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**Analysis of Clocked Sequential Circuits**

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**Example of a Sequential Circuit**

D flip-flops

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**Example: Start with A=0, B=0, x=0.**

A(next)=0 B(next)=0 Y(next)=0

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**What are A(next), B(next) and y(next) given that A=1, B=1 and X=1?**

D flip-flops

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Alternate State Table

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State Diagram Each circle is a state When x=1, y=0.

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State Diagram Each circle is a state When x=0, y=1.

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**Detects 0 in the bit stream of data**

Output is a 0 as long as input is a 1. The first 0 after a string of 1 transfers the circuit back to 00.

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Summary

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**Model a Clocked Sequential Circuit with Verilog**

Use parameter to define constants Form the next state from x and the current state Form the output fork.....join construction S0 S2 S1 S3

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Parameter S0 S2 S1 S3 Define states with parameter

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**Update States S0 S2 S1 S3 If reset is 0, set state to S0.**

If reset is 1, update state with next_state. S1 S3

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**Implement the States Using State Diagram**

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fork....join

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**Right click to add state and next_state to the waveform shown right**

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Valid Mealy Output S0 S2 S1 S3

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**Mealy Glitches Glitiches occur because x changes before**

the next rising clock edge S0 S2 S1 S3

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**General Structure of a Mealy Machine**

Output depends on input as well as the state The circuit is in. (variable: state) Synchronize the input with clock when using Mealy machine.

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Moore Machine Output depends on the state, but not the input.

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**Synthesis of Sequential Circuit**

Start with a state diagram Assign Binary Values to the States

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**State Diagram for Sequence Detector**

Problem statement: Design a circuit that detects a sequence of three or more consecutive 1’s in a string of bits coming through an input line

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**State Diagram of the Word Description**

Start with S0. if input is 0, stay in S0. if input is 1, go to S1. Once in S1 if input is 1, go to S2. if input is 0, go to S0. Once in S2 if input is 1, go to S3. Once in S3, if input is 1, stay in S3, set output to 1. Output is 1 when the circuit is in S3, 0 otherwise.

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**Assign Binary Value to the States**

1 2 3 4 5 6 7 Use two flip-flops to represent states. Let x be the input And y be the output

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**Assign Binary Value to the States**

1 2 3 4 5 6 7

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**Circuit Implementation**

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**Sequence Detector Circuit**

Random Number generator

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Measurement Results B A x y clk

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STATE DIAGRAM AND STATE TABLES

STATE DIAGRAM AND STATE TABLES

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