2000-03-29 16:38:44 +04:00
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<HTML><HEAD><TITLE>
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A Kernel Model for Precision Timekeeping
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</TITLE></HEAD><BODY><H3>
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A Kernel Model for Precision Timekeeping
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</H3><HR>
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<P>The technical memorandum: <I>A Kernel Model for Precision
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Timekeeping</I><A
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HREF="http://www.eecis.udel.edu/~mills/database/memos/memo96b.ps">
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(PostScript) </A>describes an engineering model which implements a
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precision time-of-day function for a generic operating system. The model
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is based on the principles of disciplined oscillators using phase-lock
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loops (PLL) and frequency-lock loops (FLL) often found in the
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engineering literature. The model uses a hybrid PLL/FLL discipline
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algorithm implemented in the kernel. The hybrid loop provides automatic
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time and frequency steering with update intervals from a few seconds to
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over one day.
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<P>The hybrid PLL/FLL has been implemented in the Unix kernels for
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several workstations, including those made by Sun Microsystems, Digital
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and Hewlett Packard. Currently, the modifications are in licensed
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kernels for Digital Unix 4.0 and Sun Solaris 2.6. Since these specific
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implementations involve modifications to licensed code, they cannot be
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provided directly. Inquiries should be directed to the manufacturer's
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representatives. In addition to the licensed kernels, the hybrid PLL/FLL
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has been implemented in the nonlicensed kernels for Linux and FreeBSD.
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The engineering model for these implementations, including a simulator
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with code segments almost identical to the implementations, but not
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involving licensed code, is available via the web at <A
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HREF="http://www.eecis.udel.edu/~mills/ntp/ntp">kernel.tar.Z</A> or by
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anonymous FTP from ftp.udel.edu in the <TT>pub/ntp</TT> directory.
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<P>The model changes the way the system clock is adjusted in time and
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frequency, as well as provides mechanisms to discipline its time and
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frequency to an external precision timing source, such as a pulse-per-
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second (PPS) signal. The model incorporates a generic system-call
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interface for use with the Network Time Protocol (NTP) or similar time
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synchronization protocol. The NTP software daemons for Version 3
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<TT>xntpd</TT> and Version 4 <TT>ntpd</TT> operate with this model
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to provide synchronization limited in principle only by the accuracy and
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stability of the external timing source. There are two new system calls
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defined in the model, <TT>ntp_gettime()</TT>, which returns a structure
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including the current time, estimated error and maximum error, and
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<TT>ntp_adjtime()</TT>, which provides a means to adjust kernel
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variables, including the current time and frequency offsets. Further
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information on the calling sequences and variable definitions are in the
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<TT>/usr/include/sys/timex.h</TT> file.
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2000-04-22 20:46:49 +04:00
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<hr><a href=index.htm>Home</a><address><a
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2000-03-29 16:38:44 +04:00
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href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a>
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</address></a></body></html>
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