Add support for universal fan controller

[marcelrv]

Adding support for universal (reversable) fan controller 24V incl the non standard CRC

[zuckschwerdt]

Thanks. Btw, the "CRC" is not CRC but actually a nibble sum without carry (xor), right?

[marcelrv]

Thanks. Btw, the "CRC" is not CRC but actually a nibble sum without carry (xor), right?

:-) i directly believe you.... The formula works I know.. sum of each 4 bits xor'ed

It is not documented anywhere, I figured it out from looking to the occurring value...
As the rotating counter is only 3 bits I tested out all possible values

If it is not a true crc...Should I rename it to checksum?

[zuckschwerdt]

Preferably rename crc to chk.
The formular crc ^= (b[j / 8] >> (4 * (1 - (j % 8) / 4))) & 0x0F; seems strange, e.g. the first 8 counts b[0] will be shifted, the first 4 times by 4 then 4 times by 0, right? That's a no-op with xor as each two xor's cancel out?

[marcelrv]

Preferably rename crc to chk. The formular crc ^= (b[j / 8] >> (4 * (1 - (j % 8) / 4))) & 0x0F; seems strange, e.g. the first 8 counts b[0] will be shifted, the first 4 times by 4 then 4 times by 0, right? That's a no-op with xor as each two xor's cancel out?

no, not really, the idea is that you xor eacht time 4 bits.
As the array is in bytes, b[j / 8] makes that we 2x select byte 0, 2x byte 1 , etc. The (4 * (1 - (j % 8) / 4))) piece makes we either shift by 4 or we don't shift.. the final & 0F makes we always only take the 4 bits. for the xor' ing

[zuckschwerdt]

Yes! I Missed the j += 4 :)
We usually write this with a byte-wise stepping and fold the nibbles, e.g chk ^= (b[j] >> 4) ^ (b[j] & 0xf), the last incomplete byte then needs a last chk ^= (b[3] >> 4).
There is a utility function for this, e.g. int sum = add_nibbles(b, 3) + (b[3] >> 4);

[zuckschwerdt]

You need to run ./maintainer_update.py to get the build system changes. Commit those changes too.

[marcelrv]

int sum = add_nibbles(b, 3) + (b[3] >> 4); did not work well, as it does not apply the xor.
I added a new function to do that, indeed in a more readable way based on the add_nibbles.

[zuckschwerdt]

int sum = add_nibbles(b, 3) + (b[3] >> 4); did not work well, as it does not apply the xor.

Oops. Confused myself there. I should have given a different example. Also sum == chk for xor is the same as full xor and then sum == 0 ;)

int sum = xor_bytes(b, 4); // xor message bytes, last byte also has the checksum
sum = (sum >> 4) ^ (sum & 0xf); // fold nibbles
if (sum != 0xa) …

The xor_nibbles is a nice addition. Remove the init, sum full bytes then fold after the loop. Again sorry for the add-with-carry mixup.