[Coco] 6 Chip ^809 Computer -> Kipper SBC

[Gene Heskett]

On Sunday 15 March 2015 09:26:36 RETRO Innovations wrote:
> On 3/15/2015 6:12 AM, Gene Heskett wrote:
[...]
> > The delays in the additional decoding will allow some narrow noise
> > to get to the /CS lines on the OEM PIA's.  Because of that, we'd
> > have to get our address from a point in parallel with the on board
> > 138's inputs.
>
> The onboard '138 is already delayed by SAM (or GIME, in a Coco3), so
> you're fine.  However, modern logic can get you 7ns guarantees, so
> that'd be my recommendation.

Thats faster than the HCT's, suggestions?

> > Some google searching says the 74HC138 is 1/10 the power, and about
> > 16ns delay.  Also available as HCT, rated at the same 16ns.  Thats
> > 4ns slower that the LS family. Now if /SLENB was held down and had
> > to wait on the extra 138 function being non-Y0 to release it, that
> > 4ns is a shrug.  But somehow we have to leave the other PIA alone
> > for $FF20, so it gets more complex.  I think...  Too early here. 
> > Way too early.  But certainly food for thought.
>
> 74F or ALS or a half dozen other families get you the speed you need,
> but I really don't think it's an issue.  If you look at the 6821 data
> sheet, it's output buffers don't come online until quite a ways after
> the start of the cycle, so the chance two devices will be on the bus
> is minimal.

But it does bear checking with a good scope, else you'll have ground 
bounce to beat all.  BTDT.

> Jim

Cheers, Gene Heskett
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