Saturday, November 7, 2015

Using Raspberry Pi with a VGA monitor

One of the most fantastic Raspberry Pi features is its ability to fit in a pocket. Whether you travel far away from home or just go for a short visit you can always have your whole workplace with you. You don't have to copy all the files to a pendrive and then look for a computer to work on, wondering if you bring back home a virus or find your secret files "backed up" on some peer-to-peer network. You don't have to carry a big case with heavy latop inside, either. All you need is to take your Pi out of your pocket and connect a mouse, a keyboard, and a TV.
But what if there is only one TV set, and suddenly everybody wants to watch TV or play the latest hit on a game console? Yes, you can use a good old (S)VGA monitor! It can be an interesting alternative to a big TV. A flat LCD monitor takes little space on desk and can be a useful way to recycle some old electronic stuff, instead of just throwing it away. You can make the world a better place by reusing an old monitor with Raspberry Pi!
I will show you how to connect Raspberry Pi running RISC OS to a regular computer monitor with VGA input, but most of the tips below should be useful for Linux users as well.

Although Raspberry Pi does not feature a VGA output, there are many adapters available, which can convert digital HDMI data stream into analogue signal required by a VGA monitor. However, you have to pay attention to two very important things.
First, it has to be an adapter with a special DAC (Digital to Analogue Converter) chip. Some adapters just pass the signal through, and they can only work with PCs or laptops which can transfer an analogue signal through HDMI port. Raspberry Pi is not designed to support such feature.
Second, search for an adapter with audio output (the most popular is mini-jack port). It's because the Pi settings required to make it work with a VGA display can make the built-in mini-jack audio output port inactive. It is possible to have video signal passed through the adapter, and sound through a built-in audio jack, but it can be very tricky and the settings described below don't work for everybody. Moreover, if an adapter has an analogue audio output, it indicates that it has some kind of a DAC chip onboard, and is not a simple pass-through.
Hint: adapters advertised to work with a smartphone (or tablet) in most cases also work with the Pi. Anyway, before you buy such adapter, read it's description carefully or, if in doubt, ask the seller whether it was tested with Raspberry Pi. If you are still unsure, you can buy PiView, which is guaranteed to work with all Raspberry Pi models, but it costs almost as much as the Pi itself. Personally, I use an adapter I bought quite cheap on eBay and it works very well.

Now for the software part. The Pi can now speak to the monitor through the adapter, but the monitor will not send back the information the Pi expects to receive, so we need add some information to the Pi's default GPU (Graphics Processing Unit) configuration. This configuration is stored on a boot partition of the RISC OS SD card in a file called CONFIG.TXT. To edit this file on a PC, simply put the card into a reader and after it mounts open CONFIG.TXT from the mounted partition in the editor of your choice. To edit it from RISC OS, open !StrongED, click on the SD card icon on the iconbar, then hold down Shift and double click on !Boot icon, then Loader and drag CONFIG/TXT to !StrongED icon on the iconbar. The reason I recommend !StrongED over !Edit is that the former one can handle PC end-of-line codes better (!Edit displays [0d] at the end of each line).
The contents of the config file for RISC OS should look more or less like this:
fake_vsync_isr=1
framebuffer_swap=0
gpu_mem=64
init_emmc_clock=100000000
kernel=RISCOS.IMG
This is a basic configuration needed to boot RISC OS on Raspberry Pi. It will look a bit different for Raspbian and other Linux distributions, but it doesn't matter now. The parameters are written in "key=value" form, each in separate line. You can find a detailed description of the configuration parameters at Embedded Linux Wiki.
What you should do is to add the following lines to the configuration file. You can place them right after the last parameter, before the first one, or somewhere in the middle:
hdmi_force_hotplug=1
hdmi_group=2
hdmi_mode=17
Those lines require some explanation. The "hdmi_force_hotplug=1" tells the Pi to use to use HDMI output even if no connection with an HDMI with display was detected. As I mentioned before, a VGA monitor can't communicate with Raspberry Pi through HDMI, it can only display a picture received via the adapter. Next option "hdmi_group=2" selects DMT (Display Monitor Timig) output mode, which is a VESA (Video Electronics Standard Association) standard for VGA monitors, and "hdmi_mode=17" sets screen resolution to 1024x768 with 70Hz refresh rate. You should tweak that parameter depending on your monitor display capabilities, using information found at RPiconfig in DMT video modes table.

Now for the sound. If you have a VGA adapter with audio output and want to use it, you should add the following lines to your configuration:
hdmi_drive=2
hdmi_force_eid_audio=1
The "hdmi_drive=2" option selects HDMI mode with sound (value "1" means "no sound") and "hdmi_force_eid_audio" makes the Pi skip detection of the display's audio capabilities. Without this option you may get some strange sound problems, like no audio signal after first boot, suddenly reappearing after reboot.

If you want to use the Pi's built-in audio jack instead (you can't have both outputs active at the same time!), you should add these lines in place of the previous two:
hdmi_ignore_edid_audio=1
disable_audio_dither=1
The first option tells the Pi that there is no digital audio device connected and forces it to use the analogue output, and "disable_audio_dither=1" reduces some nasty noise you can sometimes get (if you have very quiet sound output you may try try to remove this parameter from the config file).

And that's it, enjoy your new Raspberry Pi display!

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