[Coco] MC-11 Status Update

[Darren A]

On Sat, Jul 18, 2015 at 6:46 PM, Darren A wrote:

> On Sat, Jul 18, 2015 at 6:26 PM, S Klammer wrote:
>
>   What clock rate are you running the
>> 68HC11 at?
>>
>>
> It's still a slow-poke running at the same 0.89 MHz as the MC-10.  It may
> be possible to give it a double-speed poke like the CoCo 3, but I'm not
> sure how that would affect the synchronized timing with the 6847 VDG.
>
>

It turns out that running the HC11 at double speed (1.78 MHz) does afftect
the synchronization window with the 6847 VDG.  At that speed, to get the
MCU to share RAM with the VDG and not produce any unwanted glitches in the
video, the synch window needs to move from the center of E high to around
the falling edge of E. This works for video modes having 32 bytes per
line.  When a 16 byte-per-line mode is activated, the synch window instead
needs to move to the rising edge of E.

I was able to update the CPLD logic to provide a "double-speed poke" and
automatically adjust the VDG synchronization based on the clock rate and
active video mode. It just barely fits as I've now exhausted all 144
macrocells.

- Darren