[ntp:questions] Embedded solutions

David Lord snews at lordynet.org
Tue Jul 8 16:24:36 UTC 2014


Paul wrote:
> On Mon, Jul 7, 2014 at 4:38 PM, David Lord <snews at lordynet.org> wrote:
>> I'd have to look this up but think board using Elan 486 used the
>> on chip high speed timer to timestamp the pps input at a gpio
>> port along with a custom ntpd on FreeBSD to obtain sub us offset.
> 
> Perhaps you're referring to this: http://phk.freebsd.dk/soekris/pps/
> Illustrated here: http://www.febo.com/time-freq/ntp/soekris/

Possibly but I've not checked your link.

This is what I was remembering from article by John Ackermann.

! Well, It's Right Up There, Anyway...
!
! This is a Soekris net4501 computer modified to use a TAPR
! Clock-Block frequency synthesizer and a TAPR FatPPS signal
! conditioner.
!
! When fed with a quality reference signal at 10MHz to drive the
! system clock, and a quality PPS signal to provide timetags, it
! can keep time to within a few hundred nanoseconds .....

! The secret to the net4501's timekeeping capability is its use
! of an AMD Elan SC520 CPU which has internal time registers with
! a resolution of about 100 nanoseconds. With a little hardware
! and software magic (figured out by Poul-Henning Kamp), this
! timer can be used to capture PPS timetags far more precisely,
| and with much less jitter, than the traditional method of using
! an RS-232 serial port control line as the input. .....

I considered a Soekris but couldn't find any UK supplier and cost
from the US was far too much considering the board has to be hacked
to replace xtal with TAPR input and connect the pps to gpio input.


David



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