316 lines
15 KiB
HTML
316 lines
15 KiB
HTML
|
<html><head><title>
|
||
|
|
How to Write a Reference Clock Driver
|
||
|
|
</title></head><body><h3>
|
||
|
|
How to Write a Reference Clock Driver
|
||
|
|
</h3><hr>
|
||
|
|
|
||
|
|
<h4>Description</h4>
|
||
|
|
|
||
|
|
<p>Reference clock support maintains the fiction that the clock is
|
||
|
|
actually an ordinary peer in the NTP tradition, but operating at a
|
||
|
|
synthetic stratum of zero. The entire suite of algorithms used to filter
|
||
|
|
the received data, select the best clocks or peers and combine them to
|
||
|
|
produce a local clock correction operate just like ordinary NTP peers.
|
||
|
|
In this way, defective clocks can be detected and removed from the peer
|
||
|
|
population. As no packets are exchanged with a reference clock; however,
|
||
|
|
the transmit, receive and packet procedures are replaced with separate
|
||
|
|
code to simulate them.
|
||
|
|
|
||
|
|
<p>Radio and modem reference clocks by convention have addresses in the
|
||
|
|
form <tt>127.127.<i>t</i>.<i>u</i></tt>, where <i>t</i> is the clock
|
||
|
|
type and <i>u</i> in the range 0-3 is used to distinguish multiple
|
||
|
|
instances of clocks of the same type. Most clocks require a serial port
|
||
|
|
or special bus peripheral. The particular device is normally specified
|
||
|
|
by adding a soft link <tt>/dev/device<i>d</i>d</tt> to the particular
|
||
|
|
hardware device involved, where <tt><i>d</i></tt> corresponds to the
|
||
|
|
unit number.
|
||
|
|
|
||
|
|
<p>The best way to understand how the clock drivers work is to study the
|
||
|
|
<tt>ntp_refclock.c</tt> module and one of the drivers already
|
||
|
|
implemented, such as <tt>refclock_wwvb.c</tt>. Routines
|
||
|
|
<tt>refclock_transmit()</tt> and <tt>refclock_receive()</tt> maintain
|
||
|
|
the peer variables in a state analogous to a network peer and pass
|
||
|
|
received data on through the clock filters. Routines
|
||
|
|
<tt>refclock_peer()</tt> and <tt>refclock_unpeer()</tt> are called to
|
||
|
|
initialize and terminate reference clock associations, should this ever
|
||
|
|
be necessary. A set of utility routines is included to open serial
|
||
|
|
devices, process sample data, edit input lines to extract embedded
|
||
|
|
timestamps and to perform various debugging functions.
|
||
|
|
|
||
|
|
<p>The main interface used by these routines is the
|
||
|
|
<tt>refclockproc</tt> structure, which contains for most drivers the
|
||
|
|
decimal equivalents of the year, day, month, hour, second and
|
||
|
|
millisecond/microsecond decoded from the ASCII timecode. Additional
|
||
|
|
information includes the receive timestamp, exception report, statistics
|
||
|
|
tallies, etc. The support routines are passed a pointer to the
|
||
|
|
<tt>peer</tt> structure, which is used for all peer-specific processing
|
||
|
|
and contains a pointer to the <tt>refclockproc</tt> structure, which in
|
||
|
|
turn contains a pointer to the unit structure, if used. For legacy
|
||
|
|
purposes, a table <tt>typeunit[type][unit]</tt> contains the peer
|
||
|
|
structure pointer for each configured clock type and unit.
|
||
|
|
|
||
|
|
<p>The reference clock interface supports auxiliary functions to support
|
||
|
|
in-stream timestamping, pulse-per-second (PPS) interfacing and precision
|
||
|
|
time kernel support. In most cases the drivers do not need to be aware
|
||
|
|
of them, since they are detected at autoconfigure time and loaded
|
||
|
|
automatically when the device is opened. These include the
|
||
|
|
<tt>tty_clk</tt> and <tt>ppsclock</tt> STREAMS modules and
|
||
|
|
<tt>ppsapi</tt> PPS interface described in the <a href="ldisc.htm">Line
|
||
|
|
Disciplines and Streams Modules</a> page. The <tt>tty_clk</tt> module
|
||
|
|
reduces latency errors due to the operating system and serial port code
|
||
|
|
in slower systems. The <tt>ppsclock</tt> module is an interface for the
|
||
|
|
PPS signal provided by some radios. The <tt>ppsapi</tt> PPS interface
|
||
|
|
replaces the <tt>ppsclock</tt> STREAMS module and is expected to become
|
||
|
|
the IETF standard cross-platform interface for PPS signals. In either
|
||
|
|
case, the PPS signal can be connected via a level converter/pulse
|
||
|
|
generator described in the <a href = "gadget.htm"> Gadget Box PPS Level
|
||
|
|
Converter and CHU Modem</a> page.
|
||
|
|
|
||
|
|
<p>By convention, reference clock drivers are named in the form
|
||
|
|
<tt>refclock_<i>xxxx</i>.c</tt>, where <i>xxxx</i> is a unique
|
||
|
|
string. Each driver is assigned a unique type number, long-form driver
|
||
|
|
name, short-form driver name, and device name. The existing assignments
|
||
|
|
are in the <a href="refclock.htm"> Reference Clock Drivers</a> page
|
||
|
|
and its dependencies. All drivers supported by the particular hardware
|
||
|
|
and operating system are automatically detected in the autoconfigure
|
||
|
|
phase and conditionally compiled. They are configured when the daemon is
|
||
|
|
started according to the configuration file, as described in the <a
|
||
|
|
href="config.htm"> Configuration Options </a> page.
|
||
|
|
|
||
|
|
<p>The standard clock driver interface includes a set of common support
|
||
|
|
routines some of which do such things as start and stop the device, open
|
||
|
|
the serial port, and establish special functions such as PPS signal
|
||
|
|
support. Other routines read and write data to the device and process
|
||
|
|
time values. Most drivers need only a little customizing code to, for
|
||
|
|
instance, transform idiosyncratic timecode formats to standard form,
|
||
|
|
poll the device as necessary, and handle exception conditions. A
|
||
|
|
standard interface is available for remote debugging and monitoring
|
||
|
|
programs, such as <tt>ntpq</tt> and <tt>ntpdc</tt>, as well as
|
||
|
|
the <tt>filegen</tt> facility, which can be used to record device
|
||
|
|
status on a continuous basis.
|
||
|
|
|
||
|
|
<p>The general organization of a typical clock driver includes a
|
||
|
|
receive-interrupt routine to read a timecode from the I/O buffer and
|
||
|
|
convert to internal format, generally in days, hours, minutes, seconds
|
||
|
|
and fraction. Some timecode formats include provisions for leap-second
|
||
|
|
warning and determine the clock hardware and software health. The
|
||
|
|
interrupt routine then calls <tt>refclock_process()</tt> with these data
|
||
|
|
and the timestamp captured at the on-time character of the timecode.
|
||
|
|
This routine saves each sample as received in a circular buffer, which
|
||
|
|
can store from a few up to 60 samples, in cases where the timecodes
|
||
|
|
arrive one per second.
|
||
|
|
|
||
|
|
<p>The <tt>refclock_transmit()</tt> routine in the interface is called
|
||
|
|
by the system at intervals defined by the poll interval in the peer
|
||
|
|
structure, generally 64 s. This routine in turn calls the transmit poll
|
||
|
|
routine in the driver. In the intended design, the driver calls the
|
||
|
|
<tt>refclock_receive()</tt> to process the offset samples that have
|
||
|
|
accumulated since the last poll and produce the final offset and
|
||
|
|
variance. The samples are processed by recursively discarding median
|
||
|
|
outlyers until about 60 percent of samples remain, then averaging the
|
||
|
|
surviving samples. When a reference clock must be explicitly polled to
|
||
|
|
produce a timecode, the driver can reset the poll interval so that the
|
||
|
|
poll routine is called a specified number of times at 1-s intervals.
|
||
|
|
|
||
|
|
<p>The interface code and this documentation have been developed over
|
||
|
|
some time and required not a little hard work converting old drivers,
|
||
|
|
etc. Should you find success writing a driver for a new radio or modem
|
||
|
|
service, please consider contributing it to the common good. Send the
|
||
|
|
driver file itself and patches for the other files to Dave Mills
|
||
|
|
(mills@udel.edu).
|
||
|
|
|
||
|
|
<h4>Conventions, Fudge Factors and Flags</h4>
|
||
|
|
|
||
|
|
<p>Most drivers support manual or automatic calibration for systematic
|
||
|
|
offset bias using values encoded in the <tt>fudge</tt> configuration
|
||
|
|
command. By convention, the <tt>time1</tt> value defines the calibration
|
||
|
|
offset in seconds. For those drivers that support statistics collection
|
||
|
|
using the <tt>filegen</tt> utility and the <tt>clockstats</tt> file, the
|
||
|
|
<tt>flag4</tt> switch enables the utility. When a PPS signal is
|
||
|
|
available, a special automatic calibration facility is provided. If the
|
||
|
|
<tt>flag1</tt> switch is set and the PPS signal is actively disciplining
|
||
|
|
the system time, the calibration value is automatically adjusted to
|
||
|
|
maintain a residual offset of zero. Should the PPS signal or the prefer
|
||
|
|
peer fail, the adjustment is frozen and the remaining drivers continue
|
||
|
|
to discipline the system clock with a minimum of residual error.
|
||
|
|
|
||
|
|
<h4>Files Which Need to be Changed</h4>
|
||
|
|
|
||
|
|
<p>A new reference clock implementation needs to supply, in addition to
|
||
|
|
the driver itself, several changes to existing files.
|
||
|
|
|
||
|
|
<dl>
|
||
|
|
|
||
|
|
<dt><tt>./include/ntp.h</tt>
|
||
|
|
<dd>The reference clock type defines are used in many places. Each
|
||
|
|
driver is assigned a unique type number. Unused numbers are clearly
|
||
|
|
marked in the list. A unique <tt>REFCLK_<i>xxxx</i></tt>
|
||
|
|
identification code should be recorded in the list opposite its assigned
|
||
|
|
type number.
|
||
|
|
|
||
|
|
<p><dt><tt>./libntp/clocktypes.c</tt>
|
||
|
|
<dd>The <tt>./libntp/clktype</tt> array is used by certain display
|
||
|
|
functions. A unique short-form name of the driver should be entered
|
||
|
|
together with its assigned identification code.
|
||
|
|
|
||
|
|
<p><dt><tt>./ntpd/ntp_control.c</tt>
|
||
|
|
<dd>The <tt>clocktypes</tt> array is used for certain control
|
||
|
|
message displays functions. It should be initialized with the reference
|
||
|
|
clock class assigned to the driver, as per the NTP specification
|
||
|
|
RFC-1305. See the <tt>./include/ntp_control.h</tt> header file for
|
||
|
|
the assigned classes.
|
||
|
|
|
||
|
|
<p><dt><tt>./ntpd/refclock_conf.c</tt>
|
||
|
|
<dd>This file contains a list of external structure definitions which
|
||
|
|
are conditionally defined. A new set of entries should be installed
|
||
|
|
similar to those already in the table. The <tt>refclock_conf</tt>
|
||
|
|
array is a set of pointers to transfer vectors in the individual
|
||
|
|
drivers. The external name of the transfer vector should be initialized
|
||
|
|
in correspondence with the type number.
