User Manual

Revision 1.0 Page 34 of 78

nRF24L01+ Product Specification

• Auto Retransmit Delay (ARD) has elapsed.

• No address match within 250µs (or 500µs in 250kbps mode).

• After received packet (CRC correct or not).

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 2Mbps data rate and 5 byte address; 15 byte is maximum ACK packet payload length for

ARD=250µs (reset value).

• For 1Mbps data rate and 5 byte address; 5 byte is maximum ACK packet payload length for

ARD=250µs (reset value).

ARD=500µs is long enough for any ACK payload length in 1 or 2Mbps mode.

• For 250kbps data rate and 5byte address the following values apply:

Table 18. Maximum ACK payload length for different retransmit delays at 250kbps

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.

ARD ACK packet size (in bytes)

1500µs All ACK payload sizes

1250µs <

1000µs <

750µs <

500µs Empty ACK with no payload