The IEEE 802.15.4 standard uses a 16-bit ITU-T cyclic redundancy check (CRC) calculated over the MAC header (MHR) and MAC service data unit (MSDU).
The standard defines the following generator polynomial:
G(x) = x16 + x12 + x5 + 1
In RX mode, RADIO will trigger the CRC module when the first octet after the frame length (PHR) is received. The CRC will then update on each consecutive octet received. When a complete frame is received the CRCSTATUS register will be updated accordingly and the CRCOK or CRCERROR events generated. When the CRC module is enabled it will not write the two last octets (CRC) to the frame Data RAM. When transmitting, the CRC will be computed on the fly, starting with the first octet after PHR, and inserted as the two last octets in the frame. The EasyDMA will fetch frame length minus 2 octets from RAM and insert the CRC octets at their correct positions in the frame.
The following code shows how to configure the CRC module for correct operation when in IEEE 802.15.4 mode. The CRCCNF is written to 16-bit CRC and the CRCPOLY is written to 0x11021. The start value used by IEEE 802.15.4 is 0 and CRCINIT is configured to reflect this.
/* 16-bit CRC with ITU-T polynomial with 0 as start condition*/
NRF_RADIO->CRCCNF = ((RADIO_CRCCNF_SKIPADDR_Ieee802154 << RADIO_CRCCNF_SKIPADDR_Pos) |
(RADIO_CRCCNF_LEN_Two << RADIO_CRCCNF_LEN_Pos));
NRF_RADIO->CRCPOLY = 0x11021;
NRF_RADIO->CRCINIT = 0;
The ENDIANESS subregister must be set to little-endian since the FCS field is transmitted from left bit to right.