nRF24L01 CRC calculation

[Dai]

Hi,

In working with nRF24L01, I am facing a problem with nRF24L01 CRC calculation.  In the spec, it is stated that CRC calculation includes address, control field and data. For address and data, the length is always in byte. While for control field, it is 9bit : 6bit length + 2bit PID + 1 bit no_ack.  I understand CRC calculation in HW is based on data stream. While for SW, all existing examples are based on byte calculation. Would it be possible to provide an example of CRC calculation based on bit?

Thanks, Dai

[ovrebekk]

Hi Dai

Can you show me the CRC function you are using today?

Normally the byte based algorithms will loop through every single bit in each byte, and as such they should be relatively easy to update for a different bit size. 

Best regards
Torbjørn

[Dai]

Hi Torbjørn,

Thanks for your reply.  Can you please refer to the wiki page below for the CRC function I am using?

https://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks

Actually, taking a closer look at the spec, I find the granularity of CRC HW implementation of nRF24L01 is also byte instead of bit. So the question is then related to the padding of the 9-bit control field. It looks the control field is padded to 16-bit before CRC calculation. Is this a correct understanding? If yes, it would be much helpful if you can specify how the padding is done?

Best Regards,

Dai

[ovrebekk]

Hi Dai

There should be no padding of the CRC field, no. It is true that the generated CRC is either 1 or 2 bytes long, but this doesn't mean that the CRC calculation itself is byte based. 

Best regards
Torbjørn

[Dai]

Hi Torbjørn，

Thanks for your confirmation. So CRC calcualtion here is NOT byte based.

Best Regards,

Dai

[ovrebekk]

Hi Dai

That's correct. I will consider the case resolved then

Best regards
Torbjørn