The second part is a 3-bit up counter that counts up to 6 which makes up the 10's place of the seconds. The first part is a 4-bit up counter that counts up to 9 which makes up the 1's place of the seconds. However on a breadboard feel free to build the circuit i have above or use a DS1307 module. I just used a 1 Hz pulse generator to speed up the Simulation time in Multisim. By that same token if we chain 15 Flip Flops we can divide the input signal frequency to get our 1 Hz signal. The reason that is important is because a J-K flip flop output Toggles at the Positive or Negative edge (depends on the FF) of the input signal, therefore the output is effectively at a frequency that is half of the original input. The reason 32.768 kHz is used is because it is higher than our maximum hearing frequency which is 20 kHz and it is equal to 2^15. One way we can generate a 1 Hz signal is by using a crystal oscillator circuit that generates a 32.768 kHz signal (like the one i designed above which is called a pierce oscillator), that we can then divide by using a chain of Flip Flops. in the next steps we will see how we can count those cycles to make up the seconds, minutes and hours of our clock. a 1 Hz clock is generating a pulse every second. The Concept behind a Digital Clock is that we are essentially counting up clock cycles.
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