Ch5_Synchronous_Logic_Kenning

Ch5_Synchronous_Logic_Kenning - Sequential Circuits Have...

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E&CE 223 Digital Circuits and Systems Page 1 Sequential Circuits ± Have considered only combinational circuits in which circuit outputs are determined entirely by current circuit inputs. ± We can include storage elements into a circuit that act like memory and store a system state . inputs outputs combinatorial circuit memory elements state ± Outputs are then a function of both the current circuit inputs and the system state.
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E&CE 223 Digital Circuits and Systems Page 2 Types of Sequential Circuits ± Two main types of sequential circuits (classification depends on how timing happens): ² Synchronous Sequential Circuits – circuit behavior is determined from the knowledge of signal values at discrete instances in time . ² Asynchronous Sequential Circuits - circuit behavior is determined by signals at any instant in time and the order in which input signals change .
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E&CE 223 Digital Circuits and Systems Page 3 Synchronous Sequential Circuits ± To control the behavior of a circuit at discrete instances in time, we will need to introduce the concept of a clock . ± It’s a periodic signal consisting of a sequence of pulses. It is used to control the times at which the storage elements in the circuit change their values. ± The storage elements used can be latches or flip-flops . inputs outputs combinatorial circuit storage elements state clock/ control clock/ control ± The clock/control connects to the storage elements which are latches or flip-flops.
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E&CE 223 Digital Circuits and Systems Page 4 Clocks ± Clocks are periodic. ± They can control when things happen because their transition from 0-> 1 and from 1 -> 0 occur at discrete instances in time. ± The 0 -> 1 transition is often called the rising edge of the clock. ± The 1 -> 0 transition is often called the falling edge of the clock. clock 1 0 rising edge (0->1 transition) falling edge (1->0 transition)
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E&CE 223 Digital Circuits and Systems Page 5 Latches ± Latches are level sensitive storage elements; ² They operate based on whether signals are at logic levels 0 or 1 , not on logic transitions from 0 -> 1 or 1 -> 0. ± Latches are not really too useful for synchronous sequential circuits (they are for asynchronous circuits), but form the basis from which flip-flops are built.
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E&CE 223 Digital Circuits and Systems Page 6 SR Latch (NOR Implementation) Illustrated ± Consider the operation of the following circuit: Q !Q R (reset) S (set)
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E&CE 223 Digital Circuits and Systems Page 7 SR Latch (NOR Implementation) Explained ± In general (one exception, see below), the outputs are complements of each other (this is why they are labeled Q and : Q ): ² When S=1, R=0 the output is Q=1, : Q=0 and the circuit in the set state . ² When S=0, R=1 the output is Q=0, : Q=1 and the circuit in the reset state .
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This note was uploaded on 12/07/2010 for the course EE ee012 taught by Professor Razarahim during the Winter '10 term at NUCES.

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Ch5_Synchronous_Logic_Kenning - Sequential Circuits Have...

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