Re: PSA: Clock drift and pkgin

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2023-12-13 22:32, Paul Koning wrote:
> > On Dec 13, 2023, at 4:04 PM, Johnny Billquist <bqt%softjar.se@localhost> wrote:
> >
> > > ...
> >
> > If we don't have lost clock interrupts, the only source of drift (assuming we don't have ntp or something else setting the system up for slow time adjustment), would be the chrystal itself. I know that the chrystal used in VAXen aren't perfect, and you do have some drift from that. But that is completely independent of load.
> > But a drift of a couple of % would not be unexpected.
>
> Um, yes, it would be unexpected.

We might disagree, but anyway. :)

> The crystal spec in that era was 0.01%, which was the typical spec for ordinary commercial grade crystals.  Nowadays better specs are not unusual, and watch crystals (32768 Hz) have always been made to tighter specs, but the tolerances for MHz-range crystals were traditionally governed by what could be delivered cheaply and what was good enough for radio applications.

Definitely that today's crystals are better. But 0.01% is basically 
100ppm. Today we have at 25 degrees celsius a 30ppm margin on our 32768 
Hz clocks in the equipment we're using at my work. If we look over the 
temperature range of -40 to +70, we're looking at roughly 200 ppm. And 
that is today's technology. Back in the 80s and early 90s. It wasn't 
that good. So expect more than 0.01%. But maybe I was a bit pessimistic 
on "a couple of %". But let's just say 0.1% then. Over one day, that's 
still more than one minute. Heck, even at 0.01%, we're talking multiple 
seconds in a day.

And if I just look at my 11/93, which I am running off a crystal (some 
issue with the power supply filters means I can't run it on line 
frequency at the moment). I have a drift of about 2.5 seconds per day on 
that machine.

> If you see a couple % drift, that's definitely not the crystal.  And, unlike PDP-11s, VAXen don't use a power line frequency reference for the system timekeeping.

Yes. The PDP-11 usually was/is much better at keeping time than a VAX.
That was known and observed already back in the 80s.
And that was all because of it using the power line frequency, which 
even today is very good, even if power companies no longer religiously 
tries to keep it truly at a specific frequency.

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol