bitfield which allows me to have loads of small (or large) data types of non-standard size.
- The interrupt is not required if working with more than 8 bytes of data as you need to wait for the buffer to clear before filling it - so might as well wait for it all to happen
- This uses 8-bit data rather than 16-bit data, if you have a working example of this with a software table based alternative please email me! the table means I can calculate the CRC in C#
- Most of code is by billysdomain - I was 80% there, missing 2 lines in my implementation, should have rtfm (more)
Anyhow, hope this helps someone. I might do a quick post on using CRC for synchronising serial streams soon, guess how that’s done :p
