UDCF(4)                   OpenBSD Programmer's Manual                  UDCF(4)

NAME
     udcf - Gude ADS Expert mouseCLOCK USB timedelta sensor

SYNOPSIS
     udcf* at uhub?

DESCRIPTION
     The udcf driver provides support for the Gude ADS Expert mouseCLOCK USB
     and the Expert mouseCLOCK USB II, receivers for the German DCF77 and the
     Swiss HBG time signal stations.  While receivers for the British MSF time
     signal station are also being made, udcf lacks support for them.

     udcf implements a timedelta sensor and the delta (in nanoseconds) between
     the received time information and the local time can be accessed through
     the sysctl(8) interface.  The clock type is indicated in the sensor de-
     scription:

           DCF77       German DCF77 time signal station (77.5 kHz longwave
                       transmitter located in Mainflingen near Frankfurt).

           HBG         Swiss HBG time signal station (75 kHz longwave trans-
                       mitter located in Prangins near Geneva).

           Unknown     The clock type has not been determined.

     The quality of the timedelta is reported as the sensor status:

           UNKNOWN     No valid time information has been received yet.

           OK          The time information is valid and the timedelta is safe
                       to use for applications like ntpd(8).

           WARN        The time information is still valid, but no new time
                       information has been decoded for at least 5 minutes due
                       to a reception or parity error.  The timedelta should
                       be used with care.

           CRITICAL    No valid time information has been received for more
                       than 15 minutes since the sensor state degraded from OK
                       to WARN.  This is an indication that hardware should be
                       checked to see if it is still functional.  The
                       timedelta will eventually degrade to a lie as all com-
                       puter internal clocks have a drift.

SEE ALSO
     intro(4), uhub(4), usb(4), ntpd(8), sysctl(8)

HISTORY
     The udcf driver first appeared in OpenBSD 4.0.

AUTHORS
     The udcf driver was written by Marc Balmer <mbalmer@openbsd.org>.

CAVEATS
     DCF77 uses a 77.5 kHz long wave radio signal transmitted from near Frank-
     furt, Germany.  Up to about 900 km, the radio signal can travel directly
     to the receiver, providing a linearly increasing time offset based on
     distance.  Due to the curvature of the Earth, beyond this distance the
     signal must bounce off the lower ionosphere (residing at approximately 70
     km elevation during the day, and 90 km at night), thus causing a non-lin-
     early increasing time offset which can only be roughly calculated using
     trigonometry.  Since the distance and transmission geometry is not known,
     the clock receivers and udcf driver currently make no effort to calculate
     this offset.  We simply assume that the offset is small.

     In Germany, the train system uses DCF77 clocks.  As the distance from
     Frankfurt increases, trains can be expected to run later.

OpenBSD 4.7                      July 5, 2008                                2
