Handbook of Computer Science(cs) and IT

 

Multiplexer

Multiplexing means transmitting a largo number of information units over a smaller number of channels or lines. A digital multiplexer is a combinational circuit that selects binary information from one of many input line and directs it to the output line. The selection of a particular input line is controlled by a set of selection line, Normally, there are 2ⁿ lines   and n selection lines whose bit combinations determine which input  is selected.

The Selection of inputs will be as of the above Table

When S_i = 0 and S_o = 0. Output of gate 2, 3 and 4 will be 0, and the output of gate 1 will depend upon /₀ (because S_o =1 and S_i = 1, thus output of gate 1= /_o • 1.1= /₀)

   S₁     S₁       s_o S_o

Combinational circuit for 4 x 1 multiplexer

 

Demultiplexer

A demultiPlexer is a circuit that receives information on a single line and transmit that information on one of 2ⁿ possible output lines. The output line to which input will be transmitted is selected by the selection line.

 

Sequential Circuit

A switching circuit whose output depends not only on the present state of its input but also what its input conditions have been in the past.

Sequential logic output depends on stored levels and also the input
levels. The memory elements are capable of storing binary information.

A sequential circuit is specified by a time, sequence of inputs, output and internal states.

 

 

 

 

Key Points

• There are two types of sequential circuits. This classification depends on the timing of their signals.
• Asynchronous sequential circuits
• Synchronous sequential circuits

 

Asynchronous Sequential Circuits

 

This is a system whose outputs depend upon the order in which its input variable change and can be affected at any instant of time.

 

Synchronous Sequential Circuits

These circuits have a clock signal as one of their inputs to force the change in output only  at discrete instance of time. As state transition in such circuits occurs

only when the clock value is either 0 or 1 or happen at rising or falling edge of the

clock.

The memory element used in the synchronous sequential circuits is called flip-flops.