1. Lecture on Flip-Flops

2. Level-Sensitive Flip-Flop
Level-sensitive flip-flop (also called a “latch”)
“Q” changes whenever clock is “high”
CLK D Q
6 Transistors
CLK
CLK D Q

3. Level-Sensitive Flip-Flop
CLK D Q
NMOS transistor often replaced with “transmission gate”
“Transmission gate” includes both NMOS and PMOS transistors because NMOS good at passing “0” and PMOS good at passing “1”
6 Transistors
CLK D Q
CLK
Transmission
Gate
8 Transistors
CLK

4. Master-Slave Edge-Triggered Flip-Flop
Can connect two level-sensitive latches in Master-Slave configuration to form edge-triggered flip-flop
Master latch “catches” value of “D” at “QM” when CLK is low
Slave latch causes “Q” to change only at rising edge of CLK QM
Master
Latch
Slave
Latch D Q
CLK
2 x 8 = 16 Transistors
CLK
CLK D QM Q

5. Master-Slave Edge-Triggered Flip-FlopQM D Q
CLK
CLK
2 x 8 = 16 Transistors

6. More Efficient Master-Slave Edge-Triggered Flip-Flop
Called a C2MOS (Clocked CMOS) design
MASTER
SLAVE
VDD
VDD
CLK
CLK D Q
CLK
CLK
8 Transistors
GND
GND

7. Using Logic Gates to Build Flip-Flops
From previous slides, you can see that it’s possible to build an edge-triggered flip-flop using just 8 transistors
In a conventional “Digital Logic” course, transistor-level flip-flop designs are not usually taught
Instead, flip-flop designs using “cross-coupled” logic gates are usually taught

8. RS-Latch as Cross-Coupled NOR Gates
If R = 1, Q resets to 0
If S = 1, Q sets to 1
If RS = 00, no change
RS = 11 is not allowed because leads to oscillation R Q Q S
S R Q
0 0
No change
0 1
1 0 1
1 1
Undefined

9. Level-Sensitive RS-Latch
“Q” only changes when CLK is high (i.e. level-sensitive)
When CLK is high, behavior same as RS latch S Q
CLK Q R
CLK S R Q
X X
No change
No change 1
Undefined

10. Level-Sensitive D-Latch
Make level-sensitive D-latch from level-sensitive RS-latch by connecting S = D and R = not D
Compared to transistor version D Q
CLK Q
18 Transistors
CLK D Q
8 Transistors

11. Master-Slave Edge-Triggered Flip-Flop
Master-Slave configuration
Compared to transistor version
MASTER
SLAVE D Q
CLK
CLK
36 Transistors
VDD
VDD
CLK
CLK D Q
CLK
CLK
8 Transistors
GND
GND

12. Alternative Edge-Triggered Flip-Flop
VDD
VDD
CLK
CLK Q D Q
CLK
CLK
CLK Q
GND
GND D
24 Transistors
8 Transistors

13. JK Flip-Flop from D-Latch
Same as RS-Latch except “toggle” on 11 J D
Latch Q K Q
CLK
JK-FF J Q
CLK J K Q
X X
No change
CLK
No change K 1
Toggle

14. Toggle Flip-Flop from D-Latch
Toggles stored value if T = 1 when CLK is high D
Latch Q T
CLK
CLK T Q T
T-FF Q X
No change
No change
CLK
Toggle