[time-nuts] First success with very simple, very low cost GPSDO, under $8

d0ct0r time at patoka.org
Mon Apr 14 12:27:11 EDT 2014

I was experimenting with the same setup for STM32 MCU. This 
microprocessor has accept the sine wave from external OCXO or GPSDO. No 
problem with this. The only thing: I was need to start MCU from slow 
"watch" crystal first. And then switch it to work to external one. In 
another case I got incorrect timing settings for MCU. Later, I decide to 
implement LTC6957-3 chip to "share" REFCLOCK source, since that chip has 
two equal CMOS-level outputs.
Unfortunately I have no tool to measure the phase noise and jitters on 
each setup.

> It turns out all of this is built into the AVR chip.   There is a 
> counter
> and logic to copy the current counter value to a register on a PPS 
> pulse
> raising edge.    The counter keeps running and every second its value 
> is
> trapped.
> I can connect the OCXO and the PPS directly to the AVR pin.  The AVR 
> has
> hardware (a fast comparator) to "square" a low amplitude sine wave and 
> trap
> the counter on a zero crossing.   So it looks like I can get rid of  
> ALL of
> the external chips.   The built in DAC is working well also but it 
> needs
> some external resisters and caps.
> No need for '74 FFs or '373' or counter chips.    I do get precision 
> timing
> with no time critical software, no 74xxx chips.


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