Mike Harrison wrote:
>
> On Wed, 22 Nov 2000 12:08:35 -0700, you wrote:
>
> >How about this:
> >
> >Typical crude ASCII art follows...
> >
> >                         2.2K
> >Oscillator output #1 ---\/\/\/\-----------
> >                                 _|_    _|_
> >                                 \ /    \ /
> >           PIC output pin ________|      |______  PIC osc. input
> >                                 _|_    _|_
> >                                 \ /    \ /
> >                         2.2K     |      |
> >Oscillator output #2 ---\/\/\/\-----------
> >
> >
> >This uses a diode bridge for the switches. You should be able to find
> >this in several forms such as a balanced mixer or 2 dual diodes in
> >sot-23 packages, etc.
> >
> >You may have to bias the oscillator input at one half of the supply
> >voltage or put a feedback resistor from the input to the output of the
> >PIC oscillator. I think about 10K ohms should work here.
> This would be dangerous as the oscs would not be synchronised to the
> PIC's clock before switching, so you run the risk of glitches  at
> switch-over which could trip up the PIC. You could probably do some
> tricks with sleep mode to reduce this risk. .
>


I was assuming that this would be used with the oscillator having two
outputs -- one divided down from the other. Even so, without looking at
the spec's for the relative timing of the two outputs there is no way of
knowing that the problem you mentioned won't exist.  :-(

I would not be surprised if there was a glitch on one edge or the other
when the switch is made since the PIC's internal delays from the
oscillator input to an output pin are not well defined.

Mike, thanks for keeping me honest.


-- Rich

> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads