Accessing ttyS0 on Raspberry Pi 3 running Ubuntu Core

jenny.murphy · February 19, 2018, 10:53am

I am running Ubuntu Core on a Raspberry pi 3.
I would like to access a device using the serial port and believe this should be available at /dev/ttyS0. However there is no communication at the moment there.

From reading some other postings, it seems like the serial port is not enabled by default on the Raspberry Pi3 and in non Ubuntu Core systems the file /boot/config.txt should be edited.

Is there an equivalent step to be taken for Ubuntu Core to get this working? I don’t seem to have any gadget snaps for ttyS0, however I am running in development mode.

dhoomakethu · February 19, 2018, 12:09pm

You might be interested in this discussion Gadget providing serial-port slot

ogra · February 19, 2018, 12:28pm

Well, this looks more like a general issue with config.txt the interface would surely be helpful too but by default the uart is disabled in the pi bootloader …

The problem witth enabling it and thus enforcing the core speed of the GPU default is described at:

… "The Linux console can be re-enabled by adding enable_uart=1 to config.txt. This also fixes the core_freq to 250Mhz (unless force_turbo is set, when it will fixed to 400Mhz), which means that the UART baud rate stays consistent. " …

The config.txt file is writable so it can be manually edited on the installed image as a workaround nd @jenny.murphy can just add the enable_uart=1 line there.

To implement it properly the option should be added to:

github.com

snapcore/snapd/blob/master/overlord/configstate/configcore/picfg.go

// -*- Mode: Go; indent-tabs-mode: t -*-

/*
 * Copyright (C) 2017 Canonical Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package configcore

This file has been truncated. show original

… then it can be set via

snap set  core pi-config.enable_uart=1

or via a respective defaults: entry in a custom gadget in the gadget.yaml …

please create a bug at:

so it can be added properly …

jenny.murphy · February 20, 2018, 12:29pm

The workaround for me involved adding

enable_uart=1 to /boot/uboot/config.txt

and

removing “console=ttyAMA0,115200” from the line in /boot/uboot/cmdline.txt

Rebooting then for the changes to take effect.

ogra · February 20, 2018, 12:32pm

try changing it to console=serial0,115200, iirc the bootloader will pick this up so you still get a serial console (in case you actually need one at least )

jenny.murphy · February 20, 2018, 12:54pm

Hi,
In cmdline.txt I have already console=tty0. Is this significant?
I don’t seem to have any /dev/serial0, /dev/serial1 mappings but I don’t need that and can work with /dev/ttyS0.
Thanks

ogra · February 20, 2018, 1:05pm

this is defining the graphical console (typically on HDMI on the pi) if you want any console output at all, you should keep it