C-UART Interface Example

This is a simple MAVLink to UART interface example for *nix systems that can allow communication between Pixhawk and an offboard computer.

This example will recieve one MAVlink message and send one MAVlink message.

Building

$ cd c_uart_interface_example/build/
$ make

This example was developed in Eclipse, and a .project file is available in this repository.

Harware Setup

Connect the USB programming cable to your Pixhawk.

If you want to be able to interact with this example in Pixhawk's NuttX shell, you'll need a Telemetry Radio or an FTDI developer's cable. See the Exploration section below for more detail.

Execution

1. Login to Pixhawk's shell

---

screen /dev/ttyACM0 57600 8N1
<press enter>

You have to pick a port name, if the above example doesn't work, try searching for it with

$ ls /dev/ttyACM*
$ ls /dev/ttyUSB*

2. Start a mavlink session on Pixhawk's USB port

---

nsh> mavlink start -d /dev/ttyACM0

Pixhawk will start dumping machine data to the shell.

Exit screen with the key sequence: Ctrl+A , K, Y

3. Run the Example Executable

---

$ cd c_uart_interface_example/build
$ ./c_uart_interface_example -d /dev/ttyACM0

Here's an example output

OPEN PORT
Connected to /dev/ttyACM0 with 57600 baud, 8 data bits, no parity, 1 stop bit (8N1)

READ MAVLINK
Got message HIGHRES_IMU (spec: https://pixhawk.ethz.ch/mavlink/#HIGHRES_IMU)
	 time: 22146030
	 acc  (NED):	 0.452423	-0.052777	-9.760157 (m/s^2)
	 gyro (NED):	-0.004391	-0.002016	-0.004369 (rad/s)
	 mag  (NED):	-0.215094	 0.394682	 1.033930 (Ga)
	 baro: 	 1018.009949 (mBar)
	 altitude: 	 -39.547714 (m)
	 temperature: 	 43.639999 C

SEND MAVLINK
Sent buffer of length 61

CLOSE PORT
Port closed

Exploration

There are a few things to explore past this example.

First you can connect via a Telemetry Radio on Telem 1 or 2, or via an FTDI on Telem 2 or Serial 4 (https://pixhawk.org/dev/wiring). Note: Serial 5's receive pin is occupied by a second NuttX shell and can't be used to receive data without reconfiguration.

With this you'll be able to start a second port for communcation, and leave the USB port available for viewing prints in the NuttX shell.

For steps 2 and 3 from the above tutorial, you'll use a different port. On the off-board computer side, the port might now be /dev/ttyUSB0. On the Pixhawk side, here the port mappings are in the table below.

PX4 UART	NuttX UART
Telem 1	/dev/ttyS1
Telem 2	/dev/ttyS2
Serial 4	/dev/ttyS6

Now add a print statement in the Pixhawk Firmware to see received messages. Build and upload this to Pixhawk.

[Firmware/src/modules/mavlink/mavlink_receiver.cpp : line 1351]
/* if read failed, this loop won't execute */
for (ssize_t i = 0; i < nread; i++) {
	if (mavlink_parse_char(_mavlink->get_channel(), buf[i], &msg, &status)) {

		/* --- REPORT HANDLING OF MESSAGE --- */
		printf("\n");
		printf("HANDLE MESSAGE\n");
		printf("MSGID:%i\n",msg.msgid);

		/* handle generic messages and commands */
		handle_message(&msg);

Screen into the NuttX shell and start a mavlink session like in the example above.

On the off-board side, in another terminal run the c_uart_interface_example executable. You should see output in the NuttX shell similar to this:

HANDLE MESSAGE
MSGID:84

Past this, you can:

• Modify the received message data type
• Modify the sent message data type