Error on changing permissions in serial port

Question

I have been trying to find a solution for 2 days but I wasn't able to find one. I want to change the permission of the /dev/ttyAMA0 but seems to be impossible. I tried with sudo chmod 777 /deb/ttyAMA0 but at the following minute the port recovered old permissions for read, write and execute. The next thing that I tried was writing an udev rule with the following code: KERNEL=="ttyAMA0", MODE="0777, but nothing end with success.

I want to read information from an antenna that is connected on the serial port ttyAMA0, the problem is that I am not able to read the information unless I will give read permissions.

When I try to sudo adduser $USER tty the output is the following:The user pi' is already a member of tty'. Moreover, when I execute a script to access the /dev/ttyAMA0 the result is:

Permission denied: '/dev/ttyAMA0'.

I have these permissions on ttyAMA0 port:

crw--w---- 1 root tty 204, 64 Jul 5 11:15 ttyAMA0

the output of lsof /dev/ttyAMA0 is the following:

agetty 800 root 0u CHR 204,64 0t0 1157 /dev/ttyAMA0
agetty 800 root 1u CHR 204,64 0t0 1157 /dev/ttyAMA0
agetty 800 root 2u CHR 204,64 0t0 1157 /dev/ttyAMA0

I used this command to kill the process:

sudo systemctl stop serial-getty@ttyAMA0.service

Now I am able to change the permissions without any automatic change in permissions each minute. The problem now is that each time I reboot the Raspberry the permissions change to the default configuration.

I switched the serial ports ttyAMA0 and ttyS0 to get a better control of the Baudrate, that is the reason why the ttyAMA0 port has crw--w---- instead of crw-rw----.

I don't know what else I can do.

Answer 1

One plausible cause of your issue is that /dev/ttyAMA0 is used as a boot console in /boot/cmdline.txt. You should remove the console=/dev/xxx,115200 if you intend to use the serial port as a general-purpose device and not as a boot console.

Answer 2

Please, don't touch the permissions, that's not a good way to get access.

Instead, add yourself to the group tty which owns the device and has read/write access:

$ ls -l /dev/ttyAMA0 
crw-rw---- 1 root tty 204, 64 Jun 20  2016 /dev/ttyAMA0

You may easily do that using:

sudo adduser $USER tty

$USER is the name you used to log in with (echo $USER on command line will show your name) so you can use that command 'as is' and do not need to change anything.

Answer 3

The permissions are changing because another process is accessing the serial link.

You need to identify and deactivate that process (ls -l /dev/ttyAMA0 will probably show the group).

Once that process is deactivated the permissions will be correctly set by Linux without you having to do anything else.