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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Errors with PCD built-in crc functions |
 Posted: Fri Oct 03, 2008 3:56 pm |
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Hello,
PCD has built in-functions for PIC24F hardware crc engine. I found some errors and a difference to command despription in the compiler manual.
- In den manual examples, the below setup_crc is said to select the CRC16 polynomial:
| Quote: | setup_crc (12, 5);
// CRC Polynomial is X16 + X12 + X5 + 1 or Polynomial = 1020h |
As you can see from the assembly code, neither the polynomial length nor the value are set correct
| Code: | setup_crc(12, 5); // Polynomial 0x1021
032FE: MOV.B #21,W0L
03300: MOV.B W0L,642
03302: MOV.B #B,W0L
03304: MOV.B W0L,640 |
Using setup_crc (16,12, 5) achieves a correct length, but still a wrong polynomial. The correct code is
| Code: | CRCXOR = 0x1021;
CRCCON = 0xf; |
- The compiler manual tells, that crc_calc() respectively crc_calc8() can be used with a single word/byte argument. Apparently this mode is yet unsupported.
- crc_calc is always stuck in an endless loop when it starts the CRC calculation
| Code: | 0279A A88640 bset.b 0x0640,#4 ; CRCGO
0279C AF8640 btsc.b 0x0640,#4 ; CRCMPT (#6) should be tested instead
0279E 37FFFD bra 0x00279a ; loops forever |
- Furthermore, there is a general problem with the CRC hardware, that an odd number of bytes can't be processed without an considerable overhead. See AN1148 for details.
Regards,
Frank |
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eoinoc
Joined: 30 Apr 2009
Posts: 16
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| Posted: Mon Oct 25, 2010 7:12 pm |
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Does anyone have an update on this topic?
I did some testing with the built in functions in CCS and the result returned from the CRC function which uses the hardware CRC in the PIC24HJ and the answers dont appear to be correct infact the result always stays at the same value.
Is anyone willing to share some drivers for the on chip CRC generator ? |
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Mon Oct 25, 2010 11:01 pm |
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Besides the reported PCD bugs, I had another reason not to use the PIC24 hardware CRC unit. It's involving a considerable overhead for byte data with an odd byte count, as already mentioned. So I fell back to the table and shift register based CRC algorithms I used with other processors.
I didn't check again the PCD built-in CRC functions and don't know, if they have substantially changed since V4.097. |
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Douglas Kennedy
Joined: 07 Sep 2003
Posts: 755
Location: Florida
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| Posted: Tue Oct 26, 2010 12:33 pm |
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Just in case it's the issue these polynomials require seed values usually
0x0000 or 0xFFFF. Most often 0x0000 is used.
// CRC Polynomial is X16 + X12 + X5 + 1 or Polynomial = 1020h
this X16+X12+X5+1 is 1021h or 0b0001 0000 0010 0001 the set bits represent the power sequence 1 is x^0 so X^5 is in position 5 starting at zero. Did they perhaps calc X16+x12+x5+0 the true 1020h |
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