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UDCF(4)		       FreeBSD Kernel Interfaces Manual		       UDCF(4)

     udcf -- Gude ADS Expert mouseCLOCK	USB timedelta sensor

     udcf* at uhub?

     The udcf driver provides support for the Gude ADS Expert mouseCLOCK USB
     and the Expert mouseCLOCK USB II, receivers for the German	DCF77.	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-

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

     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.

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

     The udcf driver first appeared in OpenBSD 4.0.

     The udcf driver was written by Marc Balmer	<>.

     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.

FreeBSD	13.0			 June 7, 2015			  FreeBSD 13.0


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