PHY 201 (Blum) 1

Other types of flip-flops and counting

See Chapter 9 and 10 in Digital principles (Tokheim)

PHY 201 (Blum) 2

Flip Flops• Flip-flops serve as the elementary

units for memory in digital systems. Two features are needed: 1. The circuit must be able to “hold”

either state (a high or low output) and not simply reflect the input at any given time.

2. But in some circumstances, we must be able to change (to “set” and “reset”) the values.

PHY 201 (Blum) 3

Remembrance of states past

• The way in which the previous state information is held is different for different types of memory

• In DRAM (dynamic random access memory), the state (1 or 0) is held by a charge (or lack thereof) remaining on a capacitor– Charges tend to leak off of capacitors, which

is why DRAM must be periodically refreshed

PHY 201 (Blum) 4

SRAM• In SRAM (static random access memory)

the history dependence is achieved via a feedback mechanism.– Feedback: the return of part of the output

to the input of a mechanism, process or system (source: Random House Dictionary).

• SRAM does not need refreshing, making it faster, but it is more expensive; typically it is reserved for caching and other high-speed situations.

PHY 201 (Blum) 5

RS Flip Flop

feedback

PHY 201 (Blum) 6

Electronics Workbench info on RS flip-flop

PHY 201 (Blum) 7

RS Flip Flop• The Q output is inverted and fed

back in as an input.• Similarly the Q’ output is inverted

and fed back in as an input. • As suggested by the names Q and

Q’, these outputs are supposed to be inverses of one another.

PHY 201 (Blum) 8

The hold operation• The S=0, R=0 is the hold “state”, the

flip flop keeps its previous outputs• Imagine Q=1 and Q’=0,

– Then Not Q’ (which is 1) is ORed with S giving a 1 for the Q output

– Then Not Q (which is 0) is ORed with R giving a 0 for the Q’ output

– The output is the same as the input (no change)

PHY 201 (Blum) 9

RS Flip Flop (hold)

0

0

1

001

01

PHY 201 (Blum) 10

The hold operation• The S=0, R=0 is the hold “state”, the

flip flop keeps its previous outputs• Imagine Q=0 and Q’=1,

– Then Not Q’ (which is 0) is ORed with S giving a 0 for the Q output

– Then Not Q (which is 1) is ORed with R giving a 1 for the Q’ output

– The output is the same as the input (no change)

PHY 201 (Blum) 11

The set operation• The S=1, R=0 is the set “state”,

the flip flop force Q=1• Imagine Q=0 and Q’=1,

– Then Not Q’ (which is 0) is ORed with S giving a 1 for the Q output

– Then Not Q (which is now 0) is ORed with R giving a 0 for the Q’ output

– The Q output is forced to be (set to) 1

PHY 201 (Blum) 12

RS Flip Flop (set)

1

01

0

01

01

1

1 00

0 1

Red input

Green initial value

Blue eventual value

PHY 201 (Blum) 13

The set operation• The S=1,R=0 is the set “state”,

the flip flop forces Q=1• Imagine Q=1 and Q’=0,

– Then Not Q’ (which is 1) is ORed with S giving a 1 for the Q output

– Then Not Q (which is 0) is ORed with R giving a 0 for the Q’ output

– The Q output is forced to be (set to) 1

PHY 201 (Blum) 14

The reset operation• The S=0,R=1 is the reset “state”,

the flip flop forces Q=0• Imagine Q=0 and Q’=1,

– Then Not Q’ (which is 0) is ORed with S giving a 0 for the Q output

– Then Not Q (which is 1) is ORed with R giving a 1 for the Q’ output

– The Q output is forced to be (reset to) 0

PHY 201 (Blum) 15

The reset operation• The S=0,R=1 is the reset “state”, the

flip flop forces Q=0• Imagine Q=1 and Q’=0,

– Then Not Q’ (which is 1) is ORed with S giving a 1 for the Q output

– Then Not Q (which is now 0) is ORed with R giving a 1 for the Q’ output

– Then Not Q’ (which is now 0) is ORed with S giving a 0 for the Q output

– The Q output is forced to be (reset to) 0

PHY 201 (Blum) 16

The undesired operation• The S=1,R=1 is the undesired “state”• Imagine Q=0 and Q’=1,

– Then Not Q’ (which is 0) is ORed with S giving a 1 for the Q output

– Then Not Q (which is now 0) is ORed with R giving a 1 for the Q’ output

– And so on– The Q and Q’ outputs are equal, which is

undesired

PHY 201 (Blum) 17

Level clocking

PHY 201 (Blum) 18

Level clocking • Adding an additional layer of AND gates and

an extra input makes the flip flop “clocked” • What used to be the S input is now S ANDed

with CLK, so the set action is now obtained only when S=1 AND CLK=1.

