Sequential logic circuits

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# Sequential logic circuits - PowerPoint PPT Presentation

Sequential logic circuits . Outline. Sequential Circuit Models Asynchronous Synchronous Latches Flip-Flops. Sequential logic circuits. The main characteristic of combinational logic circuits is that their output values depend on their present input values.

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### Sequential logic circuits

Outline
• Sequential Circuit Models
• Asynchronous
• Synchronous
• Latches
• Flip-Flops

Sequential logic circuits

Sequential logic circuits
• The main characteristic of combinational logic circuits is that their output values depend on their present input values.
• Sequential logic circuits differ from combinational logic circuits because they contain memory elements so that their output values depend on both present and past input values

Sequential logic circuits

Sequential logic circuits

Sequential circuits can be Asynchronous or synchronous.

Asynchronoussequential circuits change their states and output values whenever a change in input values occurs.

Synchronoussequential circuits change their states and output values at fixed points of time, i.e. clock signals.

Sequential logic circuits

Sequential Circuit Models

Universal model

Sequential logic circuits

Combinational circuit model

Mealy machine model

Moore machine model

Sequential logic circuits

Sequential Circuit Models

Sequential logic circuits

Memory Devices

LatchesA latch is a memory element whose excitation signals control the state of the device. A latch has two stages set and reset. Set stage sets the output to 1. Reset stage set the output to 0.

Flip-flopsA flip-flop is a memory device that has clock signals control the state of the device.

Sequential logic circuits

Latch

Flip-flop

Sequential logic circuits

Latch and Flip-Flop Devices

Sequential logic circuits

Inverter Chains

Ring oscillator

Sequential logic circuits

Latches

RS Latch

The RS latch is the basic memory element consists of two cross-coupled NOR gates. It has two input signals, S set signal and R reset signal. It also has two outputs Q and Q'; and two states, a set state when Q = 1 and a reset state when Q = 0 (Q' = 1)

Sequential logic circuits

RS Latch excitation table

Sequential logic circuits

State, Clock, Setup Time, and Hold Time

The Clocking event can be either from low to high or from high to low. The input signal around clocking event must remain unchanged in order to have a correct effect on the outcome of the new state.

• Tsu: the minimum time interval preceding

the clocking event during the input signal

must remain unchanged

• Th: the minimum time interval after edge

of the clocking event during the input signal

must remain unchanged

Sequential logic circuits

Timing Diagram of RS-Latch

Sequential logic circuits

JK Latch

Sequential logic circuits

Level-Sensitive Latches

A level-sensitive latch is a latch with an additional enable input.

RS latch

Sequential logic circuits

RS Latch with Enable

Sequential logic circuits

D Latch

Q+ = D

Sequential logic circuits

Flip-Flops

A flip-flop is a level-sensitive latch with a clock input.

RS flip-flop

Q+ = S +R'Q

Sequential logic circuits

T (Toggle) flip-flop

Q+ = TQ' + T'Q

Sequential logic circuits

Master Slave Flip-Flops

A master slaveflip-flop consists of two latches and an inverter.

Master-slave RS flip-flop

Sequential logic circuits

Master-Slave JK Flip-Flops

Sequential logic circuits

Positive Edge-Triggered Flip-Flops

Positive edge-triggered RS flip-flop timing diagram

Sequential logic circuits

Positive Edge-Triggered Timing

A circuit that generates a positive edge-triggered timing signal can be constructed as follows:

Sequential logic circuits

Exercises

page 425, 6.1-6.6, 6.9, 6.10, 6.12, 6.13, 6.14, 6.17, 6.24, 6.25

Sequential logic circuits