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A library for the RFM69 radio module.
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README.md

plainRFM69

This is a library for HopeRF's RFM69 radio series. The main goal of this library is to use the hardware as efficiently as possible. With this goal in mind, the used approach is different from that of the already existing libraries for this radio module.

Approach

The library is composed of two parts, the first; bareRFM69 provides a bare bones interface to the hardware. It does nothing by itself and only provides methods to interact with the hardware. Most of the configuration parameters are listed in the header file. Constants are used for easy configuration and to prevent having to look up parameters in the datasheet. It uses the the new SPI transaction system.

The second part; plainRFM69, provides the user with more relevant functions for sending messages. It allows for easy sending and receiving of packets. It contains an internal buffer for received messages. Both receiving and transmitting packets is done using the RFM69's AutoMode system. Which means the radio spends as little time as possible in the transmitter phase and that received packets can be retrieved from the module efficiently. This also allows the send methods to be non-blocking, as it is unnecessary to wait until the radio completes transmission.

Internal Workings

The plainRFM69 class provides several methods to configure the radio module with its recommended parameters. The module is used in packet mode. Variable length and address filtering is supported.

The AutoMode system (described on page 42 of the RFM69CW datasheet) allows the radio to change its mode based on a start condition, it then enters the Intermediate Mode until an exit condition is met.

Receiving

By default, the radio is listening for packets. The AutoMode is configured such that the radio goes to standby when a packet is received. This means that the radio waits until that packet is retrieved by the microcontroller before it starts listening for packets again. Retrieval of this packet is done with the poll() method, which writes the packet to the internal buffer of plainRFM69. Once the packet had been read, the radio automatically returns to listening to packets.

Sending

When a packet is to be transmitted, the radio is placed into standby mode. The AutoMode is configured such that the radio automatically switches to transmitter mode when there are bytes in the FIFO. It leaves transmission mode after the packet is completely sent. This ensures that the transmitter is enabled exactly for the time necessary to sent the packet. The next time poll() is called, the radio is configured for receiving packets with the AutoMode, as described in the previous paragraph.

Interrupt

If the radio has received a packet, it waits in the Intermediate Mode until the packet is read from the FIFO. When a packet is transmitted, the radio is in the Intermediate Mode until the packet is completely sent. The radio module can represent whether it is in the Intermediate State on a digital IO pin (DIO2). In a slightly different way the same can be achieved using the DIO0 pin from the radio module, see the PingPongHPdio0 example on how to use that pin.

The microcontroller can attach an interrupt to this pin and call the poll() method on every change. This means that if a packet is received, the intermediate state is entered, DIO2 will switch from LOW to HIGH, resulting in an interrupt trigger on the microcontroller, this calls poll() which reads the packet from the FIFO, after this the radio automatically returns to receiving state. The same method is possible with transmissions, where the poll() switches the radio back to receiver mode after the transmission is complete. This means that the microcontroller does not have to wait until the transmission is complete, as this is also managed by the same interrupt and subsequent poll() call. So the method to send a packet does not block until the transmission is complete.

Testing & Performance

The library was developed and tested on two Teensy 3.1's with RFM69CW's attached. The pins used are described in the examples. The library is tested with Arduino 1.0.6, 1.8.5. The DIO0 example was developed using AdaFruit Feather M0 with the RFM69HCW radio module. Arduino and Moteino are also known to work.

All tests were performed with the radio modules located next to each other. The measured results are of course dependent on the parameters chosen, the values from the examples show below use a GFSK modulation at 300000 bps.

Maximum_Speed

Sends 64 byte packets from the sender to the receiver. For every message sent a counter is incremented. This counter is repeated in the message to create the payload. At the receiver the payload is checked and the counter is used to check whether there were missed packets.

Total packets: 101600
Packetloss count: 1
Per second: 416

This example can also be used to verify that the internal buffer is working. For example when a delay is introduced in the loop of the receiver.

Maximum_PingPong

Sends 64 byte packets from the sender to the receiver, which echoes the packet, the sender waits for this echo and checks the counter and message payload.

Successful pingpongs: 123468
Incorrect pingpongs: 0
Timeout pingpongs: 0
Successful / second: 188
Average ping (uSec): 5313

Buffering

The internal buffer of bareRFM69 is demonstrated in the BusyMan example. In this example the interrupt mechanism is used and the receiving end is very busy in the loop, this results in several messages queueing up in the internal buffer.

We are busy, doing important things!
Oh look, there are packets!
Packet (4): 2035
Packet (4): 2036
Packet (4): 2037
Packet (4): 2038
Packet (4): 2039
Packet (4): 2040
We had 6 packets in the buffer.

Usage

As said, this library was created to be efficient. The goal was not really to be user friendly. The examples were tested and are working without any known bugs. Feel free to use it, but if it breaks you get to keep both parts, if you have a fix for the problem feel free to share it.

The examples should provide pointers on how to use this, I suggest looking at Minimal and MinimalInterrupt, where the latter should be the most minimal implementation to build on. The internal buffering is shown in the BusyMan example.

The header files for both bareRFM69 and plainRFM69 provide explanation of the methods. If using a high power module, be sure to call setHighPowerModule() to enable the high power functionality to be used.

Other libraries that might interest you are Radiohead or LowPowerLabs' RFM69 which have seen extensive testing.

The PCB's I designed to connect the radio modules to the Teensy can be found in extras/hardware/.

License

MIT License, see LICENSE.md.

Copyright (c) 2014 Ivor Wanders

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