L05-4up

L05-4up - Sequential Logic adding a little state 6.004...

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L05 – Sequential Logic 1 6.004 – Fall 2010 9/23/10 Sequential Logic: adding a little state Lab #1 is due tonight (checko ff meeting by next Friday). Quiz #1 tomorrow (Fri) in section. L05 – Sequential Logic 2 6.004 – Fall 2010 9/23/10 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 C B A Y 0 0 0 0 0 0 1 1 0 1 0 0 0 1 1 1 1 0 0 0 1 0 1 0 1 1 0 1 1 1 1 1 What other building blocks do we need in order to compute? L05 – Sequential Logic 3 6.004 – Fall 2010 9/23/10 Something We Can’t Build (Yet) What if you were given the following design specification: When the button 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 button press button light What makes this circuit so di ff erent 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 – Fall 2010 9/23/10 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 – Fall 2010 9/23/10 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 – Fall 2010 9/23/10 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, force storage cap to new voltage To read: precharge bit line, turn on access fet, detect (small) change in bit line voltage N-channel fet serves
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