Re: new NTP/PPS patch available

Harald Koenig (koenig@tat.physik.uni-tuebingen.de)
Fri, 6 Dec 1996 09:30:07 +0100 (MET)

> Now it's also in the interrupt routine (of the serial driver). I'm
> afraid you are talking about new features I haven't heard about. Is
> there an option to get the time-stamp of an interrupt? That would be
> what I need, but currently the time-stamp is only taken if it's
> definitely needed, and not for every serial interrupt.
>
> No, I was talking about adding such an option. Yes, the disadvantage is
> that you will be taking a time-stamp for every interrupt on that irq
> (instead of just when the carrier detect line changes). But the place
> where you are doing the time-stamp will be much cleaner, and result in
> more accurate times for the PPS calculations.

first for CPUs without cycle counter where you have to read the timer counter
etc. it's an absolute nono because of the slow timer access (at least 5 or more
in/outs)

also taking the time in the serial driver only will give a constant offset
of some few usecs which isn't too bad (at least for what we get right now)
and this can easily be calibrated (which you'd have to do anyway if you're
really interested in this absolute accurance).

the only variable amout of time might the polling of several UARTs when
using shared interrupts which you won't do anyway when interested in a
single microsecond...

Harald

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