sequential ckt

# sequential ckt - ECSE 323 Digital System Design Sequential...

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ECSE 323 Digital System Design Sequential Circuits Katarzyna Radecka [email protected]

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Acknowledgements Material used in this set of slides was based on Fundamentals of Digital Logic with VHDL Design ” by S. Brown and Z. Vranesic
Sequential Circuits Definition of sequential circuit Synchronous sequential circuit Asynchronous sequential circuit Memory elements Classification: latches and flip-flops Latches Basic latch Gated latch Effect of propagation delays Flip-flops Asynchronous sequential circuit Asynchronous behavior Analysis of asynchrounous circuits __________________________________________________ ECSE-323/Department of Electrical and Computer Engineering/McGill University/ Prof. Marin. Revised 2005-02-14. Figures taken from Fundamentals of Digital Logic with VHDL Design, S. Brown and Z. Vranesic, 2 nd Edition, McGraw Hill.

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New Challenges So far we learned: How to represent our concept of a digital design in form of a Boolean functions and truth tables How to minimize above expressions in order to obtain the optimal digital circuit How to implement a digital circuit using basic logic gates like NOT, AND, OR, NAND, NOR, XOR, etc. Is that enough to build any digital circuit, or are we missing something? NO! Example: Implement the following game You move from START to FINISH on the n x n grid one square at the time The only restriction is that the current move cannot be in the same direction that the previous one In order to implement the above game we need means of remembering the previous move on the grid We need a memory (sequential element) which is not a part of the combinational gate library we have discussed so far START FINISH Current location Previous move Next allowed move Next forbidden move
Definition of Sequential Circuit Sequential circuits are the ones, in which the values of the outputs depend on: The present values of the inputs The past behavior of the circuit This is the contrast to the combinational circuits, where the output is the function of, and only of, the present input In such circuits storage elements store the values of the signals The contents of the storage elements represent the state of the circuit Sequential logic is used to construct computer memory and other types of storage elements, as well as finite state machines There are two types of sequential circuits: Synchronous Asynchronous

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Synchronous Sequential Circuits Synchronous sequential circuits are circuits controlled ( synchronized ) by a clock signal Clock can be any periodic signal consisting of pulses Any change in the logical levels of storage component follow the change of the clock Changes in the state of the system can occur on either positive or negative clock pulse (but generally designers avoid to use both polarities in the same system) The input to the storage element has to stabilize before the next clock occurs
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## This note was uploaded on 11/22/2010 for the course ECSE ecse 323 taught by Professor Redacka during the Winter '07 term at McGill.

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sequential ckt - ECSE 323 Digital System Design Sequential...

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