In a search for a cost effective HDMI to VGA solution for the RaspberryPi this post came closest:http://www.linusakesson.net/hardware/beagleboard/vga.php
This created some inspiration to break it free form the Beagle-only approach. So lets see if it is possible to make it universal.
The original project uses software to generate the H/Vsync signals and thereby it uses up 2 valuable GPIO's. So we need to generate these signals with a couple cheap and simple oscillators (NE556)
Now that looks pretty easy, though we are not there yet. Lets see what the idea is of this schematic.
The RGB, H/Vsync and all the ground connections, thats all pretty straightforward. Then we see 2 transistors to make the correct H/Vsync levels. But there is no H/V clock source for the signals on the board. The maker chose to generate them with software but maybe it is more convenient to do that electronically with a NE556 dual timer IC.
The R-2R type resistor networks will do the D/A conversion for the RGB signals. And a +5V powersource is also needed
So lets first have a look at the J4 and J5 to see what signal goes where and replace it wit a HDMI connector
For this we need the pinout of the OMAP3530 CPU that's on the beagleboard
Pinout OMAP3530 (PDF, not very helpfull)
VGA pinout / VGA Vesa Ddc pinout
RaspberryPi Schematics R1.0
The signals that need to present:
The pins that need to be connect:
The Signals that are present on the RaspberryPi (see page2 BCM2835 HDMI):
How to wire it up:
Richard Stallman, president of the Free Software Foundation, launched the development of the free software operating system GNU in 1984. The GNU/Linux system (essentially GNU with Linux added) is used on tens of millions of computers today. Stallman also founded the League for Programming Freedom in 1989, which campaigned against legal threats to programming – principally software patents.