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

Chris Albertson albertson.chris at gmail.com
Sun Apr 13 00:30:54 EDT 2014

On Sat, Apr 12, 2014 at 9:48 AM, Magnus Danielson <
magnus at rubidium.dyndns.org> wrote:

> In fact a 5 bit counter is enough, and then a '373 to sample it. The
> enable to the 373 needs to be synchronous to the 5/10 MHz clock, so a pair
> of DFFs ('74) is needed to synchronize the PPS and another pair to create
> the single cycle enable.
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

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.

Chris Albertson
Redondo Beach, California

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