unit 4 clocked flip flops
Post on 22-Nov-2014
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Objectives:
• Given input logice 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.
Logic circuits are classified into two groups:
Combinational logic circuits
Sequential logic circuits
Basic buildingblocks include:
Basic building blocksinclude FLIP-FLOPS:
LOGIC CIRCUITS
Logic gates make decisions
Flip Flops have memory
FLIP-FLOPS
S
R
Q
Q
•Memory device capable of storing one bit
•Memory means circuit remains in one state after condition that caused the state is removed.
•Two outputs designated Q and Q-Not that are always opposite or complimentary.
•When referring to the state of a flip flop, referring to the state of the Q output.
FLIP-FLOPS
•To SET a flip flop means to make Q =1
•To RESET a flip flop means to make Q = 0
S
R
Q
Q
Symbol
Truth Table
SET
RESET
FLIP-FLOPS
OUTPUTQ
OUTPUTNOT Q
resetinput
setinput
+V5V
NPNNPN
1k1k
1k1k
1k1k
1k1k
•The flip flop is a bi-stable multivibrator; it has two stable states.
•The RS flip flop can be implemented with transistors.
R-S FLIP-FLOP
Symbols:
Truth Table:
Set
Reset
S
R
Q
Q
Normal
Comple-mentary
FF
Mode of Operation Inputs Outputs
S R Q Q’
Prohibited 0 0 1 1 Set 0 1 1 0 Reset 1 0 0 1 Hold 1 1 Q Q’
NOTE: Active-LOW inputs
R-S FLIP-FLOPActive-Low
Q NOT
Q
RESET
SET
7400
7400
NAND LATCH
DEMORGANIZED NAND LATCH
NAND LATCH Q
Q NOT
SET RES Q NOT-Q MODE 0 0 1 1 PROHIBITED 0 1 1 0 SET 1 0 0 1 RESET 1 1 NO CHG HOLD
RESET
SET
1. Logic gates make decisions, flip flops have ____________________?
2. One flip flop can store how many bits?
3. What are the two outputs of a flip flop?
4. When referring to the state of a flip flop, we’re referring to the state
of which output?
5. What does it mean to SET a flip flop?
6. What does it mean to RESET a flip flop?
TEST
Memory
1
Q Q-NOT
Q
Q = 1
Q = 0
What is the mode of operation of the R-S flip-flop (set, reset or hold)?What is the output at Q from the R-S flip-flop (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
CLOCKED R-S FLIP-FLOP
Set
Reset
S
R
Q
Q
FF
ASYNCHRONOUS
Outputs of logic circuit can change state anytime one or more input changes
Set
Reset
S
R
Q
Q
FF
ClockCLK
SYNCHRONOUS
Clock signal determines exact time at which any output can change state
Astablemultivibrator
ClockDigital signal in the form of a rectangular or square wave
A clocked flip flop changes state only when permitted by the clock signal
TRIGGERING OF FLIP-FLOPS
• Level-triggering is the transfer of data from input to output of a flip-flop anytime the clock pulse is proper voltage level.
• Edge-triggering is the transfer of data from input to output of a flip-flop on the rising edge (L-to-H) or falling edge (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 triggeringNegative-edge triggering
H
Ltime
NGT-Negative Going TransitionPGT-Positive Going Transition
CLOCKED R-S FLIP-FLOP
Symbols:
Truth Table:Mode of operation Inputs Outputs
Clk S R Q Q’
Hold + pulse 0 0 no change Reset + pulse 0 1 0 1 Set + pulse 1 0 1 0 Prohibited 1 1 0 0
NOTE: Active-High inputs
Set
Reset
S
R
Q
Q
Normal
Comple-mentary
FF
ClockCLK
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
POSITIVE EDGE TRIGGERED R-S FLIP-FLOPSymbols:
Truth Table:
CLK SET RES Q NOT-Q MODE PGT 0 0 NO CHG HOLD PGT 0 1 0 1 RESET PGT 1 0 1 0 SET PGT 1 1 1 1 INVALID
Q NOT
Q
EDGE TRIGGERED R-S FLIP FLOPSET
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
POSITIVE EDGE TRIGGEREDR-S FLIP-FLOP
TIMING DIAGRAMS
0
0
0 NO CHG
1
1 0
1 1
SET
RESET
ILLEGAL
CLK R S Q
C
R
S
Q
NEGATIVE EDGE TRIGGERED R-S FLIP-FLOPSymbols:
Truth Table:
CLK SET RES Q NOT-Q MODE PGT 0 0 NO CHG HOLD PGT 0 1 0 1 RESET PGT 1 0 1 0 SET PGT 1 1 1 1 INVALID
Q NOT
Q
EDGE TRIGGERED R-S FLIP FLOPSET
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
EDGE DETECTOR
NEGATIVE EDGE TRIGGEREDR-S FLIP-FLOP
TIMING DIAGRAMS
0
0
0 NO CHG
1
1 0
1 1
SET
RESET
ILLEGAL
CLK R S Q
C
R
S
Q
TEST
1. Type of flip flop where the outputs of circuit can change state anytime
one or more input changes? ASYNCHRONOUS
2. Type of flip flop where the clock signal controls when any output can
change state? 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 can stop the clock and examine one set of input and output conditions.
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