Category Archives: Raspberry Pi

Embedded, Linux, Raspberry Pi

Raspberry Pi with HDMI – VGA converter

I have an early 2012 version of Raspberry Pi (generous gift from the Fedora Project) which sometimes tends to give some problem when a lot of current is being sourced from the USB. In addition to current issues with USB as well as HDMI, I faced a basic display problem when I used my HDMI-VGA converter for display which is not powered externally.

First things first, the converter won’t work directly and its almost sure that you will have to make changes to the config.txt (which holds the boot time specifications and is the key file to tell the Pi during boot time about any configurations that it should take care of) Here is what my config.txt looks like

arm_freq=800
force_turbo=1
gpu_mem=128
disable_overscan=1
start_file=start_x.elf
fixup_file=fixup_x.dat
hdmi_force_hotplug=1
hdmi_group=2
hdmi_mode=47
hdmi_drive=2

The values will obviously be different for you according to your Pi and display configuration. Have a look at http://elinux.org/RPiconfig for a very detailed explaination. Some things of interest to us here are hdmi_force_hotplug=1 just pretends that a HDMI device is always attached. The hdmi_group specifies the HDMI type whether its a DMT type output (mostly used in computer monitors) or if its CEA (which is used for TV monitors). The value 2 is for DMT.  The next is hdmi_mode which is very monitor specific. For example 47 corresponds to a monitor supporting a resolution of 1440X900 at 60Hz. You can check the wiki and find the mode for your monitor.  The hdmi_drive just chooses between HDMI and DVI output modes. You can save this as config.txt in your boot partition and ho! the converter works.

But hold on! As expected, it  the other devices connected also draw some amount of current from the USB ports which because of the infamous polyfuses between the USB supply line and ports limit the current to around 140mA. This is annoying as I don’t even know the specifications of my converter (its a cheap one from Amazon which did not come with absolutely any documentation.) The only way is to open it and measure the current draw myself. Being too lazy for that, I found a lot of solutions on the web to get my power supply of pi straightened out and amazingly the simplest one worked for me (/me says ‘he he he!’ with an evil grin) Here’s what you can try (in order) –

  • Try a better power supply – Use the ones which have ratings for 1A and above The usually are faithful. Some phone chargers will work and mostly the USB supply from laptops is not sufficient. I use my Samsung phone’s charger which has a rating of 5V, 850mA It works until I use more devices like Wi-Fi/Bluetooth dongle etc.
  • Buy the externally powered USB hub – If the simple thing above doesn’t work (usually when you are connecting more than two heavy devices – like a non-powered HDMI-VGA converter and a HDD) you need to find an externally powered USB hub. This is the safest bet (recommended) and usually not exciting if you are of the hacker kind. There is a list of good ones on http://elinux.org/RPi_VerifiedPeripherals#Powered_USB_Hubs
  • But this is pretty overkill for daily mundane tasks like a Wi-Fi dongle + some other USB peripheral. So the more interesting way is to short the polyfuses. I have heard that they no longer exist on the Model B rev 2.0 which seems good, but the main power still has polyfuses (shorting them is a bad idea I think) So for older models, you can get most of peripherals working when you short the polyfuses.
  • If you are more adventurous kind, you can get your Pi powered from an external ATX power supply which you can scavenge from some old PC (as I did) I am pretty much planning to power everything including the pi and a small fan from a Pentium 2 processor (slot package) from my ATX supply and I hope it will work.

I managed to get mpd running on my Pi and have setup my ownCloud on it so that I can use it as a small storage cloud. The mpd combined with my Droid MPD client on an Android phone has essentially made it my remote music station. I am waiting for my relay board to come so that I can maybe tweet-a-lightbulb at home sometime 🙂 I shall keep posting updated about small hacks and things I learned with the Pi.

Here’s an old post which I found interesting on polyfuses on Pi and from where I learnt some stuff about it – http://theiopage.blogspot.ca/2012/06/increasing-raspberry-pis-usb-host.html

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