Block Diagram of Digital Clock
One of the most popular applications of counters is the digital clock. Digital Clock is a time clock that displays the time of day in hours, minutes and seconds. To construct an accurate digital clock, a highly accurate basic clock frequency is required. For battery-operated digital clocks, the basic frequency is normally obtained from a quartz-crystal oscillator. Digital clocks operated from the a.c power line can use the 50Hz power frequency as the basic clock frequency. In either case, the basic frequency has to be delivered down to a frequency of 1 Hz or 1 pulse per second (PPS). The basic diagram for a digital clock operating from 50 Hz is shown below:
The 50 Hz signal is sent through a shaping circuit to produce square pulses at the rate of 50pps. The 50pps waveform is fed into a MOD-50 Counter which is used to divide the 50 PPS down to 1pps. The 1pps signal is then fed into the seconds which are used to count and display seconds from 0 through 59. The BCD counter advances one count per second. After 9secs, the BCD counter recycles to 0, which triggers the MOD-6 counter and causes it to advance one count. This continues for 59 seconds when the MOD-6 counter is in the 101 (5) count and the BCD counter is at 1001 (9), so the display reads 59 seconds. The next pulse recycles the BCD counter to 0, which in turn recycles the MOD-6 counter to 0.