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.
--
WBW,
V.P.
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