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= Pulse-Per-Second (PPS) Signal Interfacing
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|image:pic/alice32.gif[]|
{millshome}pictures.html[from 'Alice's Adventures in Wonderland', Lewis Carroll]
Alice is trying to find the PPS signal connector.
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== Related Links
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== Table of Contents
* link:#intro[Introduction]
* link:#opsys[Operating System Support]
* link:#use[Using the Pulse-per-Second (PPS) Signal]
'''''
[[intro]]
== Introduction
Most conventional time sources (radio clocks and GPSes) are connected
using a serial or USB port operating at speeds of 9600 bps. The
accuracy using typical clock-radio timecode formats, where the on-time
epoch is indicated by a designated ASCII character such as
carriage-return +<cr>+, is normally limited to 100 μs; NMEA streams
from GPSes have a similar limit. Some are worse; the SiRF line of
consumer-grade GPSes, for example, has a long-period wander of over
100 ms.
Using carefully crafted averaging techniques, the NTP algorithms can
whittle this down to a few tens of microseconds. However, some time
sources produce a pulse-per-second (PPS) signal which can be used to
improve the accuracy to a few microseconds. This page describes the
hardware and software necessary for ntpd to use the PPS signal.
The simplest way to collect a PPS signal is from a GPS over a serial
handshake pin, typically DCD; Linux supports this. On FreeBSD systems
(with the PPS_SYNC and pps kernel options) it can be connected
directly to the ACK pin of a parallel port. Clock radios are more
complicated; their PPS signal levels are usually incompatible with
serial port interface signals. Note that NTPsec no
longer supports connection via the RD pin of a serial port.
[[opsys]]
== Operating System Support
Both the serial and parallel port connection require operating system
support, which is available in a few operating systems, including
FreeBSD, Linux, and Solaris. The kernel interface described on the
link:kernpps.html[PPSAPI Interface for Precision Time Signals] page is
the only interface currently supported. Older PPS interfaces based on
the +ppsclock+ and +tty_clk+ streams modules are no longer supported.
The interface consists of the +timepps.h+ header file, which should be
in the /usr/include/ or /usr/include/sys directory of your file
system.
== PPS Driver
PPS support is built into some drivers, in particular the NMEA
driver, and may be added to other drivers in future. Alternatively,
the PPS driver described on the link:driver_pps.html[PPS Clock
Discipline] page can be used. It operates in conjunction with another
source that provides seconds numbering. The selected source is
designated as a 'prefer peer' using the +prefer+ option, as described on
the link:prefer.html[Mitigation Rules and the +prefer+ Keyword]
page. The prefer peer is ordinarily the radio clock that provides the
PPS signal, but in principle another radio clock or even a remote
Internet server could be designated preferred. Note that the +pps+
configuration command has been obsoleted by this driver.
[[use]]
== Using the Pulse-per-Second (PPS) Signal
The PPS signal can be used in three ways: using the NTP
grooming and mitigation algorithms, or using the kernel PPS
signal support described in the link:kern.html[Kernel Model for
Precision Timekeeping] page, or via the shared-memory interface. The presence
of kernel support is automatically detected during the NTP build process and
supporting code automatically compiled. Regardles of mechanism, the PPS signal
must be present and within nominal jitter and wander tolerances. Additionally,
the prefer peer must be a truechimer; that is, survive the sanity checks and
intersection algorithm. Finally, the offset of the system clock relative
to the prefer peer must be within ±0.4 s. The kernel maintains a
watchdog timer for the PPS signal; if the signal has not been heard or
is out of tolerance for more than some interval, currently two minutes,
the kernel discipline is disabled and operation continues as if it were
not present.
An option flag in the driver determines whether the NTP algorithms, the
kernel support, or the shared-memory interface is enabled (depending upon
availability). For historical reasons, the NTP algorithms are selected by
default, since performance is generally better when using older, slower
systems. However, performance is generally better with kernel support or
shared-memory support when using newer, faster systems.
'''''
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