Four-Bit Modulo-16 JK Binary Counter
An integral part of human life is competitions. Whether it is a high school baseball game, the Olympics, or a national presidential election; people compete. Essential to competitions, as it may occur to you, is the ability to keep scores so to know who is leading, who is lagging, and how much a team needs to work to win. As a result, humans are experienced score keepers.
Though not as aspiring as humans, digital electronics such as household appliances are also good at keeping counts. Your microwave oven, for example, is equipped with a timer that keeps count so you don’t eat your food cold. Many water softening units keep count of how many gallons used so to tell you when it’s time to change your filter.
What usually gives digital electronics the ability to keep track of processes is a type of microchip known simply as a counter. There are many different types of counters in use today, and different manufacturers prefer different counters for their products. In particular, our main interactive circuit depicts a counter that is able to count from zero to fifteen.
Our interactive counter is special because it is a “4-bit modulo-16 JK Flipflop counter.” All the technical sounding stuff in the quotation marks is just a way of distinguishing among counter designs. 4–bit means each number is represented by four bits, like 0011 for the number three. Modulo–16 means the counter goes through sixteen different states and so can count from zero to fifteen. And lastly, JK flipflop means we use JK flipflops to build the counter. Table 1 below shows the truth table of our counter.
| Present State | NEXT STATE |
| E = 0 | E = 1 |
| zyxw |
ZYXW |
ZYXW |
| 0000 |
0000 |
0001 |
| 0001 |
0001 |
0010 |
| 0010 |
0010 |
0011 |
| 0011 |
0011 |
0100 |
| 0100 |
0100 |
0101 |
| 0101 |
0101 |
0110 |
| 0110 |
0110 |
0111 |
| 0111 |
0111 |
1000 |
| 1000 |
1000 |
1001 |
| 1001 |
1001 |
1010 |
| 1010 |
1010 |
1011 |
| 1011 |
1011 |
1100 |
| 1100 |
1100 |
1101 |
| 1101 |
1101 |
1110 |
| 1110 |
1110 |
1111 |
| 1111 |
1111 |
0000 |
Table 2
Design and Analysis
By simply looking at the counter in the main interactive circuit, you can probably see the pattern by which we build the counter: for each bit we use one JK flipflop and one AND gate. As such, if we wanted to extend our counter from 4–bit to 7-bit, we would just add three more flipflops and three more AND gates to the circuit.
From an analysis point of view, the reason this counter works is because JK flipflops toggle when both inputs are asserted (i.e. J=1, K=1). So, for example, on every cycle of the clock the least significant bit W always toggles. The reason the other bits don’t toggle has to do with the AND gates that are in the way. The output of an AND gate is asserted only when all the inputs are asserted. As a consequence for the counter, a bit will only toggle when all the less significant bits that precede it are asserted. Play with the
circuit to verify this truth.
Boolean Function
Getting the switching expression for this counter can be easy or it can be difficult. Here is the easy non–rigorous derivation.
From the circuit (Figure 1) we can see that both J and K are always connected to the same incoming signals for all the flipflops. This means J and K will always be equal, either to 1 or to 0, but never be different from each other. Therefore, we are interested in cases where the JK flipflops toggle.
Figure 1: JK Counter with labeled AND gates
For the least significant flipflop, whose output is W, it is clear that unless E = 1 no toggling will occur. And so we say
J0 = K0 = E
For the second least significant flipflop, whose output is X, AND gate #1 is the input of J1 and K1. So since the output signal of the AND gate is 1 only when both E and W are 1, we say
J1 = K1 = E • W.
We follow the same pattern to get the output for J2 and K2. AND gate #2, which feeds into J2 and K2, is the combination of signal X and signal E • W. And so
J2 = K2 = (E • W) • X.
For J3 and K3 we use the output of AND gate #3. Therefore we have
J3 = K3 = (E • W • X) • Y.
The output of AND gate #4 does not feed into any of the flipflops in our circuit. Instead it serves as the output carry of the counter. The importance of the output carry bit is this: if we had two four–bit counters, for example, and we wanted to connect them to create a larger eight–bit counter; then we would connect the output carry of one of the four–bit counters into the Enable input of the other four–bit counter. The two connected four–bit counters would then behave as an eight–bit counter.
| J0 = K0 = E |
| J1 = K1 = E • Q0 |
| J2 = K2 = E • Q0 • Q1 |
| J3 = K3 = E • Q0 • Q1 • Q2. |
Figure 2: JK Counter with outputs re-labeled
From this pattern you can extend the counter to any number of stages you need.
