In message <[EMAIL PROTECTED]>, Warner Losh writes:
>In message <60546.987709317@critter> Poul-Henning Kamp writes:
>: Use the pps driver and you get microsecond jitter with nanosecond
>: resolution.
>
>While I usually see microsecond jitter, I have seen it as high as a
>few milliseconds when the interrupt load on the machine was high and
>the cpu was slow.
>
>I setup a system for a user here for pps (pulse per second signal),
>and he was quite upset to see the occasional spike in his data. Fast
>interrupts reduced the occurance of spikes from a few an hour to one a
>day.
The BIOS misuse of SMM mode can give you jitter in the 1msec range
and there is not much you can do about it. I found out when I
clocked a motherboard with a 14.318 derived from a Rb, and timed
1Hz pulses derived from a Cs. Every 400 seconds I ran into the
SMM interrupt for about 10 seconds, and all my measurements were
late by 800-900 microseconds :-(
Intel doesn't care much for precision timing...
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
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FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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