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[Solved] PIC24 hardware CRC-16-CCITT (poly 0x1021) calc

 
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allenhuffman



Joined: 17 Jun 2019
Posts: 554
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[Solved] PIC24 hardware CRC-16-CCITT (poly 0x1021) calc
PostPosted: Thu Nov 14, 2019 9:41 am     Reply with quote
I see others trying to figure this out in this forum and at Microchip's.

I am looking into the CCS hardware CRC routines. Since the CRC used by the .hex files is CRC-16-CCITT, I wanted to use the same one in the PIC24.

The polynomial is 0x1021, listed here:

https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations_of_cyclic_redundancy_checks

Here is the online CRC calculator I used to verify that my C# implementation for the .hex file worked:

https://crccalc.com/

If you go to that site, and select "Input type: hex" then give it this hex string:

Code:
AABBCCDDEEFF


…then click "Calc CRC-16", it shows a value down in the "CRC-16/XMODEM" line. Click on that and it will sort to the top.

That was the value (initialize to 0x0000) that matched the .hex file CRC for me.

When I try to do this using the CCS library, I am not getting a match:

Code:
    // CRC test
    unsigned int16 crc;
   
    // https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations_of_cyclic_redundancy_checks
    // CRC-16-CCITT
    setup_crc (16, 12, 5); // 0x1021 polynomial values.
   
    crc_init (0x0000);

    unsigned int8 data[6] = { 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff };
   
    crc = crc_calc16 (data, 6, 8); // 8-bit data.   

    printf ("CRC is %04x\r\n", crc);


From the Wiki page, it shows the polynomial values of:

Quote:
x^16 + x^12 + x^5 + 1


So I am using setup_Crc(16, 12, 5). I initialize to 0 and then calculate over the even number of bytes (6), specifying that they are 8-bit values.

The website shows 0x53F5 for the result, by my code shows 0x05f8.

I am obviously missing a step, or using an incorrect polynomial or init value.

Anyone used this?
_________________
Allen C. Huffman, Sub-Etha Software (est. 1990) http://www.subethasoftware.com
Embedded C, Arduino, MSP430, ESP8266/32, BASIC Stamp and PIC24 programmer.
http://www.whywouldyouwanttodothat.com ?

Last edited by allenhuffman on Thu Nov 14, 2019 10:14 am; edited 1 time in total
PCM programmer



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Posts: 21708

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PostPosted: Thu Nov 14, 2019 9:52 am     Reply with quote
In their example file, Ex_crc_hw.c, CCS has this line in main():
Code:
 setup_crc(16, 12, 5, 0);   //0x1021 CRC CCITT

The example file is dated 8/9/2018.
allenhuffman



Joined: 17 Jun 2019
Posts: 554
Location: Des Moines, Iowa, USA

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PostPosted: Thu Nov 14, 2019 9:59 am     Reply with quote
PCM programmer wrote:
In their example file, Ex_crc_hw.c, CCS has this line in main():
Code:
 setup_crc(16, 12, 5, 0);   //0x1021 CRC CCITT

The example file is dated 8/9/2018.


Thank you. That solved it. I really need to stop reading the CCS manual. The examples show:

Code:
Examples:

setup_crc (12, 5);                  // CRC Polynomial is X12 + X5 + 1
setup_crc(16, 15, 3, 1);           // CRC Polynomial is X16 + X15 + X3 + X1+ 1


…so they include the x^# digit but not the +1 at the end, and don't show a zero.

Thanks! I looked at that example code, and since it used different values I figured the 0 was part of that different polynomial.
_________________
Allen C. Huffman, Sub-Etha Software (est. 1990) http://www.subethasoftware.com
Embedded C, Arduino, MSP430, ESP8266/32, BASIC Stamp and PIC24 programmer.
http://www.whywouldyouwanttodothat.com ?
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