[Coco] It's a small win, but a win nonetheless

[Gene Heskett]

On Saturday 11 March 2017 23:03:07 RETRO Innovations wrote:

> On 3/11/2017 4:18 PM, Gene Heskett wrote:
> > Do not forget that if the input r on the comparator's input is 10
> > megs, your taking a look at it withe a 10x scope probe, or a dvm
> > lead, either the scope or the dvm is another 10 meg r, so the
> > resting voltage you measure is really twice that.
>
> I agree with your premise, but it must not be twice, because when I
> remove the probe, it still works fine.  The 10M resistor to ground is
> still installed, though.
>
> > If that 1uf is a small electrolytic, its leakage could account for
> > that.
>
> It could, I will see if I can find a 1uF in another type to try.
>
> > Also the comparators input current has to flow thru that r so you
> > might want to switch comparator types from one with a starved
> > current pnp input stage, to one with depletion mode fet inputs,
> > which have femtoamps for input currents. Neither of us has gear to
> > measure that low a currant.
>
> True, but I am going for ultra low parts count, and if I can use the
> comparator of the uC, it'll save an IC and a few passives.  The design
> is price sensitive.
>
> > Which input is getting the hysteresis feedback?
>
> +
>
> > Needs 2 stop bits? More positive framing sync, but will the music
> > tolerate that?  I dunno.
>
> Well, it would be nice, but the Coco does not provide any stop bits
> :-) And, I can't add any.
>
> Jim

Without fiddling with the rom code. But, I am quite sure the protocol 
includes a start bit AND a stop bit.  And the stop bit is probably a few 
microseconds wider than it has to be because its busy at that point, 
fetching the next byte to send. So a loss of sync, giving you an AA 
instead of a 55, (that is a one bit time slippage) may be symptomatic of 
the uC burning too much time putting the previously received byte away. 
Optimizing the uC code might be required by dropping into assembler for 
that.

Cheers, Gene Heskett
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