07 Sequential Circuit Design

07 Sequential Circuit Design - EE2000 Logic Circuit Design...

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1 EE2000 Logic Circuit Design Sequential Circuits Synchronous Sequential Logic Design
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2 Example (revisit) ± A system with one input x and one output z such that z = 1 iff x has been 1 for at least three consecutive clock times ± Input sequence: 010011001010001110101100… ± The system must store in its memory information the last three inputs of x in order produce output based on that ± What is stored in memory is the state of the system ± This circuit is called sequence recognizer
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3 Sequence Recognizer ± A circuit that recognizes the occurrence of a particular sequence of bits ± One input X and one output Z ± The circuit recognize the occurrence of the sequence of input bits “111” on X ± i.e. if previous two inputs to the circuit were “11” and current input is a “1”, Z equal to 1 ± Otherwise, Z equal to 0 ± Input sequence X : 01011001000 111 ± Output sequence Z : 0000000000000 1
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4 Requirements ± Require a combinational circuit ( A ) to determine the output ± If the current input is 0, output must be 0 ± If the current input is 1, what will be the output? Depends on the previous two input! ± Require memories ( B ) to remember the history of input sequence ± e.g. what are the previous two inputs? ± Output 1 if previous two inputs were “11” (and current input is 1), output 0 if previous inputs were not “11” ± Require a combination circuit ( C ) to “update” the input history
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5 Sequential Circuit Structure Input X Output Z Present states Q i A C B B
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6 Long Sequence Recognizer ± How to recognize the occurrence of a long sequence of input bits “11010101100110”? ± This match sequence is 14 bits long ± Need 13 flip-flops to remember the previous 13 inputs ± Not a good design ± Remember the state instead of memorizing the previous input sequence!
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7 Outline ± Concept of States ± Mealy and Moore Machines ± Design of Sequential Logic Circuits ± Two Examples ± Sequence Recognizer ± Modulo- n Counter
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8 Concept of States ± The past history is regarded as states ± The most common device used to store the state is flip-flop ± A flip-flop can remember a single bit of information (two states) ± How many states can n FFs be represented? ± A set of flip-flops are used to constitute the state of a sequential circuit ± If the value stored in the flip-flops change, the states of the circuit is changed
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9 States ± Due to the repeat of the inputs, the input can be group into classes ± These classes summarize the effect of past inputs on present and future outputs ± Represented by auxiliary variables S i ± Also known as states ± The number of states are usually finite ± Two state machines: Mealy machine, and Moore machine
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10 Mealy Machine ± A state is represented by a circle ± Transitions between states are indicated by directed lines connecting the circles S i S i S j S j X / Z State S i S i S i For the present state S i , if the next input is x , the state will change to S j and produce an output z
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11 Mealy Machine Example ± Can you tell the meaning of this state diagram ?
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This note was uploaded on 01/11/2011 for the course EE 2000 taught by Professor Vancwting during the Fall '07 term at City University of Hong Kong.

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07 Sequential Circuit Design - EE2000 Logic Circuit Design...

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