• This helps control when the setting occurs and keeps this action in sync with other operations occurring in the circuit.

PHY 201 (Blum) 19

Edge triggering

Two RS flip-flops in a Master-slave arrangement.

PHY 201 (Blum) 20

Edge Triggering• Feeding the outputs of one clocked

RS flip-flop into a second flip-flop in which the clock input is inverted results in an edge-triggered flip flop

• The first flip-flop acts as a level clocked flip-flop, that is, setting and resetting occur only when the CLK input is 1.

PHY 201 (Blum) 21

Edge triggering (cont.)• During this period, the second RS

flip-flop is getting the inverse of the CLK and so is in the no change state.

• When the CLK goes to 0, the first flip-flop goes into its no change state and the second flip-flop can be set or reset.

PHY 201 (Blum) 22

Edge triggering• What is setting or resetting the

second flip-flop are the outputs of the first flip-flop and they are held fixed.

• This way the inputs to the second flip flop can not vary through the course of the clock’s cycle.

• Whatever they were when the clock switched is what is important.

PHY 201 (Blum) 23

JK Flip Flop

PHY 201 (Blum) 24

Making an RS into a JK• Recall that the R=1, S=1 input into

an RS flip flop puts it into the undesired state.

• An additional feedback mechanism turns this undesired state of the RS flip flop into the “toggle” state of the JK flip flop.

PHY 201 (Blum) 25

JK• The inputs into the first AND gate

are now – J (used to be S), CLK and Q’ for the

upper – K (used to be R), CLK and Q for the

lower

• Since Q and Q’ are inverses, at most one of the AND gates will yield a 1.

PHY 201 (Blum) 26

The toggle state• If J and K and the CLK are all 1, the the

outcome is determined by Q and Q’• If Q=1 and Q’=0, then the lower AND

gate will produce a 1 and Q’ will be made equal to 1 and Q will be set to 0 (just the opposite of what they were)

• This is called the toggle state.

PHY 201 (Blum) 27

WorkBench JK Help

PHY 201 (Blum) 28

Counting

Decimal Most Least

0 0 0 0

1 0 0 1

2 0 1 0

3 0 1 1

4 1 0 0

5 1 0 1

6 1 1 0

7 1 1 1

Note that the least significant bit toggles every row.

The next bit toggles every two rows.

The higher bit toggles every four rows.

Etc.

PHY 201 (Blum) 29

Counting

Decimal Most Least

0 0 0 0

1 0 0 1

2 0 1 0

3 0 1 1

4 1 0 0

5 1 0 1

6 1 1 0

7 1 1 1

The middle bit toggles every two rows.

More precisely, the middle bit toggles as the least bit changes from 0 to 1 (the negative edge).

PHY 201 (Blum) 30

Ripple Counter

U1A

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2 U1B

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2 U2A

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2 U2B

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2

V1

0.05kHz 5 V

V25 V

U3

DCD_HEX

J1

Key = Space

PHY 201 (Blum) 31

Ripple Counter Description

• The J and K inputs of each flip-flop are all high. Thus each flip-flop will toggle when the clock input goes through a negative edge.

• The output of one JK flip-flop becomes the clock input of the next JK flip-flop. The when the first flip-flop’s output changes from 1 to 0, the second flip-flop sees a negative edge at its clock input. And so it toggles in response.

PHY 201 (Blum) 32

Ripple Counter

This flip-flop toggles most often making it the least significant bit.

U1A

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2 U1B

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2 U2A

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2 U2B

7476N

1J4 1Q 15

~1Q 141K16

~1CLR

3

1CLK1

~1PR

2

V1

0.05kHz 5 V

V25 V

U3

DCD_HEX

J1

Key = Space

PHY 201 (Blum) 33

Program Counter

• One use of a counter in a computer is the program counter. The program is stored in memory. After one line of code has been executed, the next line to be executed is typically the next word (or group of words in memory). The program counter “points” to a place in the memory, it is incremented to point to the next location and then it holds that value until the ALU is ready for the next line.