[quoting]

> On a related note, does anyone have any recommendations for how to
> externally drive a PIC oscillator in the RC mode,

  Think carefully about why you thought a capacitor would be
appropriate.  I'd like to know the thought process, because you are
obviously confused about something here.

[me]

My thought process was that the OSCin pin, while in RC mode, has two states:

(1) Floating, until the pin reaches an upper threshhold value.
(2) Output low, until the pin reaches a lower threshhold value.

Assuming an initial state with the pin grounded and the oscillator output low, my thinking is that the following sequence would occur:

(1) The external oscillator's output goes high.  The OSCin pin which is capacitively coupled to it should go high as well until...

(2) The OSCin pin reaches the upper threshhold, whereupon the PIC will try to ground it.  The cap will then remain in that state (external-oscillator end at VDD; OSCin-end at ground) until...

(3) The external oscillator's output goes low.  The end of the cap attached to the PIC would then go somewhat negative, but the clamp-diodes on the PIC would limit the amount, so effectively both ends of the cap would be at ground potential until the external oscillator goes high again (step 1).

[quoting]

> and/or  recommendations on the construction of a low-cost, low-power,
> reliable 32KHz oscillator?

  *Very easy!*  Look at one of your old PC (actually, AT or 386, 486
etc.) motherboards.  Locate the 32,768Hz crystal and the 4069 chip which
implements the oscillator.  Trace out the circuit, includes a couple of
caps and resistors.  Copy it.  Voil‡!

[me]

I was wondering if anyone had a schematic handy; also, I thought motherboards usually used an RTCC chip which contains the oscillator as well as the clock, some RAM, etc.  I'd just be looking for the oscillator.

[quoting]
  I am assuming that you expect that using the RC mode reduces the pin
count devoted to the oscillator.  I can't see it, certainly not on the
16C84 where in RC mode, the Xtal out becomes an instruction clock
output.  Maybe this works differently on some 8-pin devices and you can
use only one clock pin.  Otherwise, you use LP mode to nobble the
feedback resistor and feed in the external clock directly to Xtal in.

[me]

On the 12Cxxx and the 14-bit derivative (the 16C505) the RC mode does indeed only use one pin.  The 16C505 does have 3 bits to select clock mode, but unfortunately they don't provide an external-input (with no clock output) mode.  Oh well, no biggie.