14.3.1.1.2 PHY Header (PHR)
The PHY header is a single octet following the SHR. The least significant seven bits denote the frame length of the following PSDU, while the most significant bit of that octet is reserved, and shall be set to zero for IEEE 802.15.4 compliant frames.
On reception, the PHR is returned as the first octet during Frame Buffer read access. While the IEEE 802.15.4-2006 standard declares bit seven of the PHR octet as being reserved, the AT86RF212B preserves this bit upon transmission and reception so it can be used to carry additional information within proprietary networks. Nevertheless, this bit is not considered to be a part of the frame length, so only frames between one and 127 octets are possible. For IEEE 802.15.4 compliant operation bit[7] has to be masked by software.
In transmit mode, the PHR needs to be supplied as the first octet during Frame Buffer write access.
In receive mode, the PHR (that is frame length greater than zero) is returned as the first octet during Frame Buffer read access and is signaled by an interrupt IRQ_2 (RX_START).