The first internet protocol we will have a look at is UDP, or the User Datagram Protocol.

UDP is a very simple protocol on top of IP that basically only adds a notion of ports to IP packets so that multiple applications can be addressed on a single host.

First we will be receiving UDP packets on the microcontroller boards. For this to work replace the content of the current application.lua with the following:

function on_udp_packet(ip, data, port)
   local stripped= data:gsub("\n", "")

   print("Received: "..stripped)
end

function setup_udp()
   local socket = net.createUDPSocket()

   socket:on("receive", on_udp_packet)
   socket:listen(5000)

   print("listening on port 5000")
end

setup_udp()

upload it, connect picocom and reset the board.

If everything went well the output should look something like the following:

[user@computer ~]$ picocom -b 115200 /dev/ttyUSB0
…
init: got IP address 192.168.94.158 via DHCP
init: startup will continue in 3s
init: handing over to application
listening on port 5000

Meaning, in this case, that the application is listening on 192.168.94.158:5000 for UDP packets.

To send packets to the server we will use the Netcat commandline utility.

To make netcat send UDP packets to your microcontroller it has to be started like the following:

[user@computer ~]$ nc -u 192.168.94.158 5000

Hint: The IP address and port you use in the command have to match the IP and port you see in picocom.

Hint: Open a new terminal emulator so that you can have picocom and netcat running at the same time.

Now whenever you enter text into the terminal running netcat and hit enter a UDP packet is sent to the microcontroller.

In picocom you should see output like the following:

[user@computer ~]$ picocom -b 115200 /dev/ttyUSB0
…
listening on port 5000
Received: Hello World from Netcat
Received: Test

Task: Adapt the application.lua to turn on an LED when the word “on” is received and off when the word “off” is received.