Protocol decoder:Nrf24l01

nrf24l01

Name	nRF24L01(+)
Description	2.4GHz transceiver chip
Status	supported
License	GPLv2+
Source code	decoders/nrf24l01
Input	spi
Output	nrf24l01
Probes	—
Optional probes	—

The nrf24l01 protocol decoder supports the protocol spoken by the Nordic Semiconductor nRF24L01 and nRF24L01+ 2.4GHz transceiver chips.

Hardware

Modules with these chips can be purchased fairly inexpensive from various online marketplaces. Most (all?) have an 8-pin header with the following pinout:

Pin	Function
1	GND
2	VCC
3	CE
4	CSN
5	SCK
6	MOSI
7	MISO
8	IRQ

The chip has two chip select pins, "CE" used to control the standby mode, and "CSN" used for SPI communication.

Protocol

The chip uses the standard SPI protocol and pins (CSN, SCK, MOSI, MISO), with the additional CE (used to control the RX/TX and standby modes) and IRQ (used to inform the SPI master about the completion of a packet reception/transmission) signals. If the number of available pins of the master are scarce, the CE pin can be tied to VCC and the IRQ pin can be left open. Polling over SPI can be used instead.

SPI commands can have variable length, the CSN signal has to stay low during the whole command, and then go high after the last byte. The first byte of a command defines the type of the command, the chip always outputs its internal status register at the beginning. The following bytes are dependent on the command type, can be register values to write into the chip or payload data to send, or empty bytes that are ignored if the command only reads the output of the chip.

Decoder

The nrf24l01 decoder stacks on top of the SPI decoder and decodes the commands to the chip and the responses of the chip, and also issues warnings for wrong/incomplete commands.

Some decoded commands in PulseView:

• 

  Decoded "Write Register" command.

• 

  Decoded payload that is going to be transmitted.

• 

  The payload on the receiver side.

sigrok-cli can be used to decode the capture in the following way:

$ sigrok-cli -i sigrok-dumps/spi/nrf24l01/nrf24l01-communication.sr \
             -P spi:cs=rpi_CSN:clk=rpi_CLK:mosi=rpi_MOSI:miso=rpi_MISO,nrf24l01
Cmd R_REGISTER "CONFIG"
Reg STATUS = "0E"
Reg CONFIG = "08"
Reg STATUS = "0E"
Cmd W_REGISTER: CONFIG = "08"
Reg STATUS = "0E"
Cmd W_REGISTER: RF_CH = "3E"
Reg STATUS = "0E"
Cmd W_REGISTER: RX_ADDR_P0 = "376774367E"
Reg STATUS = "0E"
...
Cmd R_RX_PAYLOAD
Reg STATUS = "40"
RX payload = "message #0"

It can be seen that the register values are hex encoded, while the payload data is, if possible, represented as ASCII characters.

If only the payload is of interest, the tx-data and rx-data annotation classes can be selected using the -A|--protocol-decoder-annotations option:

$ sigrok-cli -i sigrok-dumps/spi/nrf24l01/nrf24l01-communication.sr \
             -P spi:cs=uc_CSN:clk=uc_CLK:mosi=uc_MOSI:miso=uc_MISO,nrf24l01 \
             -A nrf24l01=tx-data
TX payload = "message #0"
TX payload = "message #1"
TX payload = "message #2"
TX payload = "message #3"
TX payload = "message #4"
TX payload = "message #5"
TX payload = "message #6"
TX payload = "message #7"
TX payload = "message #8"
TX payload = "message #9"

$ sigrok-cli -i sigrok-dumps/spi/nrf24l01/nrf24l01-communication.sr \
             -P spi:cs=rpi_CSN:clk=rpi_CLK:mosi=rpi_MOSI:miso=rpi_MISO,nrf24l01 \
             -A nrf24l01=rx-data
RX payload = "message #0"
RX payload = "message #1"
RX payload = "message #2"
RX payload = "message #3"
RX payload = "message #4"
RX payload = "message #5"

Warnings issued for erroneous commands:

$ sigrok-cli -i sigrok-dumps/spi/nrf24l01/nrf24l01-test-missing-bytes.sr \
             -P spi:cs=CS:clk=CLK:mosi=MOSI:miso=MISO,nrf24l01
Cmd R_REGISTER "CONFIG"
Reg STATUS = "00"
missing data bytes
Cmd W_TX_PAYLOAD_NOACK
Reg STATUS = "00"
missing data bytes

Resources

• Nordic Semiconductor nRF24L01 (datasheet)
• Nordic Semiconductor nRF24L01+ (datasheet)
• github.com: nrf24l01 (various nRF24L01(+) projects and examples)
• Travis Goodspeed: Promiscuity is the nRF24L01+'s Duty (using the nRF24L01 in a pseudo-promiscous mode)
• Cyber Explorer: Sniffing and decoding NRF24L01+ and Bluetooth LE packets for under $30 (contains information about the modulation used by the nRF24L01)