[time-nuts] Most accurate small crystal

jimlux jimlux at earthlink.net
Thu May 27 13:24:00 UTC 2010

Poul-Henning Kamp wrote:
> In message <AANLkTimNBz5wwBJ8dR0_eNZrFPxcKWlklxFCa7cx6yV9 at mail.gmail.com>, Jim 
> Palfreyman writes:
>> I have timed the accuracy of this internal clock and have found it to be
>> pretty good so far. 17 days ago it was ticking over at 21.8 sec past the
>> minute and a quick visual inspection today and it was still *very* close to
>> that. I will confirm it properly tonight.
> Being able to model the clock of each device precisely is a very important
> part of the security of these devices.
> The important part there, is that the clock runs at a constant rate,
> but the exact value of the rate is not important, as long as the
> bounds are known.

I don't know that it needs to be all that precise..

The way it works is that your fob steps through a list of (pseudo)random 
numbers, one per minute.  The server steps through the same list.  If 
you "call in" with a particular number, the server doesn't just check 
the current step on the list, it checks some (configurable) range of 
steps on either side.  If it gets a match, it resets the server's 
"current step" to the one that matched.

The approach is very similar to the "rolling code" used in garage door 
openers and wireless key entry systems for cars.

See patent 4,885,778.

I don't know if they actually try to model the clock rate.  I think if 
you get too many "misses" you just get another fob.

The thing steps at once per minute. Say you allow up to 1 minutes error, 
and that the XO has 10 ppm error (e.g. 1 second/day).. that's a minute 
every 2 months.

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