Review Logisim - Design a shift register
Objectives
- Review Logisim in the CS201 lab material and particularly Introduction to Logisim
- Review D-flip-flop and Multiplexers (MUX-4)
- Build a 4-bit shift register circuit
Review D-Flip-Flop
The D flip-flop(shown below) lets the input at D propogate through to Q on the positive going edge of the clock. The present state-next state chart illustrates the behavior of the D flip-flop. Basically Q(t+1) = D, which is the characteristic equation.
Characteristic Table Excitation Table ==================== =================== D Q(t+1) Operation Q(t) Q(t+1) D ==================== =================== 0 0 Reset 0 0 0 -------------------- ------------------- 1 1 Set 0 1 1 ==================== ------------------- 1 0 0 ------------------- 1 1 1 ===================
Review Multiplexers (MUX-4)
A multiplexer is a combinational logic circuit designed to select one of several input lines to a single common output line accordig to the selection control signals. For example, 2-1 Multiplexer has one control signal, 4-1 Mutiplexer has two control signals, 8-1 Multipleaxer has three control signals and so on.Here is a demonstration of 4-1 Multilexer with four input lines D0, D1, D2, D3 for your reference.
Truth Table of a 4-1 Multiplexer (MUX 4) ======================================== Input Selection Output ---------------------------------------- S1 S0 Q D0 0 0 D0 D1 0 1 D1 D2 1 0 D2 D3 1 1 D3 -----------------------------------------
Build a 4-bit Shift Register
We can build a 4-bit shift register circuit using four D-flip-flops and four multiplexers (MUX-4).We can have selection inputs control over it.
Select Inputs Function
S1 S0
------------------------
0 0 Store
0 1 Shift left
1 0 shift right
1 1 Load
Here is the design diagram:
Assignments
Copyright: Department of Computer Science, University of Regina.