Address: 127.127.22.u
Reference ID: ATOM
Driver ID: ATOM-PPS
Serial Port: /dev/ppsu; 9600 baud, 8-bits,
no parity
Features: tty_clk, ppsclock
This driver furnishes an interface for pulse-per-second (PPS) signals produced by a cesium clock, radio clock or related equipment. It can be used to remove accumulated jitter and retime a secondary server when synchronized to a primary server over a congested, wide-area network and before redistributing the time to local clients.
In order for this driver to work, the local clock must be set to within +-500 ms by another means, such as a radio clock or NTP itself. The PPS signal is connected via a serial port and gadget box consisting of a one-shot and RS232 level converter. When operated at 38.4 kbps with a SPARCstation IPC, this arrangement has a worst-case jitter less than 26 us.
There are three ways in which this driver can be used. The first way
uses the LDISC_CLKPPS line discipline and works only for
the baseboard serial ports of the Sun SPARCstation running SunOS 4.x. In
order to use this option, the -DPPS flag must be included in the
DEFS_LOCAL line of the distribution configuration file
./Config.local. The PPS signal is connected via the gadget
box to the carrier detect (DCD) line of a serial port. The signal is
activated by a fudge flag3 1 command following the
server command in the configuration file. This causes the
ppsclock streams module to be configured for that port and
to capture a timestamp at the on-time transition of the PPS signal. This
driver then reads the timestamp directly by a designated
ioctl() system call. This provides the most accurate time
and least jitter of any other scheme. There is no need to configure a
dedicated device for this purpose, which ordinarily is the device used
for the associated radio clock.
The second way uses the LDISC_CLKPPS line discipline and
works for any architecture supporting a serial port. In order to use
this option, the -DCLK flag must be included in the
DEFS_LOCAL line of the distribution configuration file
./Config.local. If after a few seconds this driver finds no
ppsclock module configured, it attempts to open a serial
port device /dev/pps%d, where %d is the unit
number, and assign the LDISC_CLKPPS line discipline to it. If the line
discipline fails, no harm is done except the accuracy is reduced
somewhat. The pulse generator in the gadget box must be adjusted to
produce a start bit of length 26 usec at 38400 bps. Used with the
LDISC_CLKPPS line discipline, this produces an ASCII DEL
character ('\377') followed by a timestamp at the on-time transition of
the PPS signal.
The third way involves an auxiliary radio clock driver which calls the PPS driver with a timestamp captured by that driver. This use is documented in the source code for the driver(s) involved.
time1 time
time2 time
stratum number
refid string
ATOM.
flag1 0 | 1
flag2 0 | 1
flag3 0 | 1
flag4 0 | 1