[time-nuts] Brooks Shera

Attila Kinali attila at kinali.ch
Mon Mar 25 13:56:34 EDT 2013

On Mon, 25 Mar 2013 06:56:30 -0700
"Tom Van Baak" <tvb at LeapSecond.com> wrote:

> > I think with these it becomes obvious where the problem lies and what
> > the solution is.

> I realize there are many cases where clock domain considerations are
> important. But why does it matter in a device that is simply doing
> long-term 1PPS statistical sampling?
> Could one of you clock domain specialists actually spell out the
> GPSDO problem for the rest of us, nanosecond-by-nanosecond?

I'm by far not an expert, but i try to explain it anyways:

The PPS sampling is exactly the problem here.
The PPS occurs at any given time relative to the 24MHz clock. This means
setup and hold times can be violated in a synchronous circuit.
In this case, the input to the 75HCT4520 is an AND connection of the 24MHz
clock and the PPS. Due to this, the clock input to the counter in the
74HCT4520 can become very short, short enough that the minimum width of
the clock pulse is violated (>15ns). Because of this, the behaviour of the
counter is undefined and can lead not only to missing one count (which
would be caught by the PI control loop as additional noise), but the output
of the D-flip flops in the counter can switch or not switch depending
on the wheather in Guatemala. Ie the output of the counter becomes
(more or less) random. Which in turn means the lower 4 bit of the
input to the PI control loop are wrong[1]. Or in terms of time, we
might be off by +/-2^4*42ns=672ns, which is a major hit against the
PI loop (like knocking it with a sledge hammer).

Additionally, it is known that logic circuits can be caught nearly
indefinitly in a meta stable state (until the next clock pulse),
if the circuit has no provisions for this. Ie the output of the
flip flops would not be 1 or 0, but something inbetween, with some
negative effects on the circuitry downstream.


			Attila Kinali

[1] I ignore the additional +/-1 of bit 5 for clarity
The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
		-- unknown

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