tutorial9 - ERG2020 A&B Tutorial 9 27/03/2007 YU Xi...

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ERG2020 A&B Tutorial 9 27/03/2007 YU Xi
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Outline • Basic concepts of sequential circuit • Time analysis • State machine
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Basic concepts • What is sequential circuit? Circuits with feedback, having memory or internal states, are called sequential circuits. • Latches and flip-flops are primitive memory elements. Each of them stores 1- bit data.
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Difference between Latch and Flip-Flop • Latches change their outputs when the inputs change. • Flip-Flops change outputs according to both the inputs AND clock
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• Level sensitive vs. Edge triggered ± Level sensitive devices sample inputs when clock is high. ± Edge triggered devices sample inputs on the event edge
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• Problems with S-R latches : 1. Change states immediately when input changes (common problem of latches) ± If input signal has glitch, output will have error. (catching problem) ± Difficult to change multiple inputs at exactly the same time (synchronization problem) 2. Existence of forbidden input (S = R = 1) / forbidden state (Q = Q’ = 0) ± In forbidden state, if both S and R are negated simultaneously, metastability or oscillation may occur.
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Time analysis • Clock signal in sequential circuit • Timing parameters 1 Propagation delay: Inputs should be stable within the duration of NOR gate delay in S-R latch. 2 Minimum pulse width: minimum duration of HIGH of the input 3 Setup time: minimum time before the clocking event by which the input must be stable 4 Hold time: minimum time after the clocking event until which the input must remain stable 5 Violation on timing parameters may lead to metastability / output oscillation (racing).
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• One simple example • Sketch the outputs of an S-R latch of the type shown in the figure below for the following input waveforms.
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This note was uploaded on 05/18/2010 for the course INFORMATIO IEG 2810AB taught by Professor Professork.w.cheung during the Spring '09 term at CUHK.

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tutorial9 - ERG2020 A&B Tutorial 9 27/03/2007 YU Xi...

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