Demultiplexers

Demultiplexers 

AU: May-11

• A demultiplexer is a circuit that receives information on a single line and transmits this information on one of 2ⁿ possible output lines.

• The selection of specific output line is controlled by the values of n selection lines.

• The Fig. 1.18.1 shows the block diagram of a demultiplexer. It has one input data line, 2ⁿ output lines, n select lines and one enable input.

Differentiate between Multiplexer and Demultiplexer

Types of Demultiplexers

1: 4 Demultiplexer

• Fig.1.18.2 shows 1 : 4 demultiplexer.

• The single input variable D_in has a path to all four outputs, but the input information is directed to only one of the output lines depending on the select inputs.

• Enable input should be high to enable demultiplexer.

1 :8 Demultiplexer

• The Fig. 1.18.3 shows 1 : 8 demultiplexer.

• The single input data D_in has a path to all eight outputs, but the input information is directed to only one of the output lines depending on the select inputs.

Expanding Demultiplexer

• To provide larger output needs we can cascade two or more demultiplexer to get demultiplexer with more number of output lines. Such a connection is known as demultiplexer tree.

Example 1.18.1 Design 1: 8 demultiplexer using two 1 : 4 demultiplexers.

Solution:

Step 1: Connect D_in signal to in D_in put of both the demultiplexers.

Step 2: Connect select lines B and C to select lines S₁ and S₀ of the both demultiplexers, respectively.

Step 3: Connect most significant select line (A) such that when A = 0 DEMUX 1 is enabled and when A = 1 DEMUX 2 is enabled.

Example 1.18.2 Implement 1: 16 demultiplexer using 1 : 4 demultiplexers.

Solution: The 1: 16 demultiplexer has 16 outputs. To select one of the 16 output, the circuit needs 4 (2⁴= 16) select lines. Each 1 : 4 demultiplexer requires 2 select lines.

Step 1: Connect two least significant select lines (S₁, S₀) to select lines of four 4: 1 demultiplexer.

Step 2: Connect one more 4 : 1 demultiplexer such that its four outputs are routed to the data inputs of the four demultiplexers. Connect higher select lines (S₃ ,S₂) to the select lines of this demultiplexer.

Example for Practice

Example 1.18.3: Construct 1 to 32 demultiplexer using two 74 X 154 ICs.

(1: 16 DEMUX).

Implementation of Combinational Logic using Demultiplexer

Example 1.18.4 Implement full subtractor using demultiplexer.

Solution:

Step 1: Write the truth table of full subtractor.

Step 2: Represent output of full-subtractors in minterm form.

• For full subtractor, difference D function can be written as

D = f(A, B, C) =∑m (1, 2, 4, 7) and B_out function can be written as,

B_out =F (A, B, C)= ∑m (1, 2, 3, 7)

Step 3: Logically OR the outputs corresponding to minterms.

With D_in input 1, demultiplexer gives minterms at the output so by logically ORing required minterms we implement Boolean functions for full subtractor.

• Fig. 1.18.6 shows the implementation of full subtractor using demultiplexer.

Example 1.18.5 Implement the following functions using demultiplexer :

f₁ (A, B, C) = ∑m(0, 3, 7)

f₂ (A, B, C) =∑m(1, 2, 5).

Solution :

f₁ (A, B, C) = Σm (0, 3, 7)

f₂ (A, B, C) = Σ m (1, 2, 5)

Implementation using 1: 8 demultiplexer.

Examples for Practice

Example 1.18.6 Implement full adder using demultiplexer. AU: May-11, Marks 5

Example 1.18.7 Implement the following functions using demultiplexer

f₁ (A, B, C) = ∑m (1, 5, 7), f₂ (A, B, C) = ∑m (3, 6, 7)

Applications of Demultiplexer

1. It can be used as a decoder.

2. It can be used as a data distributor.

3. It is used in time division multiplexing at the receiving end as a data separator.

4. It can be used to implement Boolean expressions.

Demultiplexer ICs