CS201 Lab: Sequential Design

Objectives


A Note


	We have worked on combinational design; that is the design of 
	circuits with no memory. This week we will start to look at 
	sequential design, which is the design of circuits with memory. 
	Sequential design will let us design much more complicated and 
	useful circuits like counters and even microprocessors.

Definition of Sequential Circuits


	Sequential circuits are circuits that have memory. 
	In other words, these are circuits where the output depends on 
	the current inputs and the previous inputs. 

	In combinational circuits the output depends only on the current inputs.

Flip Flops

Flip-Flops are basic storage/memory elements. A sequential circuit consists of a combinational circuit and storage elements (flip-flops) that together form a feedback system. We cannot continue with anymore theory until we look at the basic memory elements used in sequential design. Some of the basic flip-flops are listed below:

  1. Set-Reset flip flop

    The most basic flip flop is the S-R flip flop. (llustrate the operation of the F/F using the diagram below ).

    
    	NOR is 1 iff all inputs are 0.
    	S=1 -> Q'=0, but then Q depends on R
    	R=0, S=1 -> Q = 1, Q' = 0. called "Set"
    	R=1, S=0 -> Q = 0, Q' = 1. called "Reset" 
    
    	R=0, S=0 -> Q(t+1) = Q(t). called "Hold"
    
    
  2. Gated S-R flip flop

    Quite often we only want the flip flop to be able to change states at certain times. For example one may have a flip-flop in an alarm system that is set by a window opening thus setting off the alarm. To prevent the alarm from going off when the alarm system is shut off we would like to disable the flip-flop when the alarm system is shut off. We can do this by adding some gates to a S-R F/F to make it a gated S-R F/F. (Illustrate the operation of the gated S-R F/F using the diagram below ).

    In practice S-R F/Fs are not used that often. The most commonly used flip-flops are the D-flip flop and the JK flip-flop. We will look at each one in more details.

  3. 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
    					===================
    
    

  4. JK flip-flop

    The JK flip-flop(shown below) is very flexible and therefore allows simpler circuit designs. Most JK flip-flops allow the signal to change on the negative edge of the clock. The present state-next state chart(shown below ) illustrates the behavior of the flip-flop. The characteristic equation is Q(t+1) = JQ'(t) + K'Q(t).

                           

    
    	Characteristic Table			Excitation Table
    	==========================            ===================
    	J  K  Q(t+1)  Operation               Q(t)  Q(t+1)  J  K
    	==========================	      ===================
    	0  0  Q(t)    No Change               0     0       0  x
    	--------------------------            -------------------
    	0  1  0       Reset                   0     1       1  x
    	--------------------------            -------------------
    	1  0  1       Set  		      1     0       x  1
    	--------------------------	      -------------------
    	1  1  Q'(t)   Complement	      1     1       x  0
    	==========================	      ===================
    
    

Sequential Design Procedure


	1. State the problem clearly

	2. Determine inputs/outputs and assign letters to inputs/outputs
	   Determine state bits and assign letters to the state bits

	3. Draw a present state-next state diagram

	4. Form the state table

	5. For each input/output draw the karnaugh maps

	6. Find the simplified expressions for the inputs/outputs using 
	   Karnaugh maps

	7. Draw the logic diagram and implement the circuit in LogicWorks 
	   (a connection diagram will be needed if the circuit is to be hardwired)

Sequential Design Example

We have now covered the basic sequential circuit elements necessary to construct sequential designs.
We will learn how to interconnect JK flip-flops to make, for example, counter circuits.


	For each flip-flop we must establish what is to be connected 
	to the JK inputs to have that particular bit count properly.
	We will look at each bit one at a time.

Here is some informattion of a JK Filp-flop for your reference.


	Characteristic Table			Excitation Table
	==========================            ===================
	J  K  Q(t+1)  Operation               Q(t)  Q(t+1)  J  K
	==========================	      ===================
	0  0  Q(t)    No Change               0     0       0  x
	--------------------------            -------------------
	0  1  0       Reset                   0     1       1  x
	--------------------------            -------------------
	1  0  1       Set  		      1     0       x  1
	--------------------------	      -------------------
	1  1  Q'(t)   Complement	      1     1       x  0
	==========================	      ===================


Conclusion

 
	Sequential circuits are circuits that have memory. 
	We examined the fundamental sequential circuits like S-R flip-flops 
	and JK flip-flops and explained the design steps necessary 
	to use these as building blocks for larger sequential circuits.
  

Assignments

Copyright: Department of Computer Science, University of Regina.