Engin112-F07-L24-latch-fflop

Engin112-F07-L24-latch-fflop - Engin112 Lectures 23,24...

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Maciej Ciesielski Department of Electrical and Computer Engineering 10/29/2007 Engin112 – Lectures 23,24 Sequential Circuits Latches and Flip-flops 10/29/2007 Engin 112 - Intro to ECE 2 Sequential vs Combinational Ckts ± Recall definition of combinational logic y Outputs are a function of inputs y Combinational delay (each gate has some propagation delay) ± Sequential circuits y Outputs depend on inputs and previous values of outputs » Outputs depend on previous state of the circuit » State is stored in memory elements (registers, latches, fliplops)
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10/29/2007 Engin 112 - Intro to ECE 3 Sequential Circuits ± So far: combinational circuits y Output of circuit depends only on inputs ± Most real-world circuits also contain memory y Pocket calculators y Counters y Computer systems, etc. ± Sequential circuit: 10/29/2007 Engin 112 - Intro to ECE 4 Flavors of Sequential Circuits ± What is the output of this sequential circuit? y Sequence of alternating 1’s and 0’s y Exhibits “asynchronous” behavior ± Circuit can change at any moment in time y Asynchronous sequential circuit y Behavior depends on physical characteristics of circuit » Speed of logic gate, signal propagation ± Very difficult to design and analyze
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10/29/2007 Engin 112 - Intro to ECE 5 Synchronous Sequential Circuits ± Synchronous sequential circuits y Storage elements change only at discrete instances of time ± Timing controlled by “clock” y Clock generator provides train of clock pulses: ± Clocked (synchronous) sequential circuit: 10/29/2007 Engin 112 - Intro to ECE 6 Storage Elements ± Binary storage device capable of storing one bit ± Latch = level-sensitive device y State changes with input when enabled (e.g., when clock = 1) y Holds last input value when disabled (when clock = 0) ± Flip-flop = edge-triggered device y State of flip-flop can only change during clock transition y Example: Flip-flops change on rising/falling edge of clock ± Why change on an edge?
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This note was uploaded on 04/16/2008 for the course ENGIN 112 taught by Professor Ciesielski during the Spring '08 term at UMass (Amherst).

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Engin112-F07-L24-latch-fflop - Engin112 Lectures 23,24...

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