MIT6_004s09_lec05

MIT6_004s09_lec05 - MIT OpenCourseWare http:/ocw.mit.edu...

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MIT OpenCourseWare http://ocw.mit.edu For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms . 6.004 Computation Structures Spring 2009
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L05 – Sequential Logic 1 6.004 – Spring 2009 2/19/09 Sequential Logic: adding a liTle state Lab #1 is due tonight (checkoff meeting by next ±hursday). modified 2/17/09 10:26 QUIZ #1 Tomorrow! (covers thru L4/R5) L05 – Sequential Logic 2 6.004 – Spring 2009 2/19/09 6.004: Progress so far… 01101 PHYSICS: Continuous variables, Memory, Noise, f(RC) = 1 - e -t/RC COMBINATIONAL: Discrete, memoryless, noise-free, lookup table functions 2.71354 volts CBAY 0000 0011 0100 0111 1000 1010 1101 1111 What other building blocks do we need in order to compute? L05 – Sequential Logic 3 6.004 – Spring 2009 2/19/09 Something We Can’t Build (Yet) What if you were given the following design specification: When the bu±on is pushed: 1) Turn on the light if it is off 2) Turn off the light if it is on The light should change state within a second of the bu±on press bu±on light What makes this circuit so different from those we’ve discussed before? 1. “State” – i.e. the circuit has memory 2. The output was changed by a input “event” (pushing a bu±on) rather than an input “value” L05 – Sequential Logic 4 6.004 – Spring 2009 2/19/09 Digital State One model of what we’d like to build Plan: Build a Sequential Circuit with stored digital STATE – ± Memory stores CURRENT state, produced at output ± Combinational Logic computes ± NEXT state (from input, current state) ± OUTPUT bit (from input, current state) ± State changes on LOAD control input Combinational Logic Current State New State Input Output Memory Device LOAD
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L05 – Sequential Logic 5 6.004 – Spring 2009 2/19/09 Needed: Storage Combinational logic is stateless : valid outputs always reflect current inputs. To build devices with state, we need components which store information (e.g., state) for subsequent access. ROMs (and other combinational logic) store information “wired in” to their truth table Read/Write memory elements are required to build devices capable of changing their contents. How can we store – and subsequently access -- a bit? ± Mechanics: holes in cards/tapes ± Optics: Film, CDs, DVDs, … ± Magnetic materials ± Delay lines; moonbounce ± Stored charge L05 – Sequential Logic 6 6.004 – Spring 2009 2/19/09 Storage: Using Capacitors We’ve chosen to encode information using voltages and we know from 6.002 that we can “store” a voltage as charge on a capacitor: Pros: ± compact – low cost/bit (on BIG memories) Cons: ± complex interface ± stable? (noise, …) ± it leaks! ± refresh To write: Drive bit line, turn on access fet,
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This note was uploaded on 11/07/2011 for the course COMPUTER S 6.004 taught by Professor Staff during the Spring '09 term at MIT.

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MIT6_004s09_lec05 - MIT OpenCourseWare http:/ocw.mit.edu...

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