The OBSERVE_TX register counts the number of packets that a transmitter has retransmitted or dropped.
This table is copied from the nRF24L01 product specification (page 55).
|PLOS_CNT||7:4||0000||R||Count lost packets. The counter is overflow protected to 15, and discontinues at max until reset. The counter is reset by writing to RF_CH.|
|ARC_CNT||3:0||0000||R||Count retransmitted packets. The counter is
reset when transmission of a new packet starts.
PLOS_CNT can be used along with a running count of transmission attempts to determine the packet loss rate.
The ARC_CNT value is used to determine when a packet is lost. When ARC_CNT equals the value of ARC set in the SETUP_RETR register, the packet being transmitted is considered lost, PLOS_CNT is incremented, and the MAX_RT interrupt fires (not necessarily in that order).