Sequential Logic Circuits

Output depends on current inputs and past outputs

• State : Collection of state variables

Sequential logic circuits are also called as finite state machines

Sequential Circuit types

• Feedback sequential circuits: Use individual logic gates and feedback loops for memory
• Clocked synchronous state machines : Use latches and flipflops as building blocks controlled by clock

Latches and Flip Flops

Latch	Flip Flop
Triggered by input level	Triggered by clock edges
Asynchronous	Synchronous
Can change state while enabled	State change only at clock edge

S-R Latch

Block Diagram	Logic diagram

S	R	Q	QN
0	0	last Q	last QN
0	1	0	1
1	0	1	0
1	1	0	0

S-R Latch with Enable

Block Diagram	Logic Diagram

S	R	C	Q	QN
0	0	1	last Q	last QN
0	1	1	0	1
1	0	1	1	0
1	1	1	1	1
x	x	0	last Q	last QN

D Latch

• Stores one bit of information
• A transparent Latch

Block Diagram	Logic Diagram

C	D	Q	QN
1	0	0	1
1	1	1	0
0	x	last Q	last QN

D Flip Flop

• Output changes only at an edge of a clock signal
• Two Types:
  • Positive Edge Triggered
  • Negative Edge Triggered

Block Diagram	Logic Diagram

D	CLK	Q	QN
0		0	1
1		1	0
x	0	last Q	last QN
x	1	last Q	last QN

J-K Flip Flop

Block Diagram	Logic Diagram

J	K	CLK	Q	QN
x	x	0	last Q	last QN
x	x	1	last Q	last QN
0	0		last Q	last QN
0	1		0	1
1	0		1	0
1	1		last QN	last Q

T Flip Flop

• Toggle at every Clock signal

Using D	Using J-K	D with Enable	J-K With Enable

Clocked Synchronous State Machines

• State Machine : Sequential Operation
• Clocked : State change with clock edge
• Synchronous : All flip flops use the same clock

Example: 3 Bit Counter using D-FlipFlops

State Diagram

Excitation Table

K-Maps

Excitation Equations

Logic circuit for Individual Excitations

Complete Logic Circuit

Circuit in Action