Digital Electronics Flip Flops | Types | Truth Table

Flip-flops are a rather unique digital device based upon the operation of combined logic gates. Flip-flops are an essential part of digital electronics. They are at the very heart of counters, timers, sequencing devices, and memories.

Flip-flops are semiconductor devices that are capable of assuming one of two stable states. Two common flip-flops types are the R-S and the J-K varieties.

R-S flip-flop

Look at Figure 1. Two NAND and two NOT gates have been configured to operate as a flip-flop. Two NOR gates have also been configured as a flip-flop.

The standard pin markings are R, S, Q, and $\overline{Q}$ . $\overline{Q}$ is pronounced “Q not”. This flip-flop configuration is called a set-reset flip-flop, or R-S flip-flop. The S pin is called set and the R pin is called reset.

R-S flip-flop, truth table, and two equivalent flip-flop circuits

Figure 1. Typical R-S flip-flop, truth table, and two equivalent flip-flop circuits. The shaded area in the truth table indicates a not-valid condition.

The operation of an R-S flip-flop is simple. When S is high, $\overline{Q}$ is also high. When R is high, Q is high. When both inputs are low, the output will represent the last input setting. Both inputs being high is not a valid value. The output cannot be determined if both inputs are high.

Figure 1 also shows a truth table of the R-S flip-flop. The basic principle behind the operation of a flip- flop is that the outputs are complementary. To say that two outputs are complementary means that when one output is high, the other is low. Flip-flops can retain their output condition.

A clocked R-S flip-flop uses a clock to synchronize the outputs. See Figure 2. With a clocked R-S Flip-flop, the output changes when there is a change in the R or S input and a pulse appears at the clock input.

By using devices that are clock driven, millions of parts can work together in unison to form an entire digital system. A digital clock is a string of pulses that varies continuously from high to low. The pulse train is the heartbeat of most digital systems.

The R-S flip-flop retains its output status even after the input is removed. This makes the clocked R-S flip- flop a good memory device.

Clocked R-S flip-flop and truth table.

Figure 2. Clocked R-S flip-flop and truth table. The shaded area in the truth table indicates a not-valid condition.

J-K flip-flop

The J-K flip-flop is operationally similar to the R-S flip-flop. The J-K flip-flop is clock driven like the clocked R-S flip-flop. The difference is that the J-K flip-flop will retain its output status when two lows are present at its inputs. Also, when both inputs are high, the outputs will toggle on and off. See Figure 3.

J-K Flip-flop truth table

Figure 3. With the J-K flip-flop, if both data inputs are high, the outputs toggle on and off.

D flip-flop

The D flip-flop, Figure 4, is similar to the J-K flip-flop except the D flip-flop does not require two inputs and the J-K does.

When an input signal is received at the input, the Q outputs will toggle after a clock signal is applied. The output state of Q and $\overline{Q}$ will not change state until the clock signal is received.

 D flip-flop and truth table.

Figure 4. The D flip-flop and truth table.

By comparing the truth tables of the R-S, J-K, and D flip-flops you can see that the D flip-flop never has an unknown state, unlike the R-S and J-K.

The R-S flip-flop has a not allowed state, and the J-K flip-flop has an output state that cannot be determined unless the prior state of the flip-flop is known. D flip-flops do not have these problems.

D flip-flop output Qs are always complementary. The J-K flip-flop can be made to simulate a D flip-flop by placing a NOT gate between its inputs.

Many flip-flops are used in binary counters. As you can see, the R-S, clocked R-S, D flip-flop, and the J-K flip-flop all have at least one of the two outputs high. They switch between these two states. Q is either high or low. Since the binary number system is composed of only 0s and 1s, you can see how the flip-flops might easily be used as the heart of a binary counting system.

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