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Decoupling capacitors for the win!

27 Feb 2016

While working on a VGA adapter for a Z80-based computer I’m building, I started out by assembling a very simple circuit for generating VGA signal based on the Parallax Propeller. I’ve put it together and loaded one of the sample programs for displaying text. It didn’t work, and I couldn’t figure out why. At first, I tried running the test program in different resolutions - that worked - but the screen was wobbly.

That led me to think that there was a problem with timings. I looked at my oscilloscope, but the timings for VSYNC and HSYNC were okay. One thing I noticed however, was that there was a lot ripple on the PSU:

I’ve then added one 100nF ceramic capacitor where the PSU meets the board, and got this:

Better, but I wonder what would happen if I added another cap of the same size to where the Propeller meets the PCB. I’ve added that, and got this:

Looks much better, doesn’t it? I loaded up the original test program, running 800x600 at 75Hz and got a nearly perfect image.

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