Wall Clock
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A Raspberry Pi Digital Wall Clock

This simple project was to replace a radio clock working from MSF with a clock based on NTP.  It just so happened that an older 10.2-inch LCD TV became free, and I wanted to have a go at programming the Raspberry Pi and learn just a little (not too much) more about Linux.

The project involved setting up the Raspberry Pi to display correctly on the TV (I converted a Windows Testcard program  to run on Linux to check the display size), using the Free Pascal and Lazarus IDE to compile a suitable wall-clock program, learning how to auto-login on the Raspberry Pi, and how to start a program automatically on the desktop (i.e. using the GUI).

I've provided the source and binary files in this Zip archive, so you can either run the program as-is, or modify it to suit your own preferences.

For this note, I assume you have already configured your Raspberry Pi to be up and running, network connected, and that it's running NTP correctly.  My Raspberry Pi runs over Wi-Fi.  Although it's not required in this application, if you want to be very precise with your Raspberry Pi's  timing you can configure it as a stratum-1 server.  The average CPU taken by the program is about 3%, so it shouldn't make the Raspberry Pi get too hot!  Although I've shown the display as wall-mounted above, you may be able to use a VESA adapter to mount the Raspberry Pi card on a monitor, and perhaps even use the 5V USB port is available to power the computer and Wi-Fi dongle.

Setting the display size

As I was using an analog TV, and not a digital computer monitor, I needed to set the scanning parameters to get the display to just fill the screen.  This is a rather fiddly business as a reboot is required after each change, so progress is not very fast!  I used the testcard program from the Zip archive to check the screen coverage.  I ended up with the following changes to /boot/config.txt.  Your values will be different.  For more options see http://elinux.org/RPi_config.txt

sdtv_mode=2		# PAL system
sdtv_aspect=3		# set 16:9 aspect ratio
overscan_left=-5	# scan margin adjustments

With those settings, the available display area is 928 x 552 pixels, an aspect ratio of 1.68:1, compared to the theoretical 1.78.  This on a NextBase 10.2-inch 800 x 480 pixel analogue TV.

Running a program automatically

Originally, I had intended to write a screen-saver, so that the digital clock would be displayed when the display was otherwise not in use.  However, I could not find a digital clock screensaver which suited me, the examples in C which I could find were too complex to modify quickly, and I could find no examples in my preferred object Pascal language.  Following helpful discussions with Stefan Enziger, Jasen Betts and others on the comp.sys.raspberry-pi Usenet newsgroup, I decided that making a program start automatically from the graphical desktop would be easier (like adding a program to the Startup group in Windows).

Auto Login at boot time

The next step was to make the Raspberry Pi automatically log me in as a user (at the main desktop) when the Pi was booted.  There are instructions here.  Condensed:

Auto Login: Edit /etc/inittab
Scroll down to: 1:2345:respawn:/sbin/getty 115200 tty1
Change to: #1:2345:respawn:/sbin/getty 115200 tty1
Under that line add: 1:2345:respawn:/bin/login -f pi tty1 </dev/tty1 >/dev/tty1 2>&1
Save the file and exit
[Optional] Install xscreensaver and set one.
[Required] Set to boot to GUI mode with: sudo raspi-config

To check this has worked, simply reboot the Raspberry Pi and watch for the desktop to appear.  You can go a little further and install a package of screensavers, set one with, say, a 1 minute timeout, and check that the screen-saver activates at one minute past desktop start time.  Remember to set the screen-saver back to None for the wall-clock program, though!  To add screen-savers to your Raspberry Pi

apt-get install xscreensaver

and you should now find a new item on the desktop Preferences menu => Screensaver.

Update for Rasbian Jessie

Raspbian Jessie no longer uses the /etc/inittab method of starting programs.  This page:


has some tips, and recommends searching Google with:

raspbian jessie auto login

Running a program when the desktop appears

This is a little more complex, and involves creating a file to run the program of your choice.  First, if you haven't already compiled the program DigitalClock, you will want to extract it from the Zip archive and make it executable:

chmod +x DigitalClock

On my Raspberry Pi, I had created a directory lazarus from the home directory (which therefore had the path /home/pi/lazarus), and placed the DigitalClock program and its source code there.  With the File Manager accessory, you should be able to run the DigitalClock program, and check that you see something resembling the photo at the start of this note.  Move the mouse and click anywhere to end the program.

Creating the required autostart file

Unfortunately, it's not as simple as on Windows - where you can create a shortcut to the program in a Startup directory.  You need to create a file named DigitalClock.desktop, in the directory .config/autostart under your home directory.  By default, that directory does not exist on a new Raspberry Pi installation, so you could use the following commands from http://www.raspberrypi.org/phpBB3/viewtopic.php?t=18968 to create it and make a file to configure the DigitalClock startup:

mkdir ~/.config/autostart
touch ~/.config/autostart/DigitalClock.desktop

You can then edit the contents of the DigitalClock.desktop file - only a few lines are required:

[Desktop Entry]

The Exec= should point to the full path of the DigitalClock executable file.  If you want to know more about entries in the .desktop file, look here.

