Before I attempted to make the digital clock, there was a series of experiments that I did with counters. This coincided with one of my third semester courses – Digital Electronics which introduced me to the fundamental principles and logic required to design various counters.
Since explaining the logic behind these counters is a bit tricky, I have skipped them in this article. However, I have discussed the design (how to do) of such counters with their logic, state tables etc., in this link.
For all these counters, the 555 astable multivibrator (my personal favorite) provides the necessary clock signals. IC7447 is used to display the binary data, generated by the counter, to the 7 segment common anode displays.
0-7 Asynchronous T-FF Counter:
I used the IC7476 JK Flip Flop IC as a T Flip Flop and implemented a 3 bit counter. Since it is 3 bits, it counts from 0 (000) to 7 (111) and cycles back to 0.
0-5 Asynchronous T-FF Counter:
Since these experiments were in the lead up to the digital clock, I would need a 0-5 counter for the 10’s position in minutes and seconds. Applying a reset condition achieves this. This counter is essentially the 0-7 counter again but with a setting to reset the counter to 0 everytime it goes to 6. So the counter goes from 0 to 5 and instead of 6, it goes back to 0.
0-5 Synchronous D-FF Counter:
A synchronous counter is one in which each flip-flop is given the same clock signal. It is also far more complicated to design than asynchronous counters due to the requirement of combinational logic to go with the flip-flops. The positive edge triggered D Flip Flops available in IC7474 are used.
Comparing this with the 0-5 asynchronous counter, one can see that synchronous logic is obviously more complicated.eo:
0-9 Asynchronous T-FF Counter:
A 0-9 counter would require 4 bits. A 4 bit counter, without any reset condition, would count from 0 (0000) to 15 (1111). For a 0-9 count (1’s position in the minutes and seconds), the reset condition is set for 10. So the counter counts from 0 to 9 and then clocks back to 0.
0-9 Synchronous D-FF Counter:
This is one of the most complicated circuits I have ever attempted. The logic behind this counter is quite difficult, as can be seen from how dense the board looks. This clearly shows that synchronous logic is not the easy route. But one point that has to be made is that, synchronous counters can be designed to count in a required way i.e., 1 need not necessarily follow 0. Synchronous counts can be 0 then 6 then 2 then 5 and back to 0 (in a circular manner). This flexibility, not possible in asynchronous logic, comes with its inherent complexity.
0-9 Counter using 7493:
Since counters are so useful and simple, the asynchronous counter has been fabricated on single chip ICs. One such IC is the IC 7493. Since using discrete flip-flop based counters like the ones above is out of the question when making a digital clock, these counters come in handy.
0 – 99 Counter using 7490 & 7493:
Another counter chip is the IC 7490. The differences between 7490 and 7493 are subtle with the 7490 being more general purpose in my opinion. These can be connected in such a way that one counter IC feeds the necessary clock signal for the other IC (asynchronous logic) to count in a decimal way.
1 – 12 Asynchronous Counter:
Asynchronous counters can count only in a sequential way but the doesn’t mean it can be designed to come back only to 0. This 1-12 counter, representing the hours in a clock, uses the T flip flops to count to 12 and reset at 13 but all bits except the LSB (least significant bit ) are reset. In other words, the counter counts to 12 (1100) and then counts to 13 (1101). When 1101 is detected, the first 3 bits are alone reset to give (0001) or 1. Thus achieving a 1-12 counter. Since the reset happens so fast, in any of these counters the reset trigger value is never seen just like how 13 is never seen here.
Counters need not just be about numbers. In fact many of the decorative lamps that on and off in a musical way are based on such counters and logic. A special counter, called a Johnson counter provides an interesting display when 4 individual LEDs represent the 4 bit data.
These experiments with counters gave the experience and knowledge that carried on into making my breadboard level flip flop based digital clock, later on.