NRF24L01P-R7 datasheet(33/78 Pages) NORDIC

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nRF24L01+ Product Specification

7.4

Automatic packet transaction handling

Enhanced ShockBurst™ has two functions for automatic packet transaction handling; auto acknowledge-

ment and auto re-transmit.

7.4.1

Auto acknowledgement

Auto acknowledgment is a function that automatically transmits an ACK packet to the PTX after it has

received and validated a packet. The auto acknowledgement function reduces the load of the system MCU

and can remove the need for dedicated SPI hardware. This also reduces cost and average current con-

sumption. The Auto Acknowledgement feature is enabled by setting the EN_AA register.

Note: If the received packet has the NO_ACK flag set, auto acknowledgement is not executed.

An ACK packet can contain an optional payload from PRX to PTX. In order to use this feature, the

Dynamic Payload Length (DPL) feature must be enabled. The MCU on the PRX side has to upload the

payload by clocking it into the TX FIFO by using the W_ACK_PAYLOAD command. The payload is pending

in the TX FIFO (PRX) until a new packet is received from the PTX. nRF24L01+ can have three ACK packet

payloads pending in the TX FIFO (PRX) at the same time.

Figure 9. TX FIFO (PRX) with pending payloads

Figure 9. shows how the TX FIFO (PRX) is operated when handling pending ACK packet payloads. From

the MCU the payload is clocked in with the W_ACK_PAYLOAD command. The address decoder and buffer

controller ensure that the payload is stored in a vacant slot in the TX FIFO (PRX). When a packet is

received, the address decoder and buffer controller are notified with the PTX address. This ensures that

the right payload is presented to the ACK generator.

If the TX FIFO (PRX) contains more than one payload to a PTX, payloads are handled using the first in –

first out principle. The TX FIFO (PRX) is blocked if all pending payloads are addressed to a PTX where the

link is lost. In this case, the MCU can flush the TX FIFO (PRX) by using the FLUSH_TX command.

In order to enable Auto Acknowledgement with payload the EN_ACK_PAY bit in the FEATURE register

must be set.

7.4.2

Auto Retransmission (ART)

The auto retransmission is a function that retransmits a packet if an ACK packet is not received. It is used

in an auto acknowledgement system on the PTX. When a packet is not acknowledged, you can set the

number of times it is allowed to retransmit by setting the ARC bits in the SETUP_RETR register. PTX enters

RX mode and waits a short period for an ACK packet each time a packet is transmitted. The time period

the PTX is in RX mode is based on the following conditions:

TX FIFO

Payload 1

Payload 2

Payload 3

Address decoder and buffer controller

SPI

Module

ACK

generator

RX Pipe

address

TX Pipe

address

From

MCU

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