[Coco] Coco3 question

[jdaggett at gate.net]

On 12 Feb 2008 at 17:14, Joe F wrote:

> Does anyone know if the coco3 has 262 or 263 scan lines.
> the 6847 spec sheet indicates 262.5 but I'm not sure it applies to the
> GIME as well. thanks.

The GIME appears to have 262 lines per frame. it is progressive scan not interlaced 
scanned. This is based on the specifications for the RGB monitor and some simple math.

Take the xtal frequency of the GIME and divide it by 1824 and you get 16699.76 Hz. Now 
divide that by 262 and you get 59.923 Hz. The CM8 specs are 15700 +/- 100 Hz for the 
horizontal frequency and 59.96 Hz. for the vertical frequency. The horizontal line frequency 
is a multiple of the vertical frame rate. Has to be. SInce timing is genreated from the main 
xtal, integer dividers are far more easier to make than a fractional dividers unless you use a 
phase acummalator to act as a divider. That becomes a bit more complex than a simple 
integer divider.  

This is the best combination of dividers that allows the GIME chip to meet the CM8 
specifications which was designed for the COCO3. 

james