Shop Learn
VGA Interface — Parallax Forums

VGA Interface


I recently connected my Prop1 to a keyboard and monitor and tried to run the demos but failed. Does anyone know of any known issues. My board is exactly like the Propeller demo board, except a very small difference. I would have bought one, but I take it the design may have been short lived for some reason, and I was a little late getting into the propeller thing.

I hooked up my scope and I could see what appears to be the syncing signals (looks pretty clean), so I do believe I wired everything ok, but I just don’t see any signal from the monitor.

The only difference to my circuit are the resistors for the D/A converter. I used a 500 ohm. pots with the wiper connected to the signal lines (r,g,b) on the vga connector. I set them to match the exact proportions as on the demo board. I figured with the standard 50 ohm impedance of the monitor, this would make no difference. I also thought that I would be able to adjust the color with the wipers.

I am using a 23” lcd monitor made by AOC. I know it works, but says no signal present. I even tried using a cable with and without 5v on pin 9.

Anybody else having this problem, or knows what I can do?


  • RaymanRayman Posts: 12,366

    Some monitors need that 5 Volt pin.

    Also make sure the ground pins are connected.

  • NickMikhay,

    What crystal are you using and do you have decoupling capacitors on all of the Propeller ground pins?

  • I run my Prop1 pro board with no issues. Hard to tell where your problem is without seeing the board schematic and code. Did you assign the correct pin group.

  • Wuerfel_21Wuerfel_21 Posts: 2,150
    edited 2021-12-16 00:25

    If the sync is right, it should start displaying something. Are you sure you didn't swap the two sync signals? Also, check the frequencies. Should be ~31kHz H / ~60Hz V

  • The monitor works with windows when I use the cable without a pin 9. I used the same pin assignments as the demo board schematic and sure that they are ok.. And, the code is the standard code from the library in the tool. The capacitors are probably not an issue, unless I have too much, since the board uses fairly small capacitors, I threw in some extra for good measure near the vga connector, and powerbus. If they were a problem I could remove them. Well, if I figure it out I will post the answer. Thanks for the help, let’s me relax the mind. Oh yes, the crystal I bought on the parallax site, and it is 5mhz.

  • RaymanRayman Posts: 12,366
    edited 2021-12-16 13:42

    I’d try running some code that outputs to serial port with 80 MHz clock. That will verify that your setup is correct if you get output in serial terminal.

    If that works then it must be something in your vga code that is wrong. Make sure the pin assignments match your setup.

  • Well, when I measured the output it was at the pin of the propeller. I then decided to check again, this time at the output connector, so to be thorough. In the end I found three points where the traces were torn, so I had to solder them together with a little wire. Checked for continuity and ran the demo again. Woof woof, meow. It all works great. The picture appears not centred, but I suppose I can configure the code to my display.

    Checked ,the pcb file and apparently I must have moved the components and detracted them from the traces at the last minute. It was only a small gap, not sure exactly now, but .05 mm or so. It seems like I should get the pcb’s redone. I would probably add a bidirectional level translator for 5v logic to pins 1-16 that I planned to use as an 8-bit expansion bus. Not sure what would be best, maybe a second tier that I can plug in if needed.

    Seems the only thing lacking from the demo board are more output pins.

  • RaymanRayman Posts: 12,366

    Glad you found the problem!

    Usually monitors have an auto-adjustment feature to center the image...

Sign In or Register to comment.