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8/12/2019 Unit 4 NAND-NOR-Clocked Flip Flops
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Digital ElectronicsDigital Electronics
Flip-Flops
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Objectives:
Given input logic levels, state the output of an RS
NAND and RS NOR.
Given a clock signal, determine the PGT and NGT.
Define Edge Triggered and Level Triggered.
Draw a Clocked F/F with and Edge Triggered
clock input and a Level Triggered clock input.
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Logic circuits are classified into two groups:
Combinational logic circuits
Sequential logic circuits
Basic building
blocks include:
Basic building blocks
include FLIP-FLOPS:
LOGIC CIRCUITS
Logic gates make decisions
Flip Flops have memory
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SEQUENTIAL
DEVICES
S
R
Q
Q
Memory device capable of storing one bit
Memory means circuit remains in onestate after condition that caused the state
is removed.Two outputs designated Q and Q-Not thatare always opposite or complimentary.
When referring to the state of a flip flop,referring to the state of the Q output.
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FLIP-FLOPS/LATCHES
To SET a flip flop/latch meansto make Q =1
To RESET a flip flop/latchmeans to make Q = 0
S
R
Q
Q
R
Q=0
No change
S Q
1
Invalid
1
0 0
1
1Q=1
0
MODE
0RESETSET
Symbol
Truth Table
SET
RESET
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R-S LATCH
Symbols:
Truth Table:
Set
Reset
S
R
Q
Q
Normal
Comple-
mentary
FF
Mode of Operation Inputs OutputsS R Q Q
Prohibited 0 0 1 1Set 0 1 1 0Reset 1 0 0 1Hold 1 1 Q Q
NOTE: Active-LOW inputs
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R-S LATCH
Active-Low
Q NOT
Q
RESET
SET
7400
7400
NAND LATCH
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Q
Q
SET
RESET
S
R
Q
Q
S Q
1
No changeQ=1Q=00 1
R
0 01 0
1 Invalid
R-S LATCH
Active-High
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1. Logic gates make decisions, SEQUENTIAL DEVICES have_______?
2. One flip flop or latch can store how many bits?
3. What are the two outputs of a flip flop and latch?
4. When referring to the state of a sequential device,
were referring to the state of which output?
5. What does it mean to SET a device?
6. What does it mean to RESET a device?
TEST
Memory
1
Q Q-NOT
Q
Q = 1
Q = 0
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What is the mode of operation of the R-S LATCH (set, reset or hold)?
What is the output at Q from the R-S LATCH (active LOW inputs)?
Mode of operation = ?
?H
L
Low
Reset
TEST
Mode of operation = ?
?L
H
Mode of operation = ?
?H
H
High
High
Hold
Set
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CLOCKED R-S LATCH
Set
Reset
S
R
Q
Q
FF
ASYNCHRONOUS
Outputs of logic circuit can
change state anytime one ormore input changes
Set
Reset
S
R
Q
Q
FF
ClockCLK
SYNCHRONOUS
Clock signal determines exact
time at which any output canchange state
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Astable
multivibrator
Clock
Digital signal in the form of a rectangular
or square wave
A clocked flip flop changes state only when
permitted by the clock signal
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TRIGGERING OF SEQUENTIAL DEVICES
L evel-tr iggeringis the transfer of data from input tooutput of a latch anytime the clock pulse is proper voltage
level.
Edge-triggeringis the transfer of data from input to
output of a flip-flop on the rising edge (L-to-H) or fallingedge (H-to-L) of the clock pulse. Edge triggering may be
either positive-edge (L-to-H) or negative-edge (H-to-L).
Level triggering
Positive-edge triggering
Negative-edge triggering
H
Ltime
NGT-Negative Going TransitionPGT-Positive Going Transition
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CLOCKED R-S FLIP-FLOP
Symbols:
Truth Table:
Mode of operation Inputs OutputsClk S R Q Q
Hold + pulse 0 0 no changeReset + pulse 0 1 0 1Set + pulse 1 0 1 0Prohibited 1 1 0 0
NOTE:Active-High inputs
Set
Reset
S
R
Q
Q
Normal
Comple-
mentary
FF
ClockCLK
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What is the mode of operation of the clocked R-S flip-flop (set, reset, hold)?
What is the output at Q from the clocked R-S flip-flop (active HIGH inputs)?
H
^
L Mode of operation = ?
?
L
^
LMode of operation = ?
?
L
^
HMode of operation = ?
?
High
Set
High
Low
Hold
Reset
TEST
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POSITIVE EDGE TRIGGERED
R-S FLIP-FLOPTIMING DIAGRAMS
S
R
CLK
Q
Q
0
0
0 NO CHG
1
1 0
1 1
SET
RESET
ILLEGAL
CLK R S Q
C
R
SQ
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NEGATIVE EDGE TRIGGERED
R-S FLIP-FLOPSymbols:
Truth Table:
Q NOT
Q
SET
RESET
CLOCK
CLK R S
0
Q
X X
1
NO CHG
X X
0
X
0
NO CHG
X NO CHG
0 NO CHG
1
1 0
1 1
SET
RESET
ILLEGAL
S
R
CLK
Q
Q
EDGE
DETECTOR
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NEGATIVE EDGE TRIGGERED
R-S FLIP-FLOPTIMING DIAGRAMS
0
0
0 NO CHG
1
1 0
1 1
SET
RESET
ILLEGAL
CLK R S Q
C
R
SQ
S
R
CLK
Q
Q
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TEST
1. Type of device where the outputs of circuit can change state anytime
one or more input changes? LATCH (asynchronous)
2. Type of flip flop where the clock signal controls when any output can
change state? Flip Flop (synchronous)
3. What do we call a digital signal in the form of a repetitive pulse or square wave?
CLOCK
4. Which is easier to design and troubleshoot, clocked or not clocked flip flops?
Clocked flip flops are easier to troubleshoot because we canstop the clock and examine one set of input and outputconditions.