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Decoders and Encoders

A decoder   is an n-input 2ⁿ output device which activates one and only one of its outputs depending on the unique input pattern, Figure 36.

  

Figure 36: Decoder.

$Y_i=m_i \cdot EN$, where m_i is the ith minterm (out of 2ⁿ total outputs). Y is active L. 

EN is ENABLE (active L).

$A_1,\ldots,A_n$ are n inputs

When EN is active, one and only one Y_i is active; all other outputs are inactive.

When EN is inactive, all outputs are inactive.

Example. Consider the 2-4 (n=2) decoder specified in the truth table:

EN	A1	A0	Y3	Y2	Y1	Y0
0	X	X	0	0	0	0
1	0	0	0	0	0	1
1	0	1	0	0	1	0
1	1	0	0	1	0	0
1	1	1	1	0	0	0

The output functions are given by

$$\begin{array}{rl} Y_0 & = \overline{A}_1 \overline{A}_0 EN \\... ... {A}_1 \overline{A}_0 EN \\ Y_3 & = {A}_1 {A}_0 EN \end{array}$$

A circuit implementation is shown in Figure 37.

  

Figure 37: Decoder circuit (n=2).

Example. Decoders can be used to implement combinational logic functions. Consider the cannonical SOP expression

$$F(A,B,C) = \sum m(1,3,4,7) .$$

A 3-8 decoder circuit implementing this function is shown in Figure 38.

  

Figure: Decoder (n=3) used to implement the logic function $F =\sum m(1,3,4,7)$.

Encoders   perform the function opposite to that of decoders. Encoders have up to 2ⁿ inputs and n outputs, and generate an n-bit word for each of the inputs.

In priority encoders,   any number of the inputs can be active, so a priority scheme selects a particular input line.

An n-m encoder block is shown in Figure 39.

  

Figure 39: Encoder.

Here, $m \leq 2^n$, EI denotes input enable, OE denotes output enable, which together with GS is used for cascading.

A truth table indicating how the priority scheme is implemented is:

EI	I2	I1	I0	GS	Y1	Y0	EO
0	X	X	X	0	0	0	0
1	1	X	X	1	1	1	0
1	0	1	X	1	1	0	0
1	0	0	1	1	0	1	0
1	0	0	0	0	0	0	1

I₂ has the highest priority, with code 11; second is I₁, with code 10; and lowest is I₀, with code 01. When the device is inactive, the code is 00.

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