from Alice’s Adventures in Wonderland, Lewis Carroll Alice is trying to find the PPS signal connector.
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.
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 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 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 PPS Clock Discipline page can be used. It operates in conjunction with another source that provides seconds numbering. The selected source is designate a prefer peer, as using the prefer option, as described on the 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.
Using the Pulse-per-Second (PPS) Signal
The PPS signal can be used in either of two ways, one using the NTP grooming and mitigation algorithms and the other using the kernel PPS signal support described in the Kernel Model for Precision Timekeeping page. The presence of kernel support is automatically detected during the NTP build process and supporting code automatically compiled. In either case, the PPS signal must be present and within nominal jitter and wander tolerances. In addition, 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 ±0.5 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 or kernel support is enabled (if available). For historical reasons, the NTP algorithms are selected by default, since performance is generally better using older, slower systems. However, performance is generally better with kernel support using newer, faster systems.