[Coco] MC-11 Status Update

[Zippster]

Very nice Darren, can’t wait to see one running some software.  :)

This is something I love about using CPLDs in a design.
You can change function quite a bit without changing the PCB, as long as
you take all the signals you might need into the package.

- Ed


> On Aug 22, 2015, at 11:46 AM, Darren A <mechacoco at gmail.com> wrote:
> 
> 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
>