Protocol decoder:nrf24l01

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nrf24l01
NRF24L01 plus module.jpg
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:

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

Clones

There exists some chips that use the same (some with some extensions) SPI protocol:

Contains three additional registers (DEMOD_CAL, RF_CAL, BB_CAL), otherwise identical.

From a quick look at the datasheet the protocol used by this chip seems to be the same as the one of the nRF24L01.

  • Beken Corporation BK2423

From a quick look at the datasheet the protocol used by this chip seems to be the same as the one of the nRF24L01. (The datasheet can be found through search engines, but no mention of this chip on the official Beken homepage.)

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

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