# set3 - TA C162 Computer Programming I Agenda What we did so...

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1 TA C162 Computer Programming I Virendra S Set3 Agenda s What we did so far? s Logic Structures without storage s e.g. Adder, Decoder, Mux, comparator etc. •What’s next? s Logic structures with storage of information s Basic storage structure s R-S latch s Gated D-latch s Register

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2 TA C162 Computer Programming I Virendra S Set3 Combinational Circuit Design- Summary Problem Statement b Truth Table b Boolean Expression b Circuit Diagram s Adder: Adding Two n-bit numbers including carry bit s Arithmetic Operations on bits s Decoder: Enables one output from 2 n at a time, for a particular combination of n input bits s Example: Letters are encoded as 8 bit codes inside the computer. s When the particular combination of bits that encodes “A” is detected, we want to activate the output line corresponding to A s Multiplexer: A multiplexer is a combinatorial circuit that is given a certain number (usually a power of two) data inputs, let us say 2 n , and n address inputs used as a binary number to select one of the data inputs. s The multiplexer has a single output, which has the same value as the selected data input. s Demorgan’s Law: AND logic to OR logic conversion and vice versa
3 TA C162 Computer Programming I Virendra S Set3 Simple Storage Element Called R-S Latch s S is used to “set” the element – s Set it to one s R is used to “reset” or “clear” the element s Set it to zero s If both R and S are one, output could be either zero or one s Quiescent” state -- Holds its previous value

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4 TA C162 Computer Programming I Virendra S Set3 Clearing the R-S latch Suppose we start with output = 1, then change R to zero. Output changes to zero. Then set R=1 to “store” value in quiescent state. 1 1 1 1 0 0 1 0 0 0 1 1
5 TA C162 Computer Programming I Virendra S Set3 Setting the R-S Latch Suppose we start with output = 0, then change S to zero Output changes to one. Then set S=1 to “store” value in quiescent state. 1 1 0 0 1 1 0 1 1 1 0 0

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6 TA C162 Computer Programming I Virendra S Set3 Quiescent state Holds previous value 1 1 0 0 1 1 1 1 1 1 0 0
7 TA C162 Computer Programming I Virendra S Set3 R-S Latch Summary R = S = 1 s Holds previous value in latch S = 0, R=1 s Set value to 1 ( Set ) R = 0, S = 1 s Set value to 0 (Reset) R = S = 0 s Both outputs equal one s Final state determined by electrical properties of gates s Don’t do it!

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8 TA C162 Computer Programming I Virendra S Set3 Gated D-Latch Two inputs: D (data) and WE (write enable) s when WE = 1 , latch is set to value of D s S = NOT(D), R = D s when WE = 0 , latch holds previous value s S = R = 1
TA C162 Computer Programming I Virendra S Set3 Register A register stores a multi-bit value. s

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## This note was uploaded on 05/14/2010 for the course CS SS ZG653 taught by Professor Shanta during the Spring '09 term at Birla Institute of Technology & Science.

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set3 - TA C162 Computer Programming I Agenda What we did so...

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