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ECEN 248: INTRODUCTION TO DIGITAL SYSTEMS DESIGN. Lecture 13 Dr. Shi Dept. of Electrical and Computer Engineering. SEQUENTIAL CIRCUITS: LATCHES. Overview. Circuits require memory to store intermediate data Sequential circuits is a circuit that has memory Flip-flop and latch - PowerPoint PPT Presentation
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ECEN 248: INTRODUCTION TO DIGITAL SYSTEMS
DESIGN
Lecture 13
Dr. Shi
Dept. of Electrical and Computer Engineering
SEQUENTIAL CIRCUITS: LATCHES
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.
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.
Sequential Circuits
Combinational circuit
Flip Flops
OutputsInputs
Nextstate Present
state
Clock
Clock: a periodic external signalClock: a periodic external signal
synchronizes when states change makes it easier to design and build large
systems
synchronizes when states change makes it easier to design and build large
systems
Cross-coupled Inverters
0
1
1
0
State 0 State 1
The system has two stable states A stable value can be stored at
inverter outputs Not possible to set a desired state
Cross-coupled Inverters (cont.)• This circuit has no stable states
Y X Z
XZ
ZY
YX
XXZYX
Time0
X
Y
Z
'1'
'0'
2 3 6 754 8 9 10 11
SR Latch
S-R Latch with NORs
1 11 00 10 0
S R Q Q’
0 1
1 0 Set
1 0Stable
0 1 Reset
0 0 Forbidden
R (reset)
Q
Q
S (set)
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
setS
R
Q
Q
S-R Latch with NORs
1 11 00 10 0
S R Q Q’
0 1
1 0 Set
1 0Stable
0 1 Reset
0 0 Forbidden
R (reset)
Q
Q
S (set)
S
R
Q
Q
tpd
S-R Latch with NORs
1 11 00 10 0
S R Q Q’
0 1
1 0 Set
1 0Stable
0 1 Reset
0 0 Forbidden
R (reset)
Q
Q
S (set)
What happens if both inputs R and S simultaneously change from 0 to 1?
Race conditions: See who runs faster
S-R Latch with NANDs
S
R
Q
Q’
0 00 11 01 1
S R Q Q’
0 1
1 0 Set
1 0 Store
0 1 Reset
1 1 Forbidden
Latch made from cross-coupled NANDsSometimes called S’-R’ latchUsually S=1 and R=1S=0 and R=0 generates unpredictable results
S-R Latches
Latch operation Latch operation enabled byenabled by
CC
Latch operation Latch operation enabled byenabled by
CC
Input sampling
enabled by gatesInput sampling
enabled by gates
NOR S-R Latch with Control Input
R’
S’Q’
Q
C’
Outputs change Outputs change when C is low:when C is low:RESET and SETRESET and SET
Otherwise: HOLDOtherwise: HOLD
Outputs change Outputs change when C is low:when C is low:RESET and SETRESET and SET
Otherwise: HOLDOtherwise: HOLD
Latch is Latch is level-sensitivelevel-sensitive, in regards to C, in regards to CLatch is Latch is level-sensitivelevel-sensitive, in regards to C, in regards to C
Only stores data if C’ = 0Only stores data if C’ = 0
S-R Latch with control input
Occasionally, desirable to avoid latch changesC = 0 disables all latch state changesControl signal enables data change when C = 1Right side of circuit same as ordinary S-R latch.
D-Latch
D Latch
Q
Q’
C
D S
R
X
Y
X Y C Q Q’
0 0 1 Q0 Q0’ Store 0 1 1 0 1 Reset1 0 1 1 0 Set1 1 1 1 1 DisallowedX X 0 Q0 Q0’ Store
0 1 0 11 1 1 0X 0 Q0 Q0’
D C Q Q’
Q0 indicates the previous state (the previously stored value)
D Latch
Q
Q’
C
D S
R
X
Y
0 1 0 11 1 1 0X 0 Q0 Q0’
D C Q Q’
Input value D is passed to output Q when C is highInput value D is ignored when C is low
D Latch
E
x
Latches on following edge of clock
E
D Q
C
x
z
z
The D latch stores data indefinitely, regardless of input D values, if C = 0
Forms basic storage element in computers
Symbols for Latches
SR latch is based on NOR gatesS’R’ latch based on NAND gatesD latch can be based on either.D latch sometimes called transparent latch
Disadvantage of Transparent Latches
D
G
Q D
G
QD
G
Q QD
G
Problems:When G=1, D passes through the entire chairWhen G=1, any glitches of D passes through the entire circuit
Problems:When G=1, D passes through the entire chairWhen G=1, any glitches of D passes through the entire circuit
Master-Slave D Flip FlopConsider two latches combined togetherOnly one C value active at a timeOutput changes on falling edge of the clock
always @(negedge clk) begin Q=D; end
0 1 0 11 1 1 0X 0 Q0 Q0’
D C Q Q’
Positive and Negative Edge D Flip-FlopD flops can be triggered on positive or negative edgeBubble before Clock (C) input indicates negative edge trigger
Lo-HiLo-Hi edge Hi-LoHi-Lo edge
Clocked D Flip-FlopStores a value on the positive edge of CInput changes at other times have no effect on output
T Flip-Flop
Positive Edge-Triggered T Flip-Flop
0 Q0 Q0’
1
Q Q’C T Created from D flopT=0 -> keep currentK resetsT=1 -> invert current
Q’0 Q0
JK Flip-Flop
Positive Edge-Triggered J-K Flip-Flop
0 0 Q0 Q0’ 0 1 0 1 1 0 1 0 1 1 TOGGLE
Q J Q’CLK K Created from D flopD = JQ’+K’QJ setsK resetsJ=K=1 invert output
Clocked J-K Flip FlopTwo data inputs, J and KJ -> set, K -> reset, if J=K=1 then toggle output
Characteristic Table
Asynchronous Inputs
Asynchronous Behavior
• Synchronous 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
Asynchronous Inputs
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
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.
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