OpenBSD-4.6/share/man/man4/udcf.4

.\" $OpenBSD: udcf.4,v 1.25 2008/07/05 12:18:27 mbalmer Exp $
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.\" Copyright (c) 2006 - 2008 Marc Balmer <mbalmer@openbsd.org>
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.Dd $Mdocdate: July 5 2008 $
.Dt UDCF 4
.Os
.Sh NAME
.Nm udcf
.Nd Gude ADS Expert mouseCLOCK USB timedelta sensor
.Sh SYNOPSIS
.Cd "udcf* at uhub?"
.Sh DESCRIPTION
The
.Nm
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,
.Nm
lacks support for them.
.Pp
.Nm
implements a timedelta sensor and the delta (in nanoseconds) between the
received time information and the local time can be accessed through the
.Xr sysctl 8
interface.
The clock type is indicated in the sensor description:
.Bl -tag -width "CRITICALXX" -offset indent
.It DCF77
German DCF77 time signal station
(77.5 kHz longwave transmitter located in Mainflingen near Frankfurt).
.It HBG
Swiss HBG time signal station
(75 kHz longwave transmitter located in Prangins near Geneva).
.It Unknown
The clock type has not been determined.
.El
.Pp
The quality of the timedelta is reported as the sensor status:
.Bl -tag -width "CRITICALXX" -offset indent
.It UNKNOWN
No valid time information has been received yet.
.It OK
The time information is valid and the timedelta is safe to use for
applications like
.Xr ntpd 8 .
.It 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.
.It 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 computer internal clocks have a drift.
.El
.Sh SEE ALSO
.Xr intro 4 ,
.Xr uhub 4 ,
.Xr usb 4 ,
.Xr ntpd 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Ox 4.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
driver was written by
.An Marc Balmer Aq mbalmer@openbsd.org .
.Sh CAVEATS
DCF77 uses a 77.5 kHz long wave radio signal transmitted from near Frankfurt,
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-linearly increasing
time offset which can only be roughly calculated using trigonometry.
Since the distance and transmission geometry is not known,
the clock receivers and
.Nm
driver currently make no effort to calculate this offset.
We simply assume that the offset is small.
.Pp
In Germany, the train system uses DCF77 clocks.
As the distance from Frankfurt increases,
trains can be expected to run later.