Fig. 8.8 A table of logic gates and symbols

Fig. 8.8 A table of logic gates and symbols

Figure 8.9(a) shows the basic inverter. Here an input of logic 1 becomes an output of logic 0. Figure 8.9(b) shows a feedback connection from the output to the input. This feedback results in an output that oscillates between 0 and 1, at a frequency that is dependent on the propagation delay time of the inverter.

Just as the basic building blocks of electronics, e.g. transistors, diodes and resistors, can be joined together to make logic gates, so those logic gates can be used as

Fig. 8.9 An inverter

Fig. 8.9 An inverter basic building blocks to make other logic devices, such as the flip-flop. A flip-flop is a switching device which has a memory. This aspect is important in the operation of sequential switching circuits where the output depends on the present input as well as the past sequence of inputs.

The flip-flop circuit of Fig. 8.10(a) shows a network of two NOR gates connected together. The output of the second gate is fed back to become one of the inputs of the first NOR gate. There are two inputs, S and R, and two outputs, Q and 'not Q'. It is usual to show the network 'cross-coupled' as in Fig. 8.10(b).

Fig. 8.10 A flip-flop circuit

The network is said to have a memory because the output is dependent on past input sequences as well as present ones. If the inputs are restricted so that S and R cannot be logic 1 simultaneously, the outputs Q and QQ (not Q), as shown in the truth table, are always true.

Figure 8.11 shows an SR flip-flop in an automatic switching circuit for head lamps.

A number of flip-flops may be used to make other devices, such as registers for holding digital codes, e.g. 1010. (This is a four-bit binary number that represents 10 in ordinary counting.)

Fig. 8.11 Automatic headlamp circuit (Toyota)

Fig. 8.12 D-type flip-flops used as 4-bit register

Fig. 8.12 D-type flip-flops used as 4-bit register

In Fig 8.12 the inputs D0 to D3 represent data, in the form of logic 0 or 1. When the clock pulses high the data will appear on the respective outputs of the register. A register will hold data until it receives a clock pulse, it will then transfer the data to the outputs which can then be used as inputs to other devices.

Thus, from the basic silicon p-n junction, transistors are made, logic gates are made from transistors, and logic gates are then used to make flip-flops. Flip-flops can then be used to make registers and many other logic circuits.

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