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EET 3350 Digital Systems Design

John Wakerly Chapter 7: 7.1 – 7.2

Sequential Circuits

Flip-Flops

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Combinational Logic Circuits

• Output set depends only on the current input set

• If the input(s) change, the output(s) may change

• History of the input behavior is not important to the future output behavior

binary values, an n-bit binary number

binary values, an m-bit binary number

n m

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Sequential Logic Circuits

• Output depends on current input and history of past inputs

• Information about the past behavior (history) of the input(s) is held in memory

• Memory is finite, holding “just enough” information as defined by design specification

n m

Size of memory depends on how much history we need to know.

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Sequential Logic Circuits

• Some of the CL outputs are used to control the memory elements

• Outputs from the memory elements are provided as inputs to the CL circuit– this is a form of feedback

• Memory may also have an external “clock” signal to control when changes can occur

n m

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Sequential Logic Circuits

• The addition of the “extra” inputs and outputs doesn’t change the basic concepts associated with the CL part of the circuit– it just has more signals

• Our approach to designing the CL circuit is the same as always

• Need to design the memory part

n m

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Sequential Logic Circuits

• Memory output is considered the circuit’s “current state” – a numerical label

• State embodies all the information about the past needed to define the current output– State variables, one or

more bits of information– Present state, now, waiting– Next state, next, with clock

binary values, an i-bit or j-bit binary number

n m

ji

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Describing Sequential Circuits

• State diagram– Graphical version of states, inputs, transitions and

outputs

Input Output

1-bit 1-bit

SequentialLogicCircuit

Clock

SL machine to recognize that the last four inputs have been 1

Seems to be an SL machine to recognize the falling edge

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Describing Sequential Circuits

• State table– For each current-state, specify next-state(s) as a

function of the present inputs– For each current-state, specify the output(s) as a

function of the present inputs

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Describing Sequential Circuits

• State table– Alternate format

More about states, tables and diagrams later.