Draw the circuit for a decoder and explain the working of this decoder. List the possible uses of the decoder circuit.

Decoders can detect a code and activate a single output to signal the presence of that code. Decoders have many applications, from producing system alerts in alarm systems to performing the task of driving multiple devices in microprocessor systems (e.g. memory).

Basic Binary Decoder
The function of the binary decoder is to determine if a given input combination has occurred. For example, if we wish to detect that 1011 occurs on the inputs of a digital circuit we must design a decoder which only outputs ‘1’ for this instance. Accordingly, a 4-input AND gate and an Inverter may be employed as illustrated in Figure.

The Three Bit Binary Decoder

In order to decode all possible combinations of three bits, eight (23=8) decoding logic gates are required. This type of decoder is called the 3-line-to-8-line decoder because they are 3 inputs and 8 outputs. Let us consider the design of such a decoder and assume that we require ACTIVE HIGH outputs. That is, for a given input combination the decoder outputs ‘1’. To illustrate lets consider Table 2-11 which list the decoding
functions and truth tables for the 3-line-to-8-line decoder.

Decoding functions and truth tables for the 3-line-to-8-line decoder

Now, we can develop a decoder based on each logic function and implement the SOP logic circuit. This is illustrated below in Figure

 

Internal Circuitry for 3-line-to-8-line decoder

It is far more convenient to use the logic symbol for the3-line-to-8-line decoder as illustrated in Figure below rather than repeating the complex internal circuitry each time.

Logic Symbol for 3-line-to-8-line decoder