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Unformatted text preview: Mark Redekopp, All rights reserved Introduction to Digital Logic Lecture 16: Combinational vs. Sequential Logic Bistables Mark Redekopp, All rights reserved Combinational vs. Sequential Outputs depend only on current outputs Outputs depend on current inputs and previous inputs Current inputs Outputs Current inputs Previous inputs Outputs Combinational Logic Sequential Logic Mark Redekopp, All rights reserved Combinational Example: Staircase Light Switch Whether or not the light is on is only dependent on the current position of the switches S1 S2 Light Mark Redekopp, All rights reserved Sequential Example: Remote Control 3 *10 30 32 + Time 1 Time 2 2 The channel is a function of the The channel is a function of the second (we must remember the second (we must remember the The channel is a function of the The channel is a function of the first button pressed and the first button pressed and the second (we must remember the second (we must remember the 3 and then use it with the 2) 3 and then use it with the 2) Inputting channel 32 Mark Redekopp, All rights reserved Sequential Logic All logic is categorized into 2 groups Combinational logic: Outputs = f(current inputs) Sequential Logic Outputs = f(current inputs, previous inputs) With sequential logic there is the idea of memory Mark Redekopp, All rights reserved Sequential Logic With combinational logic the outputs only depend on what the inputs are right now 7 4 3 It doesnt matter what the inputs were previously A0 A1 A2 A3 B0 B1 B2 B3 S0 S1 S2 S3 283 Mark Redekopp, All rights reserved Sequential Logic Suppose we have a sequence of input numbers on X[3:0] that are entered over time that we want to sum up Possible solution: Route the outputs back to the inputs so we can add the current sum to the input X Problem 1: No way to initialize sum Problem 2: Outputs can race around to inputs and be added more than once per input number Possible Solution Outputs can feedback to inputs and update them sum more than once per input A0 A1 A2 A3 B0 B1 B2 B3 S0 S1 S2 S3 283 X0 X1 X2 X3 Z0 Z1 Z2 Z3 9, 3, 2 Mark Redekopp, All rights reserved...
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This note was uploaded on 11/08/2009 for the course EE 101 at USC.
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