A13 processor have no HDMI and VGA output as his big brother A10, so we had to work out something using the LCD interface it have.
We browsed IC video DACs and there is plenty of them, but they are expensive and didn’t fit well OLinuXino low cost concept, so we decided to give a try and make simple R ladder video DAC.
A13 LCD interface is 18 bit this means we will need 3 channels x 6 bit Video DAC.
All VGA monitors have 75 ohm terminator on their input lines and the video signal is 0.75V for white level and 0V for black. So knowing this we could make resistor ladder DAC with R, 2R, 4R, 8R, 16R, 32R connected in parallel with RGB 0-5 bits. The values are easy to calculate when we know the LCD buffer output voltage 3.3V and the voltage we want to have when all bits are “1” i.e. 0.75V. The resistors using the 1% precission values picked were 549 ohm, 1050 ohm, 2180 ohm, 4370 ohm, 8660 ohm, 17800 ohm.
The adapter was routed and pcb prototypes were run. The assembled adapter was attached to LPC1788 board with 4.3″LCD with tweaking the output to match VGA lock frequency and the result was perfect.
The picture which appears on LCD was color stripes with small ball which you can move by tilting the board (using on-board accelerometer), as you can see from the picture the colors are perfect and image is standing still with no glitches and jitters.
So now we have low cost VGA solution for our A13-OLinuXino and can move forward with the PCB schematic finish and routing. Note that the VGA signals go through buffers which are gated i.e. we can enable and disable the VGA output and A13-OLinuXino will be able to work with both LCDs and VGA.
EDIT: schematic is included here below: