85 lines
4.6 KiB
HTML
85 lines
4.6 KiB
HTML
<html><head><title>
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Pulse-per-second (PPS) Signal Interfacing
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</title></head><body><h3>
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Pulse-per-second (PPS) Signal Interfacing
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</h3><hr>
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<P>Some radio clocks and related timekeeping gear have a
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pulse-per-second (PPS) signal that can be used to discipline the local
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clock oscillator to a high degree of precision, typically to the order
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less than 20 <font face=Symbol>m</font>s in time and 0.01 PPM in
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frequency. The PPS signal can be connected in either of two ways: via
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the data leads of a serial port or via the modem control leads. Either
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way requires conversion of the PPS signal, usually at TTL levels, to
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RS232 levels, which can be done using a circuit such as described in the
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<A HREF=gadget.htm>Gadget Box PPS Level Converter and CHU Modem</A>
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page.
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<P>The data leads interface requires regenerating the PPS pulse and
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converting to RS232 signal levels, so that the pulse looks like a
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legitimate ASCII character to a serial port. The <TT>tty_clk</TT> line
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discipline/streams module inserts a timestamp following this character
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in the input data stream. The <A HREF=driver22.htm>PPS Clock
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Discipline</A> driver uses this timestamp to determine the time of
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arrival of the PPS pulse to within 26 us at 38.4 kbps while eliminating
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error due to operating system queues and service times.
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<P>The modem control leads interface requires converting to RS232 levels
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and connecting to the data carrier detect (DCD) lead of a serial port.
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The <TT>ppsclock</TT> and <TT>ppsapi</TT> streams modules capture a
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timestamp upon transition of the DCD signal. Note that the
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<TT>ppsclock</TT> module functionality has been subsumed by the new
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<TT>ppsapi</TT> interface specification, which is supported by the NTP
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daemon. As the latter is expected to become an IETF cross-platform
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standard, it should be used in new configurations. The PPS Clock
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Discipline driver reads the latest timestamp with a designated system
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call or interface routine to determine the time of arrival of the PPS
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pulse to within a few microseconds. Alternatively, if provisions have
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been made in the kernel for PPS signals, the signal is captured directly
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by the kernel serial driver without using the PPS driver.
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<P>The <TT>tty_clk</TT> module is included in the NTP software
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distribution, while the <A
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HREF=http://www.eecis.udel.edu/~mills/ntp/ntp/ppsclock.tar.Z><TT>
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ppsclock</TT></A> module can be obtained via the web at that link or by
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anonymous FTP from ftp.udel.edu in the <TT>pub/ntp</TT> directory. Both
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the <TT>tty_clk</TT> and <TT>ppsclock</TT> modules are described in the
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<A HREF=ldisc.htm>Line Disciplines and Streams Drivers</A> page.
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Directions for building the modules themselves are in the
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<TT>./kernel</TT> directory. Directions on how to configure
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<TT>ntpd</TT> to operate with these modules is described in <A
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HREF=build.htm>Building and Installing the Distribution </A>page.
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<P>The PPS driver is operates in conjunction with another reference
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clock driver that produces the PPS pulse, as described in the <A
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HREF=prefer.htm>Mitigation Rules and the <TT>prefer</TT> Keyword
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</A>page. One of the drivers described in the <A
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HREF=refclock.htm>Reference Clock Drivers</A> page furnishes
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the coarse timecode used to disambiguate the seconds numbering of the
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PPS pulse itself. The NTP daemon mitigates between the radio clock
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driver and <TT>PPS</TT> driver as described in that page in order to
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provide the most accurate time, while respecting the various types of
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equipment failures that could happen.
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<P>For the utmost time quality, some Unix system kernels support a PPS
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signal directly, as described in the <A HREF=kern.htm>A Kernel Model
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for Precision Timekeeping </A>page. Specifically, the ppsclock module
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can be used to interface the PPS signal directly to the kernel for use
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as discipline sources for both time and frequency. These sources can be
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separately enabled and monitored using the <TT>ntp_adjtime()</TT> system
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call described in that page and the <TT>/usr/include/sys/timex.h</TT>
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header file. The presence of these kernel provisions is automatically
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detected and supporting code compiled.
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<P>In some configurations may have multiple radio clocks, each with PPS
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outputs, as well as a kernel provisions for the PPS signal. In order to
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provide the highest degree of redundancy and survivability, the kernel
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PPS discipline, <TT>tty_clk</TT> module, <TT>ppsclock</TT> module and
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kernel modifications may all be in use at the same time, each backing up
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the other. The sometimes complicated mitigation rules are described in
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the Mitigation Rules and the <TT>prefer</TT> Keyword page.
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<hr><a href=index.htm>Home</a><address><a
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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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