1. ECEN 248: INTRODUCTION TO DIGITAL SYSTEMS DESIGN
Lecture 13
Dr. Shi
Dept. of Electrical and Computer Engineering

2. SEQUENTIAL CIRCUITS: LATCHES

3. Overview Circuits require memory to store intermediate data
Sequential circuits is a circuit that has memory
Flip-flop and latch
Static Random Access Memory (SRAM)
Dynamic Random Access Memory (DRAM)
Sequential circuits use a clock signal to determine when to store values.
credential:
bring a computer
die photo
wafer :
This can be an hidden slide. I just want to use this to do my own planning.
I have rearranged Culler’s lecture slides slightly and add more slides. This covers everything he covers in his first lecture (and more) but may
We will save the fun part, “ Levels of Organization,” at the end (so student can stay awake): I will show the internal stricture of the SS10/20.
Notes to Patterson: You may want to edit the slides in your section or add extra slides to taylor your needs.

4. The story so far ... Combinational circuits
No way of remembering or storing information after inputs have been removed.
To handle this, we need sequential logic capable of storing intermediate (or final) results.

5. Sequential Circuits Inputs Combinational circuit Outputs Flip Flops
Next state
Present state
Clock
Clock: a periodic external signal
synchronizes when states change
makes it easier to design and build large systems

6. Cross-coupled Inverters
The system has two stable states
A stable value can be stored at inverter outputs
Not possible to set a desired state
State 1
State 0

7. Cross-coupled Inverters (cont.)
This circuit has no stable states

8. SR Latch

9. S-R Latch with NORs S-R latch made from cross-coupled NORs
R (reset) Q
S R Q Q’
0 0 Forbidden
1 1
1 0
0 1
0 0
1 0 Set
0 1 Reset Q
0 1
Stable
S (set)
1 0
S-R latch made from cross-coupled NORs
If Q = 1, set state
If Q = 0, reset state
S=1 and R=1 generates unpredictable results
reset
set S R Q

10. S-R Latch with NORs R (reset) Q Q S (set) S R Q Q’ 0 0 Forbidden 1 1
1 1
1 0
0 1
0 0
1 0 Set
0 1 Reset Q
0 1
Stable
S (set)
1 0

11. S-R Latch with NORs
R (reset) Q
S R Q Q’
1 1
1 0
0 1
0 0
0 0 Forbidden
1 0 Set
0 1 Reset Q
0 1
Stable
S (set)
1 0
What happens if both inputs R and S simultaneously change from 0 to 1?
Race conditions: See who runs faster

12. S-R Latch with NANDs S Q Q’ R S R Q Q’ 0 0 0 1 1 0 1 1 1 1 Forbidden
0 0
0 1
1 0
1 1
1 1 Forbidden
1 0 Set
0 1 Reset
0 1
Store
1 0
Latch made from cross-coupled NANDs
Sometimes called S’-R’ latch
Usually S=1 and R=1
S=0 and R=0 generates unpredictable results

13. S-R Latches

14. NOR S-R Latch with Control Input
Latch is level-sensitive, in regards to C
Only stores data if C’ = 0 R’ Q C’ Q’
Latch operation enabled by C S’
Outputs change when C is low:
RESET and SET
Otherwise: HOLD
Input sampling enabled by gates

15. S-R Latch with control input
Occasionally, desirable to avoid latch changes
C = 0 disables all latch state changes
Control signal enables data change when C = 1
Right side of circuit same as ordinary S-R latch.

16. D-Latch

17. D Latch
Q0 indicates the previous state (the previously stored value) X S D Q C Q’ R Y
X Y C Q Q’
Q0 Q0’ Store
Reset
Set
Disallowed
X X 0 Q0 Q0’ Store
X 0 Q0 Q0’
D C Q Q’

18. D Latch D Q C Q’ X S R Y D C Q Q’ 0 1 0 1 1 1 1 0 X 0 Q0 Q0’
X 0 Q0 Q0’
D C Q Q’
Input value D is passed to output Q when C is high
Input value D is ignored when C is low

19. D Latch D Q C E E Latches on following edge of clock x x z z
The D latch stores data indefinitely, regardless of input D values, if C = 0
Forms basic storage element in computers

20. Symbols for Latches SR latch is based on NOR gates
S’R’ latch based on NAND gates
D latch can be based on either.
D latch sometimes called transparent latch

21. Disadvantage of Transparent Latches
Problems:
When G=1, D passes through the entire chair
When G=1, any glitches of D passes through the entire circuit

22. Master-Slave D Flip Flop
Consider two latches combined together
Only one C value active at a time
Output changes on falling edge of the clock
clk)
begin Q=D; end
D C Q Q’
X 0 Q0 Q0’ 22

23. Positive and Negative Edge D Flip-Flop
D flops can be triggered on positive or negative edge
Bubble before Clock (C) input indicates negative edge trigger
Lo-Hi edge
Hi-Lo edge

24. Clocked D Flip-Flop Stores a value on the positive edge of C
Input changes at other times have no effect on output 24

25. T Flip-Flop 25

26. Positive Edge-Triggered T Flip-Flop
Created from D flop
T=0 -> keep current
K resets
T=1 -> invert current T C Q Q’
Q0 Q0’ 1
Q’0 Q0

27. JK Flip-Flop 27

28. Positive Edge-Triggered J-K Flip-Flop
CLK Q Q’
Created from D flop
D = JQ’+K’Q
J sets
K resets
J=K=1  invert output
0 0
Q0 Q0’
0 1
0 1
1 0
1 0
1 1
TOGGLE 28

29. Clocked J-K Flip Flop Two data inputs, J and K
J -> set, K -> reset, if J=K=1 then toggle output
Characteristic Table 29

30. Asynchronous Inputs 30

31. Asynchronous Behavior
Effects on the output are synchronized with the CLK input.
Asynchronous behavior
Effects on the output are synchronized with the CLK.
Only used for set/reset 31

32. Asynchronous Inputs 32

33. Asynchronous Inputs Note reset signal (R) for D flip flop
If R = 0, the output Q is cleared
This event can occur at any time, regardless of the value of the CLK 33

34. Summary Flip flops are powerful storage elements
They can be constructed from gates and latches!
D flip flop is simplest and most widely used
Asynchronous inputs allow for clearing and presetting the flip flop output
Multiple flops allow for data storage
The basis of computer memory!
Combine storage and logic to make a computation circuit
Next time: Analyzing sequential circuits.
credential:
bring a computer
die photo
wafer :
This can be an hidden slide. I just want to use this to do my own planning.
I have rearranged Culler’s lecture slides slightly and add more slides. This covers everything he covers in his first lecture (and more) but may
We will save the fun part, “ Levels of Organization,” at the end (so student can stay awake): I will show the internal stricture of the SS10/20.
Notes to Patterson: You may want to edit the slides in your section or add extra slides to taylor your needs.