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I'm using +3.3V / GND / TX pins to power this 7-segment display with Serial Port connection. It works, but during booting time, the display displays random things, probably because it receives random values on GPIO TX...

I would like to set the 7-segment display ON only when my Python code starts, and OFF by default (during booting time).

Is there a +3.3V or +5V GPIO pin that is default to OFF (during boot), and that I can power ON with my code?

or

Is there a way to prevent GPIO TX to send random values during booting time?

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1 Answer 1

Reset to default
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By default the UART TX gpio is used during boot to log boot messages. After boot the UART (RX/TX gpios) is used to allow login.

The simplest way on Raspbian to disable boot messages and login is by using the raspi-config option.

sudo raspi-config
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  • Untested but you might also affect this by removing console=ttyAMA0 from /boot/cmdline.txt.
    – goldilocks ♦
    Commented Apr 4, 2015 at 22:04
  • I'm on Arch Linux @joan so I couldn't test raspi-config. What does it do? Does it do more or less do the same thing than remove console=ttyAMA0 in /boot/cmdline.txt? I tried this and it works (except that on plugging the Pi power, there might be some 1 or 2 random characters fired to TX... How to stop that?)
    – Basj
    Commented Apr 4, 2015 at 22:24
  • raspi-config edits /boot/cmdline.txt to remove the console field as above and edits /etc/inittab to comment out the final line which spawns a login process on the serial link. There is nothing you can do in software about the spurious power-up characters. Also when the port is opened a spurious start bit is generated. Again you can't do anything about that in software.
    – joan
    Commented Apr 4, 2015 at 22:33
  • @joan What's the equivalent of removing the final line of /etc/inittab with systemd / archlinux ?
    – Basj
    Commented Apr 4, 2015 at 22:48
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    I have no idea. Arch should have something similar. The line is T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
    – joan
    Commented Apr 4, 2015 at 22:51

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