[ntp:questions] help with setting up NTP on windows with a USB GPS

Dave Hart davehart at gmail.com
Tue Dec 1 16:02:54 UTC 2009


On Dec 1, 13:58 UTC, Dave Baxter wrote:
> What colateral effects will the serialpps.sys driver have on the other
> com port, as that will be used for controling the radio (data lines
> only.)   Also, what effect would it have on, or be affected by, the CPU
> loading of the needed DSP routines that kick off once every few seconds.

The effect compared to using serial.sys is quite similar.
serialpps.sys adds a new "ioctl" which software other than ntpd won't
know about or use.  Until the first time that ioctl is called by a
given process, serial.sys behaves identically.  After the first call
to that ioctl, the interrupt handler for "modem status" line changes
(including DCD) squirrels away a system timestamp and cycle counter
value at each DCD clear->asserted transition, retrievable via the same
ioctl.

serialpps.sys timekeeping performance compared to the "user-mode PPS"
hack is relatively marginal under optimal circumstances, as David
Taylor has demonstrated.  The more loaded the box, the more likely
serialpps.sys's interrupt-time timestamping of DCD will be
substantially more accurate than ntpd doing the same thing from user
mode.

Cheers,
Dave Hart




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