Harold Hallikainen wrote:
>        Though not what was originally requested (2 speed clock with one
>being crystal, the other being RC), you could possibly do a 2 speed RC by

Which reminds me that I did not see the orignal posting. I suggested using
a 74S66F switch to change crystals, but for RC, is is possible that a
collector-switched bipolar transistor stage will do the trick. This is
pure imagination so far, but it may work. The scheme is:

PIC + parallel resonant crystal + 2 small capacitors connected as usual.

Coupling cap from Xout to base of additional NPN. Emitter of NPN through
~2k7 to GND. Collector of NPN through 10K to PIC IO pin. Collector of NPN
through coupling cap to Xin. Base of NPN polarized from collector through
~150k. The Xin coupling capacitor should be very small (the same order of
magnitude as the crystal 'pi' capacitor on that side).

Now, when the PIC is reset the IO pin is tristate and the NPN collector
has no current into it. The NPN arrangement should allow the PIC Xtal
oscillator to start as usual in Xtal mode. To switch to RC mode, set the
PIC pin to output and HIGH at the same time. Now the NPN stage is biased
and the crystal appears as NFB across its input and output. This will
hopefully stop the crystal oscillation and then the ac coupled 'gate ring'
oscillator formed by the NPN stage its coupling capacitors and the
internal PIC oscillator gate should take over at a frequency that is to be
determined (but should be obscenely low - tens of kHz probably).

Problem: the conducting NPN will draw more current than the PIC ;-)
Solution (?) replace NPN stage with a single tristate inverting CMOS gate
(4S24?F I think - I've never seen one though they certainly exist) and AC
couple its output only this time. Steer the enable pin with the PIC IO
pin.

Last alternative: Use a 'A' series CMOS inverter gate as external inverter
and steer by switching power using PIC IO pin, hoping for sufficient
isolation between in and out for the Xtal to oscillate with power off.

If anyone will have success with this scheme (which I have no time to try
myself) then please report.

Peter

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