[Coco] CoCo RGB video timing and levels

[Theodore Evans (Alex)]

On Aug 24, 2004, at 10:36 AM, jdaggett at gate.net wrote:

> Looking at the Service manual, it looks as the if the GIME chip is 
> running a pixel
> clock of 3.579 MHz or 280nS. In 640x200 two color mode, one read 
> covers eight
> pixels and the VDU has to read every 8th pixel or a total of 80 reads. 
> In 640x200 4
> color mode the VDU has to read every 4th pixel and makes a total of 
> 160 reads.
> Plenty of time even at a buss rate of 1.78 MHz.

At 640x200 the GIME writes at a rate of 14.28 million pixels per 
second.  This certainly seems like a pixel clock of 14.28 MHz to me.  
It reads from memory at a rate of 1.78 MHz  (16-bits at a time).  
Neither one of these corresponds to a pixel clock of 3.579 MHz.  It is 
true that the CPU and video read cycles are interleaved and in fact the 
CoCo is quite capable of doing one of each every cycle.

-- 
Theodore (Alex) Evans | 2B v ~2B = ?