数字设计基础双语课件(第4章).ppt
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1、 4.Flip-flops and counters 4.1 Sequential circuits 4.2 Memory design using gates 4.3 Flip-flops4.4 Registers4.5 Counters1 4.1 Sequential circuits 1.Concept of sequential circuitThe logic circuits whose outputs depend not only on the present logic input values but also on previous logic input and out
2、put values are called sequential circuits.Counter is a common example of sequential circuit.2 4.1 Sequential circuits(2)Asynchronous sequential circuitA sequential logic circuit that does not use a synchronizing clock signal is called asynchronous sequential circuit.2.Classification of sequential ci
3、rcuit(1)Synchronous sequential circuitA sequential logic circuit whose output changes are initiated by an input clock signal and in which the outputs change upon the required clock signal transition is called a synchronous sequential circuit.3 4.2 Memory design using gates A logic circuit can mainta
4、in a constant output value by the use of feedback whereby the output is connected to the inputs to reinforce the output value.Set-reset memory design )R=0,S=1,Q=0;)R=1,S=0,Q=1;From the circuit,we can get:Once the output has been forced to a 0 or a 1,the input can return to a 1 and the output will re
5、main unchanged.4 4.2 Memory design using gates It is often convenient to have both a true output Q and a complementary output Q which in normal operation of the memory circuit has the opposite logical value to Q.The memory design shown below is called a latch.The latch can store one binary value,but
6、 its outputs will change when one of the inputs changes to a 0.Disadvantage of latch:5 4.2 Memory design using gates Sometimes we have active high inputs memory design.6 4.3 Flip-flops Normally we want the output changes to be synchronized with a clock signal.Such memory designs are called flip-flop
7、s.A flip-flop can store a single bit by producing an output of a 0 or a 1 continuously until changed by conditions on the inputs and a clock signal transition.1.Flip-flopFlip-flops are the basic building block of sequential circuits.There are several types of flip-flops.7 4.3 Flip-flops 2.R-S flip-f
8、lopThe R-S flip-flop has two inputs,named S and R.S for set,R for reset.(1)Truth tableQ+indicates the value of Q after the activating clock transition.Q-is the value before the clock transition.However,only after a specified clock transition occurs will the outputs take on the required values,before
9、 which the outputs can not change even if the S or R inputs change.8 4.3 Flip-flops(2)Characteristic equationCharacteristic equation describes the relationship between the Q output and inputs.The characteristic equation of R-S flip-flop is:Q+=Q R+S(3)Level triggeringThe simplest form of clock activa
10、tion is level triggering.When the clock becomes a 1,the outputs assume their values according to the inputs.9 4.3 Flip-flops)When CP=1,the values on S and R enter the circuit and the outputs will assume the corresponding input values.)When CP=0,the outputs of gates G3 and G4 will be at a 1 irrespect
11、ive of the logic levels on S and R.Level triggered R-S flip-flop10 4.3 Flip-flops If we simply want to store the value of one binary digit,then D flip-flop is needed which has only one data input to specify a 0 or a 1 to be stored.3.D flip-flopThe Q output simply becomes the value on the D input aft
12、er the activating clock transition.11 4.3 Flip-flops(1)Characteristic equationQ+=DTruth table(3)Edge triggering(2)Truth tableThe output changes on a transition of the clock signal and the inputs are allowed to change at other times without affecting the output.This form of clock activation is called
13、 edge triggering.12 4.3 Flip-flops Master-slave flip-flopIn a master-slave flip-flop,two flip-flops are used in cascade.The first one,the master,captures the input values when clock is a 1.The second one,the slave,takes on the masters output on the falling edge of the clock.13 4.3 Flip-flops Though
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