VGA output is a tiny bit bad, any ideas?

Chris_D

Hi guys,

I am working on a homemade circuit board using the same schematic layout as the demo board for my VGA output.· It works good but there are times when certain areas of the display are a bit scrambled.· About the best way to describe it is that in the areas of display that are bad, it looks like a different font - perhaps normal instead of bold.·

The bad area is not always in the same place.· If I take my finger and rub it accross the surface mount resistors, I can make bad areas and I can make bad areas look good.· This makes me think it is a "problem" with my circuit board or the construction thereof.·

I am thinking that perhaps a coating on the board would fix the problem but don't have anything to try.·· But, even if I did have something, I would ask here first just to make sure.·

Does anyone have any idea what could be causing this?·

Chris

Toby Seckshund

I am a bit puzzled about the finger affecting what should be a low impedance circuit, the vid feeds should be 75 Ohms at the output and effectivly low Ohms at the Prop end of the 240 and 470 Ohm reistors. Either you have extremely conductive fingertips or the fault is something else affecting other unconnected inputs (very Hi Z) to the prog which are doing the unwanted things.

Does the same thimg happen if one of the VGA demo progs are run or is it just with yours?

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Ale

I'd check if all connections, solder joints, are well.. connected. That sounds like ground is not routed properly or not enough decoupling have been used.

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Bob Lawrence (VE1RLL)

I don't think it's your layout. I took a piece of PC board wired to a prototyping board for the Hybrid /Hydra boards (allows me to plug it into the connector on the board) and both were wired to a VGA connector. To look at it it's nothing but a rats nest of wires, a real mess but it works fine.

Phil Pilgrim (PhiPi)

It sounds lik a bad solder joint. I'll bet one of your DAC resistors is little more than capacitively coupled. When you press on it, it makes a better connection. Try displaying red, green, and blue separately and see if one color is affected but not the others.

-Phil

Chris_D

I didn't think to try some of the demo programs to see if the problem follows software.· I will try that the next chance I get.

As for bad solder joint, certainly possible, but the force I am applying to the resistors is a very light touch.· In any case, I will hit each of the pads with a soldering iron to see if that clears things up as well.

To clarify, I am an absolute rookie with electronics, but have been tinkering for a couple of years.· I use decoupling caps on ICs because everyone tells me to.· Seeing the phrase "Decoupling" here makes me want to understand more about this.· Can someone explain what decoupling means relative to what might be happening with my circuit?

Thanks

Chris

Toby Seckshund

The decoupling caps act as "short term batteries" as the current demands of the chip is made up of high current spikes of demand. The meter will only give an indication of the average used. Because the rising and falling of these peaks are so swift the power rails are not just the wires from the supply, they are a complex mix of resistance, inductance and stray capacitance. Left to their own devices their voltage would dip down, spike up and generally give very poor stability. Putting capacitors across the rails, AS CLOSE TO THE IC AS POSSIBLE, will smooth out most of these traits. A large electrolytic gives a large charge resevoir, but possibly a poor high frequency reaction. A small value ceramic will give a small charge resevoir but a fast responce. That is why there is a mix of values and types in most designs. Tantalum gives a reasonable compromise.

The wires, and tracks, form resistance and inductors and so should be as fat and short as possible. A good regulator IC will fight a lot of these problems, and cause a lot of them if they are not decoupled properly too!

These patches of "not nice video" are they horizontal, vertical or really blobs that can be moved?

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Chris_D

Thanks for the description Toby,

Blobs would be the best way to describe them.· I am using text mode and I think the resolution works out to 100 chars by 48 lines (again, just an estimate as I have fogotten since setting up that part of the program).· The problem areas are around clusters of characters and the "Blob" does move. It can move on its own as in it is in a different place when I power up or it can be moved by running my finger over the resistors.

I don't know if it is bad enough to show up on a photo, but I will try to take a picture the next time it is very noticable.·

Thanks again!

Chris

Toby Seckshund

When moved, by your paw, does the affected area stay reasonably static or does it go back to it's starting position as soon as it is withdrawn ?

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Bob Lawrence (VE1RLL)

Just to add to my point about layout.

The Hybrid board doesn't have a VGA connector so I quickly made something up to participate in the Turbulence Demo . I was able to use that ratsnest to run the Turbulence Demo with no problems( that I am aware of)

Re: Turbulence is the successor of craft. It's a microcontroller demo based on the Propeller chip from Parallax. External I/O consists of stereo sound (at line level) and a VGA signal.

www.linusakesson.net/scene/turbulence/index.php

ElectricAye

Chris,

Toby did a great job of explaining decoupling, but you might also want to browse this article on wikipedia if you still have questions.
en.wikipedia.org/wiki/Decoupling_capacitor

One other thing, when you touch your resistors, are you sure that you aren't charged up from dancing across a carpet, brushing balloons on your cat, rubbing elbows with Casimir Effect sweater-wearing passersby or something? I doubt that would have anything to do with it, but it's just a thought.

jrjr

Chris,

It is most likely that your use of standard timings in VGA is showing some
monitor sync peculiar reactions.

The finger may be 'shaping' the sync signals enough to slide into an acceptable range.

First, try another monitor if you have one, or try trimming the H/V sync pulse widths slightly.

Where is the error showing ?

Top of Screen ?
Left edge of screen ?

Please describe the symptom better and location.

jr

Toby Seckshund

jrjr is correct.

If the active part of your VGA has wrong timings and some of it falls into the sync/blanking bits then the monitor will suffer from timing problems and/or clamping problems (this where the monitor assumes black should be, and nails its levels to that point)

Stick in a bog standard "demo" prog and see if the problem is still there ( and get the monitor to do an "AUTO")

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