[Coco] Coco 3 pixel clock

Steve Bjork 6809er at bjork-huffman.net
Tue Oct 9 10:50:25 EDT 2007

The Pixel Clock rate for RGB (and Video) are based on the master 
clock of 28.63636 MHz (NTSC version).  For a 640 dot mode the pixel 
clock is 14.31818 MHz and for a 320 dot mode the clock is 7.15909 
MHz.  As far as changing the pixel clock, it's locked to the master clock.

It should be noted that the 512 dot mode uses the same clock rate as 
the 640 mode (as does 256 uses the 320) but the unused dots take the 
boarder color in place of graphics data.  This is also true with the 
different horizontal (line) modes too. The 225 line mode does not 
have extra lines over the 192 mode, the unused lines of the 192 mode 
take the color from the boarder too.

In my games Super Pitfall and Z-89 (Zaxxon for the CoCo 3), they both 
used the 256 by 192 mode to save on memory and update time.  But the 
graphics have the same dot aspect ratio as the 320 by 225 mode 
because they use the same pixel clock.

Steve (Zaxxon) Bjork

At 07:01 PM 10/8/2007, you wrote:
>         Does anyone know the pixel clock rate for the RGB output?
>         and if it can be programmatically changed??
>Christopher R. Hawks,HAWKSoft

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