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<HTML><HEAD><TITLE>
A Kernel Model for Precision Timekeeping
</TITLE></HEAD><BODY><H3>
A Kernel Model for Precision Timekeeping
</H3><HR>
<P>The technical memorandum: <I>A Kernel Model for Precision
Timekeeping</I><A
HREF="http://www.eecis.udel.edu/~mills/database/memos/memo96b.ps">
(PostScript) </A>describes an engineering model which implements a
precision time-of-day function for a generic operating system. The model
is based on the principles of disciplined oscillators using phase-lock
loops (PLL) and frequency-lock loops (FLL) often found in the
engineering literature. The model uses a hybrid PLL/FLL discipline
algorithm implemented in the kernel. The hybrid loop provides automatic
time and frequency steering with update intervals from a few seconds to
over one day.
<P>The hybrid PLL/FLL has been implemented in the Unix kernels for
several workstations, including those made by Sun Microsystems, Digital
and Hewlett Packard. Currently, the modifications are in licensed
kernels for Digital Unix 4.0 and Sun Solaris 2.6. Since these specific
implementations involve modifications to licensed code, they cannot be
provided directly. Inquiries should be directed to the manufacturer's
representatives. In addition to the licensed kernels, the hybrid PLL/FLL
has been implemented in the nonlicensed kernels for Linux and FreeBSD.
The engineering model for these implementations, including a simulator
with code segments almost identical to the implementations, but not
involving licensed code, is available via the web at <A
HREF="http://www.eecis.udel.edu/~mills/ntp/ntp">kernel.tar.Z</A> or by
anonymous FTP from ftp.udel.edu in the <TT>pub/ntp</TT> directory.
<P>The model changes the way the system clock is adjusted in time and
frequency, as well as provides mechanisms to discipline its time and
frequency to an external precision timing source, such as a pulse-per-
second (PPS) signal. The model incorporates a generic system-call
interface for use with the Network Time Protocol (NTP) or similar time
synchronization protocol. The NTP software daemons for Version 3
<TT>xntpd</TT> and Version 4 <TT>ntpd</TT> operate with this model
to provide synchronization limited in principle only by the accuracy and
stability of the external timing source. There are two new system calls
defined in the model, <TT>ntp_gettime()</TT>, which returns a structure
including the current time, estimated error and maximum error, and
<TT>ntp_adjtime()</TT>, which provides a means to adjust kernel
variables, including the current time and frequency offsets. Further
information on the calling sequences and variable definitions are in the
<TT>/usr/include/sys/timex.h</TT> file.
<hr><a href=index.htm><img align=left src=pic/home.gif></a><address><a
href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a>
</address></a></body></html>
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