1240891706 - Switching Networks Computer Organization CDA...

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Computer Organization CDA 3103 Dr. Hassan Foroosh Dept. of Computer Science UCF © Copyright Hassan Foroosh 2004 inputs outputs system Switching Networks ± A simple model of a digital system is a unit with inputs and outputs: ± Digital systems consist of an interconnection of electronic switches i.e. a switching network ± A switching network performs a set of logical functions in either a combinational or sequential fashion. Combinational vs. Sequential ± Combinational: ± No Feedback ± O/P’s defined completely in terms of I/P’s. ± Sequential: ± With feedback ± Network goes through different states ± New state depends on I/P’s and current state. T n 1 X X M n 1 Z Z M T n 1 X X M n 1 Z Z M M Sequential logic ± Sequential systems ± Exhibit behaviors (output values) that depend not only on the current input values, but also on previous input values ± In reality, all real circuits are sequential ± The outputs do not change instantaneously after an input change ± Why not, and why is it then sequential? ± A fundamental abstraction of digital design is to reason (mostly) about steady-state behaviors ± Look at outputs only after sufficient time has elapsed for the system to make its required changes and settle down
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Synchronous sequential digital systems ± Outputs of a combinational circuit depend only on current inputs ± After sufficient time has elapsed ± Sequential circuits have memory ± Even after waiting for the transient activity to finish ± The steady-state abstraction is so useful that most designers use a form of it when constructing sequential circuits: ± Memory of a system is represented as its state ± Changes in system state are only allowed to occur at specific times controlled by an external periodic clock ± Clock period is the time that elapses between state changes it must be sufficiently long so that the system reaches a steady-state before the next state change at the end of the period B A C Clock Difference between combinational and sequential logic ± Combinational: ± input A, B ± wait for clock edge ± observe C ± wait for another clock edge ± observe C again: will stay the same ± Sequential: ± input A, B ± wait for clock edge ± observe C ± wait for another clock edge ± observe C again: may be different Representations ± Some we've seen already ± digital interpretation of analog values ± transistors as switches ± use of a clock to realize a synchronous sequential circuit ± Some others we will see ± switches as logic gates ± truth tables and Boolean algebra to represent combinational logic ± Combinational logic gates ± encoding of signals with more than two logical values into binary form ± state diagrams to represent sequential logic ± hardware description languages to represent digital logic ± waveforms to represent temporal behavior Boolean Algebra ± Deals with a set of variables (operands ) combined with a set of operators .
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This note was uploaded on 08/22/2010 for the course CDA 3101 taught by Professor Staff during the Fall '07 term at University of Central Florida.

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1240891706 - Switching Networks Computer Organization CDA...

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