Dave VanHorn wrote:

> > The obvious thing to do is to tap into the xtal output.
> > So, can somebody tell me what sort of electronic wizardry I have
> > to do to make the 4MHtz crystal drive some sort of gate so I can
> > divide by 4 and drive the ISD chip???
>
>
>Careful.
>
>Check the spec, there should be an output drive spec for that pin. 

Concidentally, I plan to divide down a PIC driven 4,096 MHz Xtal to 1,024 MHz to feed a A/D (MAX110), for maximum 50Hz reject.

I plan to use 1/4 HC393, which is a nice little 16 pin device containing two /2 and two /8 small ripple counters. (its sister 390 has /2 and /5 counters)

Looking at the spec (Motorola) I see that the *maximum* input capacitance is 10pF.

>In many cases, that output is designed to only drive a crystal.

And a 30pF (typ) capacitor!

>Also, remember to check the effect of that gate capacitance on the osc 
>circuit.

Yep, just reduce the cap you would normally use on that pin, with the nominal capacitande of the clock input you are driving. (something 7pF ? )

Being a CMOS device, it has negligible high DC resistance.  (do not use TTL...)

We also have to take in count the average voltage on thet pin; should we use HCT or HC?
Need to measure, or it it stated somewhere?
Anyone?

Regards
/Morgan
Morgans Reglerteknik, HŠllekŒs, 277 35 KIVIK
  tel 0414-446620, fax 70331, mrt@iname.com