[time-nuts] have 10MHz need 19.5Mhz

[Bob Camp]

Hi

The Pi is simply a standard ARM CPU. The clock system in it is well documented. How you get *at* the clock system may or may not be so well documented. There are bits you flip to switch it from external crystal to external oscillator. The gain of the stage and the function of the output pin are impacted by the bits. By far your best option would be to figure out how to flip the appropriate bit. Baring that, ignore the output pin and just drive the input pin. Trying to put a differential signal in there will just mess things up. 

If you don't need video you *might* be able to change the clock frequency. The other things that would be impacted are the USB (needs 48 MHz from somewhere) and the serial baud rates. Since the ethernet hangs of of a USB port it's clock would not be impacted. 

Bob

On Jun 2, 2013, at 2:59 PM, Chris Albertson <albertson.chris at gmail.com> wrote:

> Recent talk about NTP servers.  It seems the limit to their accuracy is the
> quality of the crystal that drives the CPU clock.  Most of them make really
> good thermometers.  I'd like to try and replace the crystal on a Raspberry
> Pi with a signal derived from a time nut quality 10MHz standard.
> 
> The Pi uses a crystal (not a TTL can, a real two lead crystal and a pair of
> 47pf caps) Both leads of the crystal attach to a pair of pins on an IC.   I
> figure I can unsolder the crystal and inject a balanced 19.5MHz signal
> directly to the IC's pins.    I know the ARM CPU just might work on a 20MHz
> clock or maybe 15MHz but the video likely would not.  I'm going to have to
> supply 19.5MHz.
> 
> The question is the best way to get from 10MHz to 19.5MHz.  I care only
> about long term (tens of seconds to days) stability.
> 
> I thought of using an AD9850 DDS chip.   You can buy these on break out
> boards very cheap on eBay but they need a 125MHz clock.    I could drive
> the 9850 with a 120MHz clock that is multiplied up from 10MHz.    what is
> the simplest 12x multiplier.   I assume getting to 125MHz from 10MHz is to
> hard.
> 
> New-AD9850-module-modest-capacity-AD9851-DDS-Function-Generator-up-to-40MHZ<http://www.ebay.com/itm/New-AD9850-module-modest-capacity-AD9851-DDS-Function-Generator-up-to-40MHZ-/400422353936?pt=LH_DefaultDomain_0&hash=item5d3b083c10>
> 
> 
> Is there a smarter and more direct way to get 19.5MHz for 10MHz?
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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