52 lines
2.8 KiB
HTML
52 lines
2.8 KiB
HTML
<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>
|