The ATmega128RFA1 and ATmega256RFR2 contain a version of this IP
on the MCU.
The radio core behaves mostly like a at86rf231, but all registers
are mapped to memory and radio states can directly generate interrupts
on the CPU.
The ATmega256RFR2 adds support for automatic retransmissions.
This has not been implemented yet.
Co-authored-by: Josua Arndt <jarndt@ias.rwth-aachen.de>