Compiling the DigitalClock software

If you want to compile the software, for example should you want to make some changes to suit your own needs, you will need to install the FreePascal compiler (fpc), and the Lazarus development system.  It's not as speedy as Embarcadero's Delphi on a Windows PC, but developing a small program like this is perfectly feasible on the Raspberry Pi.  As fpc/Lazarus is also available for Windows, you could develop and test on Windows (or the Apple Mac), and then transfer the source for the .pas and .lfm files to the Raspberry Pi.  I ended up creating skeleton applications on the Raspberry Pi, adding event handlers to the form, and cutting and pasting the code for the event handlers from my Windows version to the Raspberry Pi.  For these short programs, typing the code in by hand is straight-forward enough.

Installing Lazarus

I followed the discussion here, which led to this most helpful page, getting a more recent version of Lazarus than that in the default Raspberry Pi Raspian OS.  [Is this still the case?]  Thanks to Jack for that page and package.  Once installed, Lazarus should appear as a new entry on the desktop Programming menu.  One hint, to keep the size of the executable down, is to disable the debug information in the Project, Project Options ..., Compiler Options, Linking menu, uncheck Generate debugging info.  Executable size is about 2 MB without the debug info, 15+ MB with!

For help with Lazarus, try this Usenet group:


which is also available through a mailing list.

How to test while developing

My procedure has been this: I log in via an X-Windows server for Windows (ming) using PuTTY for authentication and can thus access the Lazarus IDE, to edit and compile and run the program.  This produces output on the X-Windows server running on the PC, completely separately from any display running on the TV.

I can also be logged in remotely from my PC via SSH to a console session, and run the graphics program to produce output on the TV screen by telling the SSH session where to display its output:

export DISPLAY=:0

Thanks to Stefan Enziger for that nugget!  Of course, you can't overwrite a program which is being actively run by the Raspberry Pi on the TV, so for that reason you may want to have a separate copy of the program which you auto-start.  I was slightly surprised to find that even though I had the DigitalClock running via autostart, I could still run the TestCard program to confirm the resolution figures I mentioned above.


V1.0.0.0   2013-Aug-12 First version, click on the form to exit the clock.
V1.0.1.2 2013-Aug-14 Make mouse cursor disappear again a few seconds after any mouse movement stops.
V1.0.2.3 2013-Aug-23 Allow mouse wheel to control the brightness - you may need to move the mouse first.
V1.0.3.4 2013-Aug-27 Save changed brightness settings across restarts.
V1.0.5.7 2014-Apr-20 Wall clock allowing either UTC or local time to be used.  For UTC, you should change the line in the DigitalClock.ini file to UTC=1 and restart the program.  For local time reset it to "UTC=0" and restart.  The time characters are slightly smaller in UTC mode, to allow for the string "utc" after the time.  Yes, correctly-speaking it should be "UTC" but that takes up even more space!
(2014-Apr-21 - corrected Zip file)
V1.1.0.8 2016-Dec-06 Choice of text colour - for black text, you should change the line in the DigitalClock.ini file to Inverse=0 and restart the program.  For white text reset it to "Inverse=1" and restart.

The downloads include full source code.

Future enhancements

At the moment, the screen brightness is fixed.  It would be nice to be able to adjust it live.  As well, perhaps for astronomy enthusiasts, a darker red text on a very dark grey background would be useful.  You have the working source code, so .... it's over to you!

One minor tweak concerns the Free Pascal "Now" function.  I noticed that over the clock change in the Autumn my wall clock continued to display summer time rather than winter time - my guess is that the Now function doesn't check for a change in the minutes offset very frequently, if at all since it was first called.  Restarting the program fixed the problem, but it would be nice to have the fix in the code.

What else have I done with the Raspberry Pi?

  • Receiving ADS-B signals on 1.09 GHz with a cheap TV dongle and feeding the results to Plane Plotter over a Wi-Fi link.  This allows you to put the receiver right up close to the antenna, avoiding an expensive (or lossy) piece of cable, and avoiding the need for a cable run at all.  Details are here.  It's not much more than following someone else's instructions, but at least I can vouch for every command on that page, and perhaps you will find something useful there.
  • A stratum-1 NTP server with microsecond-level offsets and a power consumption of less than 4 watts.  the page has information about interfacing a GPS receiver to the Raspberry Pi, choosing between various receiver options, and how to compile NTP from the source.  I hope you will find the page interesting, and perhaps build a GPS-locked stratum-1 NTP server yourself.


My knowledge of Linux is very limited, but at least I am learning a little due to my purchase of the Raspberry Pi.  This project would have been impossible without the patient help of folks on various mailing lists and Usenet newsgroups, to whom I am very grateful.  Offering this Web page and my software is my way of actively saying many thanks!

Copyright © David Taylor, Edinburgh   Last modified: 2016 Dec 06 at 18:38