[Coco] On John Linnville's Color Computer Game Master Cartridge Design

[John W. Linville]

It's really not that complicated -- even a PAL would be waste, much
less a CPLD. No special programming needed either. Hammers and nails,
I guess... :-)

John

On Wed, Aug 02, 2017 at 01:39:01PM -0500, Zippster wrote:
> Hmm, was not aware of that.  I haven’t seen any details of the design.
> 
> Easy enough to handle with some modern programmable logic in the loop, but it sounds like john didn’t use any.
> A latch in combination with the ready pin and programming technique would do with old school components I suppose
> from the sound of it, though I haven’t really looked at that data sheet.
> 
> - Ed
> 
> 
> > On Aug 2, 2017, at 1:23 PM, RETRO Innovations <go4retro at go4retro.com> wrote:
> > 
> > At CocoFEST, John and I discussed potential additional opportunities for the SN76489 sound IC within the Coco community. After recovering from the fest, I ordered a batch of the ICs and I sent out a PCB design to attach the unit to a Coco.  While the boards arrived many weeks back, I have been busy with other things and, as discussed on both the podcast and CocoTALK, I've been a bit unmotivated to finish the design.
> > 
> > Well, I decided I should at least get the unit working, so I would at least be knowledgeable about the IC and its capabilities.  The datasheet specifies a quite lengthy cycle time needed to store data into the unit, but I was undeterred.
> > 
> > And, after 3 nights working (fighting) with this IC, I think people should give more credit to John for his design.  He didn't just wire up this IC to the Coco expansion bus, he performed some magic to successfully map it into the Coco address range.
> > 
> > The SN76489 takes 32 4MHz cycles to successfully store a value in a register, which is 8uS, though that might be worst case, as I see 3uS typical.  In any event, this need to hold the data valid for so long makes a simple connection to the Coco bus more complex (essentially, unless one wants to HALT the CPU while the write completes, it appears one needs to store the data in a faster "buffer register" and then hold the bus signals to the IC until the IC signals the write is complete (there is a "READY" pin)).
> > 
> > I thought I'd point this out, since I originally did not have a full appreciation for John's accomplishment.  As it stands, my original PCB won't work, but I was able to verify correct operation by attaching the IC to a microcontroller and successfully pushing sound out of it that way (I know the chips are good).
> > 
> > Jim
> > 
> > -- 
> > RETRO Innovations, Contemporary Gear for Classic Systems
> > www.go4retro.com
> > store.go4retro.com
> > 
> > 
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> 
> 
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-- 
John W. Linville		Someday the world will need a hero, and you
linville at tuxdriver.com			might be all we have.  Be ready.