14.3.3.3 Automatic FCS Generation
The automatic FCS generation is enabled by setting the TX_AUTO_CRC_ON bit in the TRX_CTRL_1 register (TRX_CTRL_1.TX_AUTO_CRC_ON) to '1'. This allows the AT86RF212B to compute the FCS autonomously. For a frame with a frame length specified as N (3 ≤ N ≤ 127), the FCS is calculated on the first N-2 octets in the Frame Buffer, and the resulting FCS field is transmitted in place of the last two octets from the Frame Buffer.
In RX_AACK mode, when a received frame needs to be acknowledged, the FCS of the ACK frame is always automatically generated by the AT86RF212B, independent of the TX_AUTO_CRC_ON bit setting.
A frame transmission of length five with TX_AUTO_CRC_ON set, is started with a Frame Buffer write access of five bytes (the last two bytes can be omitted). The first three bytes are used for FCS generation; the last two bytes are replaced by the internally calculated FCS.