Subject: Re: NTP and PPS_SYNC option
To: john heasley <>
From: Greg Troxel <>
List: tech-kern
Date: 12/15/2005 08:10:09
I don't really understand the PPS API, but there are two separate
things lurking here:

 driving the system time off the hardware clock.  This can be
 frequency, where the system clock advances at the rate controlled by
 the PCI hardware clock, rather than the normal counters.  Or, it can
 also be phase where the PCI clock provides absolute time.  There has
 been talk of "timecounter" support being added to NetBSD, and this
 may relate.

 Using a 1 pps signal to discipline the local clock (which is
 conventional) in the kernel, or to take precise on-second snapshots
 of system clock time for NTP to use in the PLL to adjust the system

If the first is implemented and in use, I'm not sure of the value in
the second.  But, the second approach may end up being less invasive,
at least in some cases.

        Greg Troxel <>