Sequential Circuits

1. Sequential Circuits

2. Sequential Circuits
• Consist of a combinational circuit to which storage
elements are connected to form a feedback path
• State: –the state of the memory devices now, also
called current state
• Next states and outputs are functions of inputs and
present states of storage elements

3. Two Types of Sequential Circuits
• Asynchronous sequential circuit
• Depends upon the input signals at any instant of time and
their change order
• May have better performance but hard to design
• Synchronous sequential circuit
• Defined from the knowledge of its signals at discrete instants
of time
• Much easier to design (preferred design style)
• Synchronized by a periodic train of clock pulses

4. SYNCHRONOUS SEQUENTIAL CIRCUITS

5. Memory elements
• Latch -—a level-sensitive memory element
• SR latches
• D latches
• Flip-Flop —- an edge-triggered memory element
• Master-slave flip-flop
• Edge-triggered flip-flop
• RAM and ROM —a mass memory element

6. Latches
• A latch is binary storage element
• Can store a 0 or 1
• The most basic memory
• Easy to build
• Built with gates (NORs, NANDs, NOT)

7. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0
0
1
0
0
0 1 Q = Q0
Initial Value

8. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1
1
0
0
0
1 0 Q = Q0
Q = Q0

9. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1 1 0
0 1 0 0
0
1
1
0
1 Q = 0
Q = Q0

10. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1 1 0
0 1 0 0 1
0 1 11
0
1
0
0 1
Q = 0
Q = Q0
Q = 0

11. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1 1 0
0 1 0 0 1
0 1 1 0 1
1 0 0
0
1
0
1
1 0
Q = 0
Q = Q0
Q = 1

12. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1 1 0
0 1 0 0 1
0 1 1 0 1
1 0 0 1 0
1 0 1
1
0
0
1
1 0
Q = 0
Q = Q0
Q = 1
Q = 1

13. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1 1 0
0 1 0 0 1
0 1 1 0 1
1 0 0 1 0
1 0 1 1 0
1 1 0
0
1
1
1
0 0
Q = 0
Q = Q0
Q = 1
Q = Q’
0

14. Latches
• SR LatchR
S
Q
Q
S R Q0 Q Q’
0 0 0 0 1
0 0 1 1 0
0 1 0 0 1
0 1 1 0 1
1 0 0 1 0
1 0 1 1 0
1 1 0 0 0
1 1 1
1
0
1
1
0 0
Q = 0
Q = Q0
Q = 1
Q = Q’
0
Q = Q’

15. SR Latch
R
S
Q
Q
S R Q
0 0 Q0
0 1 0
1 0 1
1 1 Q=Q’=0
No change
Reset
Set
Invalid
S
R
Q
Q
S R Q
0 0 Q=Q’=1
0 1 1
1 0 0
1 1 Q0
Invalid
Set
Reset
No change

16. FlipFlops(Controlled Latches)
• Digital Devices consist of logic gates can preserve 1 bit (1
or 0)
• Types:
•S-R FlipFlop
•D FlipFlop
•J-K FlipFlop
•T Flipflop

17. S-R FlipFlops
• SR Latch with Control Input
C S R Q
0 x x Q0
1 0 0 Q0
1 0 1 0
1 1 0 1
1 1 1 Q=Q’
No change
No change
Reset
Set
Invalid
S
R
Q
Q
S
R
C
S
R
Q
Q
S
R
C

18. D FlipFlop
D Q
QCLK
I/P O/P
CLR

19. J-K Flip-Flops
D = JQ’ + K’Q
J Q
QK

20. T Flip-Flops
D = TQ’ + T’Q = T  Q
J Q
QK
T D Q
Q
T
D = JQ’ + K’Q
T Q
Q

21. THANK U