# Lec-06-SeqImpl - Sequential Logic Implementation Sequential...

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CS 150 - Fall 2000 - Sequential Logic Implementation - 1 Sequential Logic Implementation Sequential Circuits Primitive sequential elements Combinational logic Models for representing sequential circuits Finite-state machines (Moore and Mealy) Representation of memory (states) Changes in state (transitions) Basic sequential circuits Shift registers Counters Design procedure State diagrams State transition table Next state functions

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CS 150 - Fall 2000 - Sequential Logic Implementation - 2 Abstraction of State Elements Divide circuit into combinational logic and state Localize feedback loops and make it easy to break cycles Implementation of storage elements leads to various forms of sequential logic Combinational Logic Storage Elements Outputs State Outputs State Inputs Inputs
CS 150 - Fall 2000 - Sequential Logic Implementation - 3 Forms of Sequential Logic Asynchronous sequential logic – state changes occur whenever state  inputs change (elements may be simple wires or delay elements) Synchronous sequential logic – state changes occur in lock step across all  storage elements (using a periodic waveform - the clock) Clock

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CS 150 - Fall 2000 - Sequential Logic Implementation - 4 In = 0 In = 1 In = 0 In = 1 100 010 110 111 001 Finite State Machine Representations States: determined by possible values in sequential storage elements Transitions: change of state Clock: controls when state can change by controlling storage elements Sequential Logic Sequences through a series of states Based on sequence of values on input signals Clock period defines elements of sequence
Example Finite State Machine Diagram Combination lock from first lecture closed not equal & new OPEN ERR open

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Can Any Sequential System be Represented with a  State Diagram? Shift Register Input value shown on transition arcs Output values shown within state node 100 110 1 1 1 1 1 0 0 0 0 0 1 0 0 D Q D Q D Q IN OUT1 OUT2 OUT3 CLK
CS 150 - Fall 2000 - Sequential Logic Implementation - 7 010 100 110 011 001 000 101 111 3-bit up-counter Counters are Simple Finite State Machines Counters Proceed thru well-defined state sequence in response to enable Many types of counters: binary, BCD, Gray-code 3-bit up-counter: 000, 001, 010, 011, 100, 101, 110, 111, 000, . .. 3-bit down-counter:  111, 110, 101, 100, 011, 010, 001, 000, 111, . ..

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CS 150 - Fall 2000 - Sequential Logic Implementation - 8 How Do We Turn a State Diagram into Logic? Counter Three flip-flops to hold state Logic to compute next state Clock signal controls when flip-flop memory can change Wait long enough for combinational logic to compute new value Don't wait too long as that is low performance D Q D Q D Q OUT1 OUT2 OUT3 CLK "1"
CS 150 - Fall 2000 - Sequential Logic Implementation - 9

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Lec-06-SeqImpl - Sequential Logic Implementation Sequential...

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