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.
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.
|(a) An RS Flip-flop||(b) Symbol|
Table 3.1: Truth Table[Ref.6]
A very special connection within the flip-flop is the
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.
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.