|
||
|
|
|
||
|
|
<p><dt><tt>./acconfig.h</tt>
|
||
|
|
<dd>This is a configuration file used by the autoconfigure scheme. Add
|
||
|
|
two lines in the form:
|
||
|
|
|
||
|
|
<p><pre>
|
||
|
|
/* Define if we have a FOO clock */
|
||
|
|
#undef FOO
|
||
|
|
</pre>
|
||
|
|
|
||
|
|
<p>where FOO is the define used to cause the driver to be included in
|
||
|
|
the distribution.
|
||
|
|
|
||
|
|
<p><dt><tt>./configure.in</tt>
|
||
|
|
<dd>This is a configuration file used by the autoconfigure scheme. Add
|
||
|
|
lines similar to the following:
|
||
|
|
|
||
|
|
<p><pre>
|
||
|
|
AC_MSG_CHECKING(FOO clock_description)
|
||
|
|
AC_ARG_ENABLE(FOO, [ --enable-FOO clock_description],
|
||
|
|
[ntp_ok=$enableval], [ntp_ok=$ntp_eac])
|
||
|
|
if test "$ntp_ok" = "yes"; then
|
||
|
|
ntp_refclock=yes
|
||
|
|
AC_DEFINE(FOO)
|
||
|
|
fi
|
||
|
|
AC_MSG_RESULT($ntp_ok)
|
||
|
|
</pre>
|
||
|
|
|
||
|
|
<p>(Note that <tt>$ntp_eac</tt> is the value from <tt>--
|
||
|
|
{dis,en}able-all-clocks</tt> for non-PARSE clocks and
|
||
|
|
<tt>$ntp_eacp</tt> is the value from <tt>--{dis,en}able-parse-
|
||
|
|
clocks</tt> for PARSE clocks. See the documentation on the autoconf
|
||
|
|
and automake tools from the GNU distributions.)
|
||
|
|
|
||
|
|
<p><dt><tt>./ntpd/Makefile.am</tt>
|
||
|
|
<dd><p>This is the makefile prototype used by the autoconfigure scheme.
|
||
|
|
Add the driver file name to the entries already in the
|
||
|
|
<tt>ntpd_SOURCES</tt> list.
|
||
|
|
|
||
|
|
<p>Patches to <tt>automake-1.0</tt> are required for the
|
||
|
|
autoconfigure scripts to work properly. The file <tt>automake-
|
||
|
|
1.0.patches</tt> can be used for this purpose.
|
||
|
|
|
||
|
|
<p><dt><tt>./ntpd/Makefile.am</tt>
|
||
|
|
<dd>Do the following sequence of commands:
|
||
|
|
|
||
|
|
<p><pre>
|
||
|
|
automake
|
||
|
|
autoconf
|
||
|
|
autoheader
|
||
|
|
configure
|
||
|
|
</pre>
|
||
|
|
|
||
|
|
<p>or simply run <tt>make</tt>, which will do this command sequence
|
||
|
|
automatically.
|
||
|
|
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p><h4>Interface Routine Overview</h4>
|
||
|
|
|
||
|
|
<dl>
|
||
|
|
|
||
|
|
<dt><tt>refclock_newpeer</tt> - initialize and start a reference
|
||
|
|
clock
|
||
|
|
<dd>This routine allocates and initializes the interface structure which
|
||
|
|
supports a reference clock in the form of an ordinary NTP peer. A
|
||
|
|
driver-specific support routine completes the initialization, if used.
|
||
|
|
Default peer variables which identify the clock and establish its
|
||
|
|
reference ID and stratum are set here. It returns one if success and
|
||
|
|
zero if the clock address is invalid or already running, insufficient
|
||
|
|
resources are available or the driver declares a bum rap.
