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

John Ackermann N8UR jra at febo.com
Sat Dec 1 00:54:50 UTC 2012

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!


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