[Coco] Coco 3 pixel clock

Christopher Hawks chawks at dls.net
Tue Oct 9 12:09:56 EDT 2007

jdaggett at gate.net wrote:
> On 8 Oct 2007 at 21:01, Christopher Hawks wrote:
>>  Does anyone know the pixel clock rate for the RGB output?
>> -------------------------
> Not with 100% certainty.  There are few possibilities. What is most likely is 11.454545MHz 
> for 640 pixel mode. This is the master clock divided by 2.5. This yields 640 pixels for display 
> and 90 pixels for border and blanking interval. A total of 730 pixels. The horizontal 
> frequency is 15.691 KHz in this configuration. In 320/256 pixel mode the pixel clock is the 
> master clock divided by 4,  7.159090MHz. This  yields 320/256 pixels for display and 
> 136/200 pixels for border and blanking. Lesser resolution modes would use the color burst 
> clock. 
>>  and if it can be programmatically changed??
> Since there is only one master clock in the Coco3, the pixel clock is slaved to resolution and 
> the master clock. to change pixel clock would require changing the master clock. This would 
> also affect color generation if done. 

Thanks Guys for the info.

	I've been playing with a 5.4" LCD display (Intec G8270) and my Coco3. I 
can run S-Video into it and get a great picture. I noticed that the 
internal conversion board (S-Video to the LCD) has a 7.15 MHz crystal 
and tried connecting the Coco3 directly to the LCD. They both have Red, 
Green, Blue, and Sync connections. (I combined Hsync and Vsync with an 
OR gate.) 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).

	This would be great! A color LCD with just a 2 chip interface to the 
Coco3. (Cost me about $45 on the internet.)

Christopher R. Hawks
Any research done on how to efficiently use computers has been long lost
in the mad rush to upgrade systems to do things that aren't needed by
people who don't understand what they are really supposed to do with them.
	-- Graham Reed, in a.s.r.

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