USB Setup on Linux

By default, when you plug in an Mbed device to a Linux machine, its serial port (TTY) will get an arbitrary path (e.g. /dev/ttyACM0), and this path can even change depending on the order you plug things in. This can be really annoying, especially if you are working with multiple Mbed devices at the same time.

This page will show you how to configure an Mbed device to have a consistent TTY path on Linux, and also how to set the permissions so that it can be accessed without root.

Setting up the TTY

First, you need to figure out what tty device corresponds to the Mbed board. It will generally be one of the /dev/ttyACMx devices, which you can list by running ls -l /dev/ttyACM*. Generally the easiest way to identify which is by running sudo dmesg -w in a console, then plugging in the device and looking at the prints produced by the kernel. (If the device has multiple TTYs, see "Devices with Multiple TTYs" below)
Then, once you have found it, get the serial number of the Mbed device from its tty:
```
udevadm info -a -n /dev/ttyACMx | grep '{serial}' | head -n1
```
(if this command does not print anything, see "Devices Without a Serial Number" below)
Create a file /etc/udev/rules.d/99-usb-serial.rules with contents like:
```
SUBSYSTEM=="tty", ATTRS{serial}=="<serial number>", MODE:="0666", SYMLINK+="tty<mbed target>", GROUP="plugdev"
```
where <mbed board> and <serial number> are replaced by the mbed target name and the serial number found above.

For example, for my NUCLEO_F429ZI board, it looks like this:
```
SUBSYSTEM=="tty", ATTRS{serial}=="066CFF343537424257254941", MODE:="0666", SYMLINK+="ttyNUCLEO_F429ZI", GROUP="plugdev"
```
Note: If you have multiple of the same Mbed board, make sure to give them different symlink names, e.g. ttyNUCLEO_F429ZI_1 and ttyNUCLEO_F429ZI_2.
If you have not done this before, add your username to the plugdev group:
```
$ sudo usermod -a -G plugdev $(whoami)
```
You will need to log out and log in for this change to take effect.

Reload udev:

$ sudo udevadm control --reload-rules
$ sudo udevadm trigger

You should now see your mbed device at the desired TTY path.
To connect to it, install minicom, then run minicom -D /dev/tty<mbed target> -b 115200 (or substitute a different baud rate if your code is not configured for 115200 baud)

Devices with Multiple TTYs

A few Mbed targets (including NRF52840_DK) have multiple TTYs from a single USB device, so the above udev rule will not work. You need to add an additional segment, DEVPATH=="*:1.0*", to select only the first TTY.

For example, for my NRF52840_DK, my udev rule looks like

SUBSYSTEM=="tty", ATTRS{serial}=="001050278063", MODE:="0666", DEVPATH=="*:1.0*", SYMLINK+="ttyNRF52840_DK", GROUP="plugdev"

Devices Without a Serial Number

A few Mbed targets (including all SparkFun Artemis boards and some older Nuvoton ones) do not have a unique serial number, so the above way of writing the udev rule will not work. Instead, you must make the rule based on the vendor and product IDs of the device, as printed by lsusb. For example, for SFE_ARTEMIS, one might do:

SUBSYSTEM=="tty", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", MODE:="0666", SYMLINK+="ttySFE_ARTEMIS", GROUP="plugdev

Note that this does not work if you have multiple of serial-number-less devices connected to a single computer. There is no way to repeatably assign the TTY name in that situation.

Setting Up Udev Rules for Programming

If your device has a debugger onboard, you may also need to install udev rules to allow that debugger to be accessed by tools like STM32Cube, OpenOCD, and PyOCD.

The easiest way to do that is to download this config file here and copy it to /etc/udev/rules.d. Then, reload udev as above.