Logic circuits are classified in two broad categories. What described thus far have been grouped as combinational logic circuits. The other category is the sequential logic circuits which are extremely valuable because of their memory characteristics. The basic building block of the sequential logic circuit is the flip-flop (F/F). Flip-flops are also called bistable multivibrators or latches.

There are several types of flip-flops : the RS F/F, the D F/F, the JK F/F and the T F/F. Among these flip-flops there are also clocked F/F and master-slave F/F. Only the simplest RS flip-flop built from NAND gates will be covered in this introductory course. The suggested applications are taken from Section 2 of the I.S.M.E.C. Electronics 13-16 Projects Manual [Ref.5]. Students must try to connect and test all the circuits in this part during their practical sessions.

RS Flip-flop

The simplest RS flip-flop can be constructed from logic gates, either two NAND gates or two NOR gates. Figure 3.1 shows an RS flip-flop wired from two NAND gates. It has two inputs S and R and two complementary output Q and which are exactly opposite to each other.

Figure 3.1
(a) An RS Flip-flop (b) Symbol

Table 3.1: Truth Table

Referring to Table 3.1, in the prohibited state both outputs are 1. This condition is not used on the RS flip-flop. The set condition means setting the output Q to 1. Likewise, the reset condition means resetting (clearing) the output Q to 0. The last row shows the disabled, or hold, condition of the RS flip-flop. The outputs remain as they were before the hold condition existed. There is no change in the outputs from the previous states, i.e. the flip-flop memorizes the previous condition.

Example 3.1

A very special connection within the flip-flop is the feedback path from the output of one NAND gate into the input of other gate. This characteristic feedback determines the truth table of the flip-flop as well as its memory property.
For instance, let us assume that S = 1, R = 1, Q = 0 and = 1 initially. If S goes low, the output of NAND gate 1 goes high. Both inputs of gate 2 are high so that = 0. This low signal is fed back to an input of gate 1 and keeps Q = 1 even if S =1 again. For the same reason, further change of S to 0 produce no effect.

Problem 3.1

Describe and explain what happens to the flip-flop in example 3.1 if R goes low, then high, then low and then high again. That is, two low pulses are fed to R after the flip-flop has finished the changes described in example 3.1.

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