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VGA SIP Adapter Hardware Issue? — Parallax Forums

VGA SIP Adapter Hardware Issue?

LombardLombard Posts: 30
edited 2014-05-26 17:51 in Propeller 1
I've been having problems getting VGA output to work on my Prop ASC board with the VGA SIP Adapter. Kuroneko helped me in another thread to narrow it down to perhaps some kind of hardware issue... I will describe my symptoms:

The i/o pins 0..7 are functional when floating, I verified all of them with a meter. I also used the bypass pads on the bottom of the ASC to eliminate the current limiting resistors. All of the led's on the VGA SIP adapter seem to work independently from one another. i.e., if I run 
PUB null
   dira[0..7]~~
   outa[0] := 1
   waitcnt(8_000_000 + cnt)
   repeat
     outa[0..7] := outa[0..7] >> 1
     waitcnt(8_000_000 + cnt)

The led's will all light up in sequence. The green ones (G1, G0) seem dimmer than they should be, but I could be wrong.

Here is the the first thing that seems problematic: I can light pins 0..1 (V and H) together, and I can also light pins 2..3 (B0 and B1) together. However, I cannot light pins 1..2 together - in that situation, I will only see pin 1 light, while pin 2 remains off.

Also, at any time that the adapter is plugged in, if I poke the solder joint for either pin 0 or 1 with a piece of wire or finger (antenna), the corresponding led will light. This only works for those two. The schematic doesn't show that V and H are routed any differently than the other pins. I'm a bit confused by that.

I'm baffled! Does this sound familiar to anyone?

Comments

  • TubularTubular Posts: 4,703
    edited 2014-05-26 14:32
    It'd be worth checking the supply from your ASC to the supply pins of your led driver, when driving the 1&2 led combination.

    For the 74LC541, Pins 10 should be within a few millivolts of Vss on the ASC. Pin 20 should be be within a few millivolts of the Vdd rail of the ASC. And there should be at least 3 volts across pin 10 & 20 at all times

    Is there a resistor somewhere in the Vdd supply to the VGA SIP adapter?
  • LombardLombard Posts: 30
    edited 2014-05-26 15:14
    Ah, you're absolutely right. I was looking at supply voltages under various led current demand, and it was sagging tremendously more than a power supply ever should for a few milliamps, suggesting that there were some secret current limiters on what I thought was a 3.3v power jumper. On the via marked "R," near the power input on the ASC I soldered in a wire for 3.3V power, because the R via was measuring 3.3 ... apparently it does have a resistor in series with it somewhere, glad I didn't damage anything. I rerouted the power to the "3V3" pin, and it's working quite nicely. :)

    Does anyone know where that board layout is? I went looking for it a couple times but came up empty handed. Thanks!
  • TubularTubular Posts: 4,703
    edited 2014-05-26 16:01
    Ok, good that it got sorted

    The 'R' pin could be a reset pin, it probably has a high value pullup resistor, perhaps the one inside the prop. With a multimeter it would read 3 volts but not under significant load...
  • kuronekokuroneko Posts: 3,623
    edited 2014-05-26 16:54
    Haven't found a layout but the schematic is available from the Wiki.
  • Martin HodgeMartin Hodge Posts: 1,246
    edited 2014-05-26 17:46
    Yes, the R pad is for an external reset switch. When grounded it will reset the Propeller. It is held high with a 10k resistor to 3v3.
  • LombardLombard Posts: 30
    edited 2014-05-26 17:51
    Haha, it is a reset pin indeed! That explains the "crashing" I was seeing occasionally :) Light up too many led's (pulling R low) and the propeller shuts down.

    Careless mistakes always seem so obvious in hindsight..
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