Chapter 8

16-bit CRC-CCITT

The dspic-utility.h header file defines two functions for calculating the 16-bit CRC-CCITT value of a stream of bytes.

These functions are based on an article by Joe Geluso, originally located at Unfortunately, it seems that that site no longer exists, but a copy of the article can be found in the Wayback Machine at

8.1  Usage

To use these functions, you can use the -I compiler option to add the dsPIC Helper Library source directory to the compiler command line, and then include the header file like so:

  #include <crc-ccitt.h>

8.2  Functions

uint16_t crc_ccitt(uint16_t crc, char byte)   function

Call this function with the first byte in the stream, using an initial crc value of CRC_CCITT_INITIAL_VALUE . The function returns the crc value to use in the call for the next byte in the stream. Once all the bytes have been processed, call crc_ccitt_normalize() to retrieve the actual CRC value.

uint16_t crc_ccitt_normalize(uint16_t crc)   function

Once all the bytes have been processed, call this function with crc set to the last value returned by crc_ccitt() to retrieve the actual CRC value.

8.3  Example

Here’s a simple example that calculates the 16-bit CRC-CCITT value for the string "123456789". A call to test() will return value 0xE5CC:

#include <crc-ccitt.h>

uint16_t test(void)
    const char* str = "123456789";
    uint16_t crc = CRC_CCITT_INITIAL_VALUE;
    while (*str)
        crc = crc_ccitt(crc, *str++);
    return crc_ccitt_normalize(crc);