I'm working on a program running on a micro controller and need to implement a self-test for the program code integrity.
For this, I let the code calculate a CRC16 checksum over the whole flash memory (program space) and transmit this value to another system via some network. The other system then has to compare the checksum against a pre-calculated value.
However, with each update, the CRC value changes. So the whole process could be simplified, if the program code can be prepared beforehand, such that the CRC16 checksum always matches a predefined value like 0 or better something like 0x1234.
Is there an easy way to achieve this?
Another way to put this: can I easily calculate a byte sequence, that I would have to add to my programs binary code (for example by changing a static array with dummy data included in the program), so that the CRC16 gives my predefined value? Can this byte sequence be included anywhere in the code or does it have to be exactly at the end?
(If necessary, I could also implement another checksum algorithm besides CRC-16.)
Thanks for your answers!
Yes, easily. For your n bytes of flash, compute the CRC-16 of the first n-2 bytes, and store that CRC in the last two bytes. Those two bytes would be appended in little-endian order for a reflected CRC, and in big-endian order for a non-reflected CRC. Then the CRC-16 of the n bytes will be a constant. That constant is known as the "residue" of the CRC. For CRC's with no exclusive-or at the end, the residue is always zero. You didn't say what CRC you're using, but you can find the residues of known CRC's (before the final exclusive-or) in Greg Cook's catalog. Or you can just see what you get.