MSF Radio Time Clock

Posted on Jan 22, 2013

The National Physics Laboratory broadcasts a time signal, previously known as the Rugby clock but now called `Time from NPL.` Its most commonly known as the MSF signal due to it originally being identified in Morse code those letters. It is broadcast from Anthorn on 60kHz. Many commercial clocks use it to automatically set themselves. I decided to convert a digital clock I bought into one set by the MSF signal. To make the project more interesting I decided to use the ATtiny2313 microcontroller with only 2k flash ROM and 128 bytes of RAM. Features: Automatically set by MSF time signal. Bright, flicker free display. Alarm with choice of 5 polyphonic melodies. Hour chime with choice of melodies and no chime between 00:00 and 08:00

MSF Radio Time Clock
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The original clock electronics used two PCBs, one for the 7 segment LEDs and another for the clock controller and support hardware. I started by tracing the connections between them and mapping out how the display worked. The display is multiplexed with simple transistor switches for the common anode. Only the middle two 7 segment displays have dots. I connected the anode transistors to the AVR with 2k2 resistors to limit current and the cathodes to a 74HC595 shift register via a ULN2803 Darlington array. I removed the controller IC and all other unused parts. The AVR uses a 12MHz crystal which is accurate enough for keeping time. In my initial tests on the bench it varied by less than one second per day, but when installed in the clock case it looses about 3-4 seconds over 24 hours. Since the time is re-synchronised with the MSF signal every night that is more than adequate. An MSF receiver module is connected to the AVR. I bought it for a few pounds and it works very well, although like most low frequency time code receivers it is extremely sensitive to noise. The multiplexed display has to be turned off while it is in use, so it is not possible to see the current time when it is being re-synchronised. I had to use a long USB cable for programming during development because the electrical noise from my PC was interfering. For details of the data format the Wikipedia page has some information about the signal...

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