15 seconds per 86400 seconds = 1 per 5760, so maybe tweaking that
value by one might give me something fairly close. In general, I like
to see less than 1 second per day error in the system clock.
If the max error on the crystal is 50 ppm, we would expect less than
4.32 seconds of error per day, still a lot higher than I like to see,
but it seems to me the kernal could be patched to have the same error
as the crystal you are using.
Your battery-backed real-time clock has less than 1 second error per
day, at least the one I have running at 75 degF. I will see what the
error is in one week.
Your goal should be Arm boards that are within 1 second per day on the
system clock. I would imagine, with the hardware you have, you could
achive that with a little creativity, it is a matter of software.
Don.
On 4/18/06, Jesse Off <> wrote:
> Don, you could also tweak the value "5085" +/- 1 where it appears in
> the kernel source file
> ./include/asm-arm/arch-ep93xx/time.h 15
> seconds a day (174PPM) seems excessive. There is 63 PPM error
> introduced by Linux wanting 100Hz and not being able to generate it
> perfectly, and another 50 PPM crystal spec for 113 PPM (9.7
> seconds/day) total potential error.
>
> NetBSD uses a precise 64Hz clock for timekeeping and in my tests
> drifts a lot less than Linux. One might be able to change Linux to
> use that 64Hz clock if timing is really critical.
>
> //Jesse Off
>
>
> > Of course this still does not solve the problem of losing over 15
> seconds
> > per day when they want to know to the nearest second when a train
> leaves the
> > station.
> >
> > Thanks!
> >
> > Don.
>
>
>
>
>
>
>
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