ESB CRC calculation

RE: ESB CRC calculation

Kenneth — Wed, 20 Mar 2024 10:54:59 GMT

I can find several cases discussing calculation of CRC here on devzone, it should be the same for all variants of the nRF24-, nRF51, nRF52-series. I suggest to take a look at them, but afaik it should be standard CRC-16-CCITT (though I have seen someone mention False at the end, if that make any difference).

Kenenth

RE: ESB CRC calculation

RLC — Wed, 20 Mar 2024 09:36:17 GMT

Hi Kenneth,

Thanks for the quick response.

You are absolutely correct. I have not found any online calculators that perform a bitwise calculation of the CRC. Hence, I created one in Excel using manual long division (attached).

I further validated my spreadsheet by deleting a bit from the Packet Control Field. The Address + (modified) Packet Control Field + Payload is then a multiple of 8-bits. This can be conveniently represented as bytes. Using the calculator at https://crccalc.com/ for CRC-16/AUG-CCITT, I get the same CRC as my spreadsheet.

As the link suggests, the MSByte and MSBit ordering can be a source of error. I've simplified this by:

1. Using the same value for all address and prefix bytes. Hence, byte-order is not an issue for the address/prefix.

2. Using 0xE7 for the address/prefix. This is 0b11100111 so bit-order is not an issue for the address/prefix.

3. Use payload of all 0x00. Hence, bit/byte order is not an issue for the payload.

Is the ESB 2-byte CRC calculated as per CRC-CCITT? The documentation suggests so but doesn't say explicitly.

Thanks

RE: ESB CRC calculation

Kenneth — Tue, 19 Mar 2024 13:17:20 GMT

Hello,

It's a long time since I looked into this, and since it's done in hardware most of the time (at least for the radio) I must admit I don't remember the details. What I remember barely is that many (at least online) crc calculators need byte aligned data, while in this case it's not. So you may need to take a look at this old thread on how to calculate crc on bit (not byte):
https://stackoverflow.com/questions/27332600/crc-bit-order-confusion

Kenneth