TMS9918A and PS/2 Keyboard

 

I've been away from my project, but this week I've made a few additions to the SBC.


Instead of using an old TRS-80 keyboard, I decided a while back to use a PS/2 keyboard (interfaced through the VIA). Things worked relatively well, except that the SBC froze if no keyboard was present. Last week, I changed from the fully software-driven PS/2 interface, to an MCU-based interface. This offloads a lot of the work, and only sends characters (instead of scan codes) via an interrupt to a buffer. 

And today, I decided to add a video component to the SBC. A Texas Instruments TMS9918A VDP (Video Display Processor). It outputs a composite signal to a standard-definition television. Although hard to see in this picture, you can see a bit of text with lots of color fringing. I will need a good monitor to see if it's a problem with my cheapy LCD monitor, or the video signal itself. I did notice some jitter in the oscilloscope, so fixing this might not be so easy.

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