PiWeather : How to communicate wirelessly between an Arduino and a Raspberry Pi

If you wonder what PiWeather is, check out the first article I wrote !

Raspberry Pi FTW !
Raspberry Pi FTW !

One of the key part of this project is to first determinate what technology to use to transmit the data and second how to encode the data to send. What I wanted was:

  • Reliability
  • Low power design
  • Good range (enough to cover a small house)
  • Ease of use
  • Something cheap
  • Something that could work simply on the Arduino and Raspberry Pi side

RF chip

A standard 433 MHz RX/TX couple

My first choice was spontaneously to pick 433MHz RX and TX as they are incredibly cheap and can reach a pretty long range if you associate them with the right antennas. The problem with these is that the very low-level side of the transmission. If you put the TX pin high, the RX pin will go high too, and that's it. You really have to do all the encoding by yourself, and if you want it to be really reliable, you must provide a way to ensure all your data were transmitted correctly (like a CRC).  I first thought I would have to cope with this and handle all this low-level part by myself on both the Pi and Arduino's sides, when I stumbled upon the one RF chip I needed: the 2.4GHz nRFL24L01+.

nRF24L01+
The nRF24L01+ used on my first prototype.

The nRF24L01+ was exactly what I needed: cheap, low power, easily connected to a Pi or an Arduino through SPI, built-in CRC, both RX and TX at the same time, multi pipe, and more!

It also is really easy to use through SPI: just give it the bytes you want to send (up to 32), and send it to the address of your choice. The same thing goes for the RX mode, it can even generate an IRQ when a message has come!

I used the library from Stanly Seow on both Arduino and Raspberry's side, and it works pretty good! One flaw of the Pi's implementation is that it doesn't use interrupts generated by the nRFL2401+, so it kind of stalls the Pi polling the chip. I modified it a bit to use interrupts from the great WiringPi library, allowing my program to sleep 99.9% of the time. I will get into these details in an other post, stay tuned 😉

Okay, so I know how data will go from my sensor units to the Pi. But what am I going to send?

Messages

So far, I've thought about sending the following data to the Pi from the sensor units:

  • Temperature
  • Pressure
  • Humidity
  • Wind speed
  • Wind direction
  • Rain metrics
  • Unit battery level, to detect low voltage

Also, my units needed some kind of address or unique ID so the Pi could easily recognize them, so I added an ID in the list of things I could possibly send. I could have decided of my own encoding format, but what if I change in the future the things I send and in which order, etc? You can see now that encoding the data in a forward compatible way is really not that simple!

I actually never thought about encoding myself the data 😀 It was just to show you that it is definitely not a piece of cake. I knew exactly what I needed to use : Google Protocol Buffers

From the Google website , "Protocol buffers are Google's language-neutral, platform-neutral, extensible mechanism for serializing structured data – think XML, but smaller, faster, and simpler". You can find libraries to use them in almost any available language, from a micro controller with 2kB of RAM running C code, to a server running PHP or Python.

So how does it work?

You first describe the messages you want to send in a file having the .proto extension:

message SensorData{
 required int32 id=1;
 optional bool binding= 2;
 optional float temperature = 3;
 optional int32 battery_level = 4;
 optional float pressure = 5;
 optional float humidity = 6;
 optional int32 type = 7;
 }

message is the keyword required to start defining a message. Then it looks a bit like a mix of a C structure and enum declaration. You define your fields and attribute them a unique ID (ex : the field temperature has the ID 3), and then add if your field is optional or required in the message. In this case, each of my sensor units will have to send its ID to the Pi during a transaction so the Pi know who it is talking to, but some sensors will retrieve temperatures, some wind speed, some pressure, so I made all the other fields optionals.
If I want in the future, to add a new data to my message SensorData, such as windspeed, I can just modify my the proto file like so:

message SensorData{
 required int32 id=1;
 optional bool binding= 2;
 optional float temperature = 3;
 optional int32 battery_level = 4;
 optional float pressure = 5;
 optional float humidity = 6;
 optional int32 type = 7;
 optional float wind_speed = 8;
 }

And you know what the great part about this is? It will be backward compatible with the existing sensor units using the "old" messages! So no need to re-flash the existing sensor units if I had optional fields in my messages, which is a very, very nice thing 🙂 I won't get more into details for GPB here, their website has tons of examples of how to use them!

To actually use these messages, you need to translate them into the language you are using. I chose to use NanoPb, which is a very good implementation of protocol buffers for embedded systems. It has a tiny footprint (less than 2kB!) and generates all the code you need to include in your project, so no need to link against a lib, which is always good news 😉

NanoPb takes the proto file and creates the associated C structures, encoding/decoding functions for my messages. The generated structure looks like this:


typedef struct _SensorData {
 int32_t id;
 bool has_binding;
 bool binding;
 bool has_temperature;
 float temperature;
 bool has_battery_level;
 int32_t battery_level;
 bool has_pressure;
 float pressure;
 bool has_humidity;
 float humidity;
 bool has_type;
 int32_t type;
} SensorData;

Let's say my sensor unit has both a temperature sensor, and a humidity sensor. If I want to send the data to the Pi, I can do like so:

 SensorData message;
 uint8_t sensorDataBuffer[32];/*Maximum size you can send with the nRF24L01*/
 pb_ostream_t stream;

 message->has_temperature=true;
 message->temperature=sensor.temp();
 message->has_humidity=true;
 message->humidity=sensor.humidity();
 message->id=42; /*The unit ID*/

/*Let's encode this with nanopb*/

 stream = pb_ostream_from_buffer(sensorDataBuffer, sizeof(sensorDataBuffer));
 pb_encode(&stream, SensorData_fields, message);

/*Now our encoded data is in the sensorDataBuffer, ready to be sent ! */
 nRF24.send(sensorDataBuffer);

It's as simple as that, and very powerful. I think protocol buffers are really the right way to go for size constrained embedded applications such as this one. But don't think they are only used for small applications, they can be so powerful that they are wildly used in different industries to send big loads of data, through nested messages for example 😉

So that's it, I introduced you the chip I use to make the communications, and the format I adopted for the messages my sensors send wirelessly. I think I will go back on this in a coming article where I will define exactly how the software I developed works.

Cheers 🙂

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