1. Synchronous Sequential Circuits
5 Chapter
Synchronous Sequential Circuits

2. Logic Circuits- Review
Combinational Circuits
Sequential Circuits
Consists of logic gates whose outputs are determined from the current combination of inputs.
Performs an operation that can be specified by a set of Boolean functions.
Employ storage elements in addition to logic gates.
Outputs are a function of the inputs and the state of the storage elements.
Output depend on present value of input + past input.

3. Overview Storage Elements and Analysis
Introduction to sequential circuits
Types of sequential circuits
Storage elements
Latches
Flip-flops
Sequential circuit analysis
State tables
State diagrams

4. 5.2 Sequential Circuits A Sequential circuit contains: Inputs Outputs
Storage elements : Latches or Flip-Flops
Combinatorial Logic:
Implements a multiple-output switching function
Inputs are signals from the
outside.
Outputs are signals to the outside.
Other inputs, State or Present State, are signals from storage elements.
The remaining outputs, Next State are inputs to storage elements.
Inputs
Combinational
Logic
Outputs
State
Next
State
Storage Elements

5. 5.2 Sequential Circuits Sequential Logic
Inputs
Combina-tional
Logic
Outputs
Storage Elements
State
Next
State
Sequential Logic
Output function Outputs = g(Inputs, State)
Next state function Next State = f(Inputs, State)

6. Types of Sequential Circuits
Depends on the times at which:
storage elements observe their inputs, and
storage elements change their state
Synchronous
Behavior defined from knowledge of its signals at discrete instances of time
Storage elements observe inputs and can change state only in relation to a timing signal (clock pulses from a clock)
Asynchronous
Behavior defined from knowledge of inputs at any instant of time and the order in continuous time in which inputs change
If clock just regarded as another input, all circuits are asynchronous!

7. 5.3 Storage Elements :Latches
Maintain a binary state (0 or 1) indefinitely as long as power is delivered to the circuit
Switch states (01 or 10) when directed by an input signal
The major difference among storage elements are in the number of inputs they possess and in the manner in which the inputs affect the binary state.
Most basic storage element
Used mainly to construct Flip-Flops
Asynchronous storage circuit
Types of latches:
SR Latches
D Latches
X = X

8. 5.3 Storage Elements :Latches
Basic (NOR) S – R Latch
Cross-coupling two NOR gates gives the S – R Latch:
Graphic Symbol
Function diagram
S (set)
R (reset) Q R S Q

9. 5.3 Storage Elements :Latches
Q t+1 R S
Q t+1=Q
No change
Reset to 0 1
Set to 1
undefined
Basic (NOR) S – R Latch
Qnext=(R+Q’ current)’
Q’next=(S+Qcurrent)’
Q’ t+1
Q t+1 Q R S 1
Q t+1=Q =0 ؟
Undefined
undefined
Function table

10. 5.3 Storage Elements :Latches
What about SR=11?
Both Qnext and Q’next would become 0,which contradicts the assumption that Q and Q’ are always complemented.
Another problem is what happen if we then make S=0 and R=0 together:
Qnext = (0+0)’=1
Q’next =(0+0)’=1
But these new values go back into the NOR gates and we then get Q=Q’=0 again
Qnext = (1+1)’=0
Q’next=(1+1)’=0
So the circuit enters an infinite loop , where Q and Q’ cycle between 0 and 1 forever.

11. 5.3 Storage Elements :Latches
Clocked S - R Latch 1 S` R` S R Q C
Adding two NAND gates to the basic Ś - Ŕ NAND latch gives the clocked S – R latch:
Has a time sequence behavior similar to the basic S-R latch except that the S and R inputs are only observed when the line C is high.
C means “control” or “clock”.

12. D Latch(Transparent Latch)
5.3 Storage Elements :Latches
D Latch(Transparent Latch)
Adding an inverter to the S-R Latch, gives the D Latch:
Note that there are no “indeterminate” states! (solves the S-R latch problem) D Q C C D Q

13. D Latch(Transparent Latch)
5.3 Storage Elements :Latches
D Latch(Transparent Latch)
Qnext=((D.C)’.Q’ current)’ Q’next=((D’.C)’.Q current)’
Next state of Q D C
No change X
Q=0, reset state 1
Q=1 , set state Q D
Q(t+1) 1
Q t+1 D 1

14. 5.4 Sequential Circuits :Flip-Flops
The latch timing problem
Master-slave flip-flop
Edge-triggered flip-flop
Other flip-flops
- JK flip-flop

15. The Latch Timing Problem
In a sequential circuit, paths may exist through combinational logic:
From one storage element to another
From a storage element back to the same storage element
The combinational logic between a latch output and a latch input may be as simple as an interconnect
For a clocked D-latch, the output Q depends on the input D whenever the clock input C has value 1

16. The Latch Timing Problem (continued)Q Y
Consider the following circuit:
Suppose that initially Y = 0.
As long as C = 1, the value of Y continues to change!
The changes are based on the delay present on the loop through the connection from Y back to Y.
This behavior is clearly unacceptable.
Desired behavior: Y changes only once per clock pulse
Clock
Clock Y

17. 5.4 Sequential Circuits :Flip-Flops
Timing
A trigger: The state of a latch or flip-flop is switched by a change of the control input.

18. The Latch Timing Problem
5.4 Sequential Circuits :Flip-Flops
The Latch Timing Problem
The key of proper operation is it to trigger it only during a signal transition (negative or positive)
A solution to the latch timing problem is to break the closed path from Y to Y within the storage element
The commonly-used, path-breaking solutions replace the clocked D-latch with:
a master-slave flip-flop
an edge-triggered flip-flop

19. Master-Slave Flip-Flop
Consists of two clocked D latches in series with the clock on the second latch inverted
What happened when c=1?
The data from D input is transferred to the master .
The slave is disabled .
Any change in the input change the master output
( Y ) but can’t effect the slave output . Y C D Q

20. Master-Slave Flip-FlopC D Q Y
What happened when C=0?
The master is disabled .
The slave is enable.
The value of ( Y ) is transferred to the slave as input .
The output ( Q ) is equal ( Y ) .
Conclusion:
The output of the F-F. can change only during the transition of clock from 1 to 0 or at Trigger by the negative edge
The output is the value stored in the master stage immediately before the negative edge.
What about positive edges?

21. 5.4 Sequential Circuits :Flip-Flops
Timing

22. Graphic Symbols

23. Other flip-flops Other F-Fs can be built using D F-F
There are four operation on a F-F
- set to 1 - Reset to 0 - toggle ( complement ) of Q - nothing
There are tow F-F
- JK F-F

24. JK Flip-Flops D = JQ’ + K’Q Q t+1 K J No change Q t+1 = Q Reset to 0 1
Reset to 0 1
Set to 1
Complement Q t+1= Q’

25. Characteristic Table

26. Characteristic Equations

28. State Equation

31. Analysis This circuit consist of : 2 D F-F A and B Input x Output Y
Qt+1 = D
A= D A
B = D B

35. State Diagram

36. Input / output
state

39. Analysis
1 D F-F ( A )
2 Input X , Y
Qt+1 = D
D = A  X  y

42. Analysis
2 JK F-F (A , B)
Input x
Q t+1 = JQ’ + K’Q