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Class Notes Set 1

# Class Notes Set 1 - Classification of Digital Circuits...

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Classification of Digital Circuits Combinational logic circuits. Output depends only on present input. Sequential circuits. Output depends on present input and present state of the circuit.

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Combinational Logic Design Procedure Start with the problem statement. Determine the number of inputs variables and the required number of output variables. Derive a truth table that defines the required relationship between input and output. Simplify each output function (Karnaugh maps). Draw the logic diagram.
Half Adder Design Example A half adder computes the sum of two one bit Boolean inputs, which can be at most 10 2 . This requires two outputs. Inputs: X, and Y. Outputs: S and C. X Y C S 0 0 0 0 0 1 0 1 1 0 0 1 1 1 1 0

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Half Adder Design Example C = XY S = X Y
Decoders A decoder is a multiple-input multiple-output logic circuit that converts coded inputs into coded outputs, where the inputs and outputs codes are different.

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Function Realization With Decoders The outputs of a binary decoder provides 2 n minterms for its n inputs, and a Boolean functions can be represented as a sum of minterms, therefore a decoder and one or more OR gates can be used to realize sums of minterms of n variables.
Multiplexers Combinational logic circuit that selects binary information from one of many input lines and directs it to a single output line. The input is selected by the binary value on the select lines.

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Demultiplexers Combinational logic circuit that receives binary information on a single input and sends this information to one of many possible output lines. The output is selected by the binary value on the select lines.
Function Realization With Multiplexers A multiplexer is basically a binary decoder whose outputs were ORed together and some extra input lines were added to each product term.

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Function Realization With Multiplexers A sum of minterms can be realized by setting the corresponding input line of the chosen minterms to 1 and setting the input lines of the remaining minterms to 0.
Transition Time Time interval between two reference points on a waveform. These reference points are usually 10% and 90% of the voltage change. Rise time ( t r ) – Time interval when waveform is changing from a logic low to a logic high level. Fall time ( t r ) – Time interval when waveform is changing from a logic high to a logic low level.

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Time it takes for a change at the input of a device to produce a change at the output of the same. t pLH is the propagation delay when the output changes from LOW to HIGH. t pHL is the propagation delay when the output changes from HIGH to LOW. t
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Class Notes Set 1 - Classification of Digital Circuits...

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