L7+Latch+&+FF - Lecture 7 Latches and Flip Flops...

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1 Digital Design Copyright © 2007 Frank Vahid Lecture 7 Latches and Flip Flops
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2 Digital Design Copyright © 2007 Frank Vahid Introduction • Beginning from this lecture, we will: – Design a new building block, a flip-flop , that stores one bit – Combine that block to build multi-bit storage – a register – Describe the sequential behavior using a finite state machine – Convert a finite state machine to a controller a sequential circuit having a register and combinational logic Note: Slides with animation are denoted with a small red " a " near the animated items
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3 Digital Design Copyright © 2007 Frank Vahid Concepts of Sequential Circuit • Sequential circuit – Combinational circuit with feedbacks – Due to the feedback, output of a sequential circuit is decided by • Present inputs, and • Past input sequence and • Past outputs sequence • Timing concepts – input-output propagation delay –c l o c k – Other timing issues
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4 Digital Design Copyright © 2007 Frank Vahid Example Needing Bit Storage • Flight attendant call button – Press call: light turns on Stays on after button released – Press cancel: light turns off – Logic gate circuit to implement this? Q Call Cancel Doesn’t work. Q=1 when Call=1, but doesn’t stay 1 when Call returns to 0 Need some form of “storage” in the circuit a a Bit Storage Blue light Call button Cancel button 1. Call button pressed – light turns on Bit Storage Blue light Call button Cancel button 2. Call button released – light stays on Bit Storage Blue light Call button Cancel button 3. Cancel button pressed – light turns off
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5 Digital Design Copyright © 2007 Frank Vahid First attempt at Implementation of Bit Storage • We need some sort of feedback – Does circuit on the right do what we want? • No: Once Q becomes 1 (when S=1), Q stays 1 forever – no value of S can bring Q back to 0 Q S t 0 t 0 Q S 0 1 0 1 0 1 0 Q t S 0 t 1 Q S 0 0 t 1 Q S 1 1 t 1 Q S 1 1 t 0 Q S 1 a
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6 Digital Design Copyright © 2007 Frank Vahid 0 0 1 R=1 S=0 t Q 1 0 1 0 R S 1 0 t 1 0 Q Second Attempt at Bit Storage – SR Latch Q S (set) SR latch R (reset) Cross-coupled NOR gates, does it do what we want? –Y e s ! 1 0 0 1 0 1 t Q S=0 R=0 t Q S=1 R=0 0 1 1 t Q R=0 S=0 1 0 1 0 0 0 1 1 X 0 Recall… a
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7 Digital Design Copyright © 2007 Frank Vahid Example Using SR Latch for Bit Storage • SR latch can serve as a bit storage, for example: – Call=1 : sets Q to 1 • Q stays 1 even after Call=0 – Cancel=1 : resets Q to 0 • But, there’s a problem. .. R S Q Call but ton Blue light Cancel but ton Bit St o r a g e Ca l l button Ca nce l button
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8 Digital Design Copyright © 2007 Frank Vahid Problem with SR Latch • Problem – If S=1 and R=1 simultaneously, we don’t know what value Q will take R= 1 S= 1 0 0 0 0 t Q R= 0 S= 0 0 0 1 1 t Q 1 1 0 0 t Q 0 1 0 1 0 1 0 1 S R Q t 1 t 0 1 Q 0 Q may oscillate. Then, because one path will be
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L7+Latch+&+FF - Lecture 7 Latches and Flip Flops...

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