rf24-serial-docs

Notes about interfacing NRF24L01(+) with USB adapter

Introduction

NFT24L01(+) is a small and cheap 2.4GHz radio receiver/transmitter controllable via Serial Peripheral Interface and produced by Nordic semiconductor.

Table of contents

• Installation
• Configuration of the adapter
• Communication between two serials
• Communication between a serial and an MCU
  • Coding
• References

Installation

Mount your NRF24L01(+) on the adapter orienting the antenna in the opposite direction with respect the USB connector (see image below).

The adapter most probably ships a CH340T USB to serial converter. This chinese chip requires installation of drivers on Windows and Mac OS operating systems, while is natively supported by the Linux kernel.

You'll find very quickly the most updated drivers for your system on the Internet. Anyhow, here is a list of links to download the drivers from:

• Linux (generally not required)
• Windows
• Mac OS
• Android

You'll also need a serial terminal emulator, like PuTTY. Install one of your choice and configure it for connection at 9600/8-N-1.

Now you're ready for starting using your adapter 😉

Configuration of the adapter

The adapter allows to set up only a subset of configuration parameters supported by the NRF24L01. In particular it is not possible to set:

• the emission power (set to 0 dBm)
• the address length (set to 5 bytes -> 40 bits)
• the data length (32 bytes, 31 for user data)
• more than one destination address

The configuration can be done via AT commands on the serial terminal. A list of available AT commands follows:

• AT?: return the current configuration
• AT+BAUD=n: set the baud rate of the serial connection; n=1-6:
  • 1 -> 4800
  • 2 -> 9600 (default)
  • 3 -> 14400
  • 4 -> 19200
  • 5 -> 38400
  • 6 -> 115200
• AT+RATE=n: set the data link speed; n=1-3:
  • 1 -> 250Kbps
  • 2 -> 1Mbps
  • 3 -> 2Mbps (default)
• AT+RXA=0Xnn,0Xnn,0Xnn,0Xnn,0Xnn: set the local receiving address (default 0xff,0xff,0xff,0xff,0xff) in hexadecimal format
• AT+TXA=0Xnn,0Xnn,0Xnn,0Xnn,0Xnn: set the target address (default 0xff,0xff,0xff,0xff,0xff) in hexadecimal format
• AT+FREQ=2.abcG: set the operating frequency; abc can be any integer number between 400 and 525 (default 2.400G). NOTE: in other contextes this parameter is called channel; the correspondance is linear such that 400 and 525 correspond to channels 0 and 125, respectively (e.g.: 409 corresponds to channel 9). NOTE: to avoid interferences with WiFi use frequencies above 2.500GHz.
• AT+CRC=n: set the CRC length, in bits; n = 8/16 (default: 16 bit)

Once set, the configuration is retained by the adapter so you'll find it again even in case of power-off of the device.

NOTE: all AT commands are written using capital letters, without spaces and using the english typeset.

NOTE: don't be afraid if you see dumb chars on the serial terminal ad output. Probably they are UTF-16 chinese chars not well displayed by your locale.

Communication between two serials

Using two adapters is straight forward. You have to open a serial connections to each of your adapters and configure them such that the remote address of one is the local address of the other. All other communication parameters such as the data speed, the frequency and the CRC must coincide.

What you write to a serial terminal is immediately sent to the other.

Communication between a serial and an MCU

The microcontroller on the adapter uses 32-bytes-wide fixed-length messages. The first byte must contain the actual user data length so only 31 bytes are available to the user. Even if the message is shorter, 32 bytes must be used. If the data to transmit is longer than 31 bytes it must be split into several messages and rebuilt after the transmission. A 32-bytes message will always be sent and expected to be received by the adapter.

Thus, an MCU connected via SPI to its own NRF24L01(+) must be programmed fullfilling these constraints in order to correctly communicate with the adapter.

Schematic of a message:

+-------+-------+-------+-------+-------+-------+-------+
|       |       |       |       |       |       |       |
|   0   |   1   |   2   |  ...  |   N   |  ...  |   31  |
|       |       |       |       |       |       |       |
+-------+-------+-------+-------+-------+-------+-------+
 Length | User                          | Empty

Coding

Example code of message serialization, in C++:

char const msg[] = "Hello, World!";

char packet[32];

packet[0] = sizeof(msg);
memcpy(packet + 1, msg, sizeof(msg));

nrf24.send(packet); // replace with the function of your choice/need

References