|
||
|
|
<p><dt><tt>refclock_unpeer</tt> - shut down a clock
|
||
|
|
<dd>This routine is used to shut down a clock and return its resources
|
||
|
|
to the system.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_transmit</tt> - simulate the transmit procedure
|
||
|
|
<dd>This routine implements the NTP transmit procedure for a reference
|
||
|
|
clock. This provides a mechanism to call the driver at the NTP poll
|
||
|
|
interval, as well as provides a reachability mechanism to detect a
|
||
|
|
broken radio or other madness.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_sample</tt> - process a pile of samples from the
|
||
|
|
clock
|
||
|
|
<dd>This routine converts the timecode in the form days, hours, minutes,
|
||
|
|
seconds, milliseconds/microseconds to internal timestamp format. It then
|
||
|
|
calculates the difference from the receive timestamp and assembles the
|
||
|
|
samples in a shift register. It implements a recursive median filter to
|
||
|
|
suppress spikes in the data, as well as determine a rough dispersion
|
||
|
|
estimate. A configuration constant time adjustment
|
||
|
|
<tt>fudgetime1</tt> can be added to the final offset to compensate
|
||
|
|
for various systematic errors. The routine returns one if success and
|
||
|
|
zero if failure due to invalid timecode data or very noisy offsets.
|
||
|
|
|
||
|
|
<p>Note that no provision is included for the year, as provided by some
|
||
|
|
(but not all) radio clocks. Ordinarily, the year is implicit in the Unix
|
||
|
|
file system and hardware/software clock support, so this is ordinarily
|
||
|
|
not a problem. Nevertheless, the absence of the year should be
|
||
|
|
considered more a bug than a feature and may be supported in future.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_receive</tt> - simulate the receive and packet
|
||
|
|
procedures
|
||
|
|
<dd>This routine simulates the NTP receive and packet procedures for a
|
||
|
|
reference clock. This provides a mechanism in which the ordinary NTP
|
||
|
|
filter, selection and combining algorithms can be used to suppress
|
||
|
|
misbehaving radios and to mitigate between them when more than one is
|
||
|
|
available for backup.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_gtlin</tt> - groom next input line and extract
|
||
|
|
timestamp
|
||
|
|
<dd>This routine processes the timecode received from the clock and
|
||
|
|
removes the parity bit and control characters. If a timestamp is present
|
||
|
|
in the timecode, as produced by the <tt>tty_clk</tt> line
|
||
|
|
discipline/streams module, it returns that as the timestamp; otherwise,
|
||
|
|
it returns the buffer timestamp. The routine return code is the number
|
||
|
|
of characters in the line.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_open</tt> - open serial port for reference clock
|
||
|
|
<dd>This routine opens a serial port for I/O and sets default options.
|
||
|
|
It returns the file descriptor if success and zero if failure.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_ioctl</tt> - set serial port control functions
|
||
|
|
<dd>This routine attempts to hide the internal, system-specific details
|
||
|
|
of serial ports. It can handle POSIX (<tt>termios</tt>), SYSV
|
||
|
|
(<tt>termio</tt>) and BSD (<tt>sgtty</tt>) interfaces with
|
||
|
|
varying degrees of success. The routine sets up the <tt>tty_clk,
|
||
|
|
chu_clk</tt> and <tt>ppsclock</tt> streams module/line discipline,
|
||
|
|
if compiled in the daemon and requested in the call. The routine returns
|
||
|
|
one if success and zero if failure.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_control</tt> - set and/or return clock values
|
||
|
|
<dd>This routine is used mainly for debugging. It returns designated
|
||
|
|
values from the interface structure that can be displayed using ntpdc
|
||
|
|
and the clockstat command. It can also be used to initialize
|
||
|
|
configuration variables, such as <tt>fudgetimes, fudgevalues,</tt>
|
||
|
|
reference ID and stratum.
|
||
|
|
|
||
|
|
<p><dt><tt>refclock_buginfo</tt> - return debugging info
|
||
|
|
<dd>This routine is used mainly for debugging. It returns designated
|
||
|
|
values from the interface structure that can be displayed using
|
||
|
|
<tt>ntpdc</tt> and the <tt>clkbug</tt> command.
|
||
|
|
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<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>
|