Meinberg DCF77C51 - Manual (Page 7)

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DCF77C51
The radio remote clock DCF77C51 has been designed for applications where only the
serial interface is used to transmit information on date and time to other devices. The
clock has an enhanced LF receiver and is manufactured using surface mounting
technology (SMT). The electronic assembly is mounted in a plastic case with four
LEDs in the front panel which let the user monitor the clock's status.
Overview
Antenna and LF Receiver
An external ferrit antenna is used to receive the signal from DCF77. Optionally, a
weather-proof antenna is available which can be mounted outdoor. A coaxial cable
which can be up to more than 100 meters in length is used to pass the antenna's output
signal to the on-board LF receiver where it is demodulated by a synchronous detector
with automatic gain control. The demodulated time marks are fed to the clock's
microprocessor.
Microprocessor Circuit
Time marks from the receiver circuit are filtered and decoded by the microprocessor.
If no errors are detected in the current time message an additional plausibility check
against the previous time message is performed. If that plausibility check passes, too,
the real time clock on the board is adjusted corresponding to the decoded time and
date. The real time clock is read periodically and it's date and time are passed to the
serial port drivers. Additionally, the microprocessor generates output pulses when the
second or minute changes. An on-board microprocessor supervisory circuit provides a
watchdog timer which lets the microprocessor recover from malfunction, along with a
power-fail comparator which resets the microprocessor if the supply voltage drops
below a specified threshold.
Buffered Real Time Clock
If the board's power supply is turned off, a gold cap capacitor on the board lets the real
time clock keep time and status for a minimum of 48 hours (typically 180 hours). This
capacitor does not need any maintenance. Alternatively, the clock can be ordered with
a lithium battery which has 10 years of life time guaranteed.
Asynchronous Serial Port
An asynchronous serial port can be used to transmit information on date and time to
other devices. The port can be set up as either RS-232 port or 20 mA current loop.