Digital ElectronicsDigital Electronics

Flip-Flops

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 condit ion 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 f l ip f lop, referr ing to the state of the Q output.

FLIP-FLOPS

•To SET a f l ip f lop means to make Q =1

•To RESET a f l ip f lop means to make Q = 0

S

R

Q

Q

Symbol

Truth Table

SET

RESET

FLIP-FLOPS

OUT PUTQ

OUT PUTNOT Q

resetinput

setinput

+V5V

NPNNPN

1k1k

1k1k

1k1k

1k1k

•The f l ip f lop is a bi-stable mult ivibrator; i t has two stable states.

•The RS f l ip f lop 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 LAT CH Q

Q NOT

SET RES Q NOT -Q MODE 0 0 1 1 PROHIBIT ED 0 1 1 0 SET 1 0 0 1 RESET 1 1 NO CHG HOLD

RESET

SET

ACTIVE-LOW R-S FLIP-FLOPTIMING DIAGRAMS

R-S FLIP-FLOPActive-High

ACTIVE-HIGH R-S FLIP-FLOPTIMING DIAGRAMS

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

LLow

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 t ime 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 Transit ionPGT-Posit ive Going Transit ion

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

^

L Mode of operation = ?

?

L

^

H Mode of operation = ?

?

High

Set

High

Low

Hold

Reset

TEST

CLOCKED R-S FLIP-FLOPTIMING DIAGRAMS

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 T RIGGERED 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 T RIGGERED 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 f l ip f lops are easier to troubleshoot because we can stop the clock and examine one set of input and output condit ions.