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): Synchronous Sequential Circuits – circuit behavior is determined from the knowledge of signal values at discrete instances in time . Asynchronous Sequential Circuits - circuit behavior is determined by signals at any instant in time and the order in which input signals change .
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)
E&CE 223 Digital Circuits and Systems Page 5 Latches Latches are level sensitive storage elements; 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)