[Coco] Coco 3 pixel clock

Mike Pepe lamune at doki-doki.net
Tue Oct 9 18:44:26 EDT 2007

Mark McDougall wrote:
> Christopher Hawks wrote:
>> It looks great (with my Hsync delay circuit to center it), but appears 
>> to be missing about every eighth dot on the screen. If I move the 
>> display horizontally, the missing dots change so I suspected it is a 
>> difference in pixel clock rates. This occurs in all resolutions (640, 
>> 320, and 256).
> If you _really_ wanted to, you could probably implement a pixel-clock 
> converter with a CPLD and some video RAM as a line buffer... there might 
> be a simpler way (I'm no video guru) but that's how you'd do it inside 
> an FPGA...
> Regards,

I sorta did something like that- I ran a 512x200 plasma display with a 
CoCo 3 (probably about 15 years ago at this point)

The pixel clock in the CoCo is of course derived from the 28.63636 
crystal. I simply "h4x0r3d" out the crystal and replaced it with a TTL 
can oscillator. I then tapped the output to divide down to something the 
plasma display could accept.

Something similar might work today. It was relatively minor surgery.


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