DCF77 is a transmitter for atomic time in Europe. It's situated in Mainflingen, Germany, near Frankfurt. With a reception radius of 2000 km the whole of Europe is covered. DCF77 operates in the low-frequency band, at 77.5 kHz (that's what the "77" in the name refers to).
DCF77 began transmitting on 1 January 1959. At that time there was a need for a simple coding scheme, requiring only the most simple electronics to decode. The coding that was devised encodes date and time in 59 bits which are transmitted 1 bit per second. A sync second completes the minute.
You can buy “analog” DCF77 clocks, digital clocks (with or without weather station), or just DCF77 receivers. Pictured is a receiver module sold by Conrad. These modules are easy to use, though you should pay attention to 2 things:
the signal output is open collector, so you'll have to place a pullup resistor (typically 10 kΩ) between the output and Vcc. Or you can use the microcontroller's internal pull-up.
the ferrite antenna rod should be placed perpendicular to the direction of Frankfurt, Germany. The more the orientation deviates from this perpendicular line, the worse the reception will be.
On this particular module the connections are as follows (left to right):
inverted DCF77-output (negative pulses), open collector, maximum 30 V, 1mA
DCF77-output (positive pulses), open collector, maximum 30 V, 1 mA
Vcc, 2.5 V to 15 V.
At 3.3 V I measured a current consumption of 90 µA, which is very low indeed. Note that you have to add the current through the pull-up resistor to that, and at 3.3 V and 10 kΩ this is another 330 µA. If you want to keep current low it's better to use the inverted output (active low), because then this 330 µA only flows on average 15% of the time, whereas this would be 85% of the time for the normal output (active high). So, with a 10 kΩ pull-up total current would be 90 + 0.15 × 330 = 140 µA when the inverted output is used, 90 + 0.85 × 330 = 370 µA when the normal output is used.