[time-nuts] Using a frequency synthesizer replacement for motherboard oscillator

Sarah White kuzetsa at gmail.com
Sat Dec 1 01:29:14 UTC 2012

On 11/30/2012 7:54 PM, John Ackermann N8UR wrote:
> On Nov 30, 2012, at 7:10 PM, Sarah White <kuzetsa at gmail.com> wrote:
>> On 11/30/2012 6:30 PM, Eric Garner wrote:
>>> the actual RTC on modern (Intel based) PC's is driven from a standard
>>> 32,768 Hz crystal attached to the PCH. some of them are in incredibly small
>>> packages now instead of the old tuning fork-in-a-can ones. peeling off the
>>> load caps and crystal from the board would allow you plenty of spaces to
>>> tack down a lead from an external synthesizer.
>> Yeah, the one on the (Soekis) example was pretty small. So far none of
>> of the replies have indicated that anyone on here has experience beyond
>> an embedded system.
> Sarah, when I was designing and protoyping the ClockBlock, I did interface it with a standard mobo (don't recall the specifics).  As someone else pointed out, the process is basically:
> 1.  Find and remove the oscillator that drives the CPU, likely something between 33 and 100 Mhz in modern systems.  It's *not* the 32.768 kHz crystal (if there still is one; I think it's actually built into thr RTC chip these days).
> 2.  Figure out which pin is the output of the oscillator module.
> 3.  Figure out the proper drive voltage (most easily based on the supply voltage of the oscillator).
> 4.  Hook the ClockBlock output to the signal pad where the oscillator used to be via small-diameter coax cable such as RG-174, connecting the coax shield to ground on the board and using a series resistor if you need to drop the signal voltage below the 3.3V minimum that the ClockBlock can provide via its voltage-select jumper.  Some math and/or experimentation may be involved; the goal is to get enough signal to drive the board, without exceeding the safe Vin rsting of whatever devices the clock is driving.
> 5.  Set the ClockBlock jumpers for the proper clock frequency.
> Have fun!
> John

John :)

Ok, wow, thanks!

I couldn't have asked for a better answer to my specific question than
one from the designer of the module itself (and more or less saying, and
confirming "yes, I've done this in testing")

Slightly unrelated but...

Any chance you could recommend a minimalist set of tools that would be
helpful for poking around so I could make sure things are wired up right
/ signaling as desired, etc?

Please don't say "logic analyzer" or "oscilloscope" because if that sort
of thing is mandatory, I'll just give up now.

I took a basic electrical engineering course nearly 20 years ago, and
have worked on a few simple controllers and even modified a computer
motherboard or two, so this won't be my first venture into such things.

... I'm just currently without ANY tools. (Not counting the dremmel
rotory tool for doing acrylic fingernails, and/or various repair &
tooling, cutting, and sanding of things that would take too long by hand)

... Well mostly none. The only decent tool I have on hand is a soldering
iron with a variable control / stand to adjust power and to have
somewhere to sit it while it warms up (also, there's a position on the
stand which is handy for holding the iron stationary so I can tin wires)

Guess that's all for now. Thanks everyone :)

P.S. Probably not doing anything like this for at least a month anyway.
Still need to sock away enough budget for "cheap computer to modify" and
the clock block itself (or some other appropriate frequency synthesizer)

P.P.S This might be my last post of the night. Friend's birthday is
today / have a party to finish getting things ready for.

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