User's Guide

Revision 1.0 Page 32 of 75
nRF24L01+ Preliminary Product Specification
• Auto Retransmit Delay (ARD) elapsed.
• No address match within 250µs.
• After received packet (CRC correct or not) if address match within 250µs.
nRF24L01+ asserts the TX_DS IRQ when the ACK packet is received.
nRF24L01+ enters standby-I mode if there is no more untransmitted data in the TX FIFO and the CE pin is
low. If the ACK packet is not received, nRF24L01+ goes back to TX mode after a delay defined by ARD
and retransmits the data. This continues until acknowledgment is received, or the maximum number of
retransmits is reached.
Two packet loss counters are incremented each time a packet is lost, ARC_CNT and PLOS_CNT in the
OBSERVE_TX register. The ARC_CNT counts the number of retransmissions for the current transaction.
You reset ARC_CNT by initiating a new transaction. The PLOS_CNT counts the total number of retrans-
missions since the last channel change. You reset PLOS_CNT by writing to the RF_CH register. It is possi-
ble to use the information in the OBSERVE_TX register to make an overall assessment of the channel
quality.
The ARD defines the time from the end of a transmitted packet to when a retransmit starts on the PTX.
ARD is set in SETUP_RETR register in steps of 250µs. A retransmit is made if no ACK packet is received by
the PTX.
There is a restriction on the length of ARD when using ACK packets with payload. The ARD time must
never be shorter than the sum of the startup time and the time on-air for the ACK packet.
For 1Mbps data rate and 5 byte address; 5 byte is maximum ACK packet payload length for ARD=250µs
(reset value).
For 2Mbps data rate and 5 byte address; 15 byte is maximum ACK packet payload length for ARD=250µs
(reset value).
ARD=500µs is long enough for any ACK payload length.
As an alternative to Auto Retransmit it is possible to manually set the nRF24L01+ to retransmit a packet a
number of times. This is done by the REUSE_TX_PL command. The MCU must initiate each transmission
of the packet with a pulse on the CE pin when this command is used.