Re: IIGS 115,200 Baud Serial Port - How?

[Michael Black <et472@ncf.ca>]

On Sat, 3 Jan 2009, David Wilson wrote:

> It is interesting to read the "ZILOG SCCZ8030/Z8530 QUESTIONS AND
> ANSWERS" Application note.
>
> Q. Can RTxC on both channels be driven from the same crystal.
> A. No. A separate crystal should be used for each channel.
> The crystal should be connected between
> /SYNC and RTxC of the respective channels. The alternate
> solution may be to use crystal on one channel
> and reflect the clock out of the TRxC output and feed
> it into another channel
But that question is ambiguous.  Clearly, the question is whether
one needs 2 crystals.  And the answer should never be "yes".

But the point is that one wouldn't put the two pins of each channel
in parallel and then put one crystal in there.

The two pins where the crystal connects are undoubtedly the input and
output of a gate that is linearized internally.  Putting the crystal
across the pins causes the gate to oscillate, and at the frequency
of the crystal.  One of those pins will be an input, one will be
an output of that gate.

So one can presumably feed the pin that connects to the input of
the gate with an external oscillator signal, certainly this is normal
procedure for that sort of situation.  ONce that is the case, then
it's a simple matter for determining which of the two pins is the
output of the gate, and then feeding that to the input pin of the
second channel.

   Michael

> It seems that Apple worked out how to do it with one crystal by using
> the /SYNC output to drive the 2nd RTxC input.