correct me if im wrong..

talk abt power consumption..
if im not mistaken. u plan to use the PIC o/p pin to bias the diodes in 1
bridge to ground out one of  the osc o/ps ?

then the PIC pins cannot be in float.. they ahve to be  high or low right?

agin.. as far as i can see u can shift b/w only 2 clocks.. which u could
have done using a gate and a divider
alok

-----Original Message-----
From: Richard Ottosen [mailto:rottosen@IDCOMM.COM]
Sent: Thursday, November 23, 2000 12:39 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Working dual-clock speeds


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.

-- Rich



Dan Michaels wrote:
>
> Roman Black wrote:
> .....
> >> Can you see a similar "easy" way to something like this with
> >> 2 different "oscillator" signals [ie, PIC_pin+diode -vs- bulky
> >> external switching logic]?
> >>
> >> I want to use one of the programmable oscillator chips - namely
> >> the ECS-300, which has divide ratios from 1/2 down to 1/256 - but
> >> unfortunately the non-divided output comes off a different pin
> >> from the divided output, so you still need a way to select
> >> between the two signals. All in all, with this part, you can get
> >> 20Mhz, 10, 5, 2.5, ............, or 16Mhz, 8, 4, 2, 1, .5, .25,
> >> ........... - nice.
> >
> >
> >Hi Dan, yeah, here is a circuit that will do what you ask,
> >it needs two PIC pins driven differentially.
> >I am not too proud of this circuit, although it might
> >be cheaper and smaller board footprint than using logic
> >gates for switching it is not very pretty. I think it should
> >be functional though, even with 1N4148s.
>
> Hi Roman, a variation on the old AND-OR logic, I see.
> Good start. The series Rs at 10K would probably be too big
> for 20 Mhz operation - 2-3K might be alright.
>
> Do you really need the pull-up diodes to the pins? Why not
> just float vs pull low.
>
> Now the stinker - can you do the switch using only 1 PIC pin?
>
> best regards,
> - dan michaels
>
> --
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