CMOS Oscillators
INTRODUCTION
This note describes veral square wave oscillators that can be built using CMOS logic elements.The circuits offer the following advantages:
•Guaranteed startability
•Relatively good stability with respect to power supply variations
•Operation over a wide supply voltage range (3V to 15V)•Operation over a wide frequency range from less than 1Hz to about 15MHz
•Low power consumption (e AN-90)
•国际民航组织
Easy interface to other logic families and elements in-cluding TTL
工作室英文
Several RC oscillators and two crystal controlled oscillators are described.The stability of the RC oscillator will be suffi-cient for the bulk of applications;however,some applications will probably require the stability of a crystal.Some applica-tions that require a lot of stability are:
1.Timekeeping over a long interval.A good deal of stability
is required to duplicate the performance of an ordinary wrist watch (about 12ppm).This is,of cour,obtain-able with a crystal.However,if the time interval is short and/or the resolution of the timekeeping device is rela-tively large,an RC oscillator may be adequate.For ex-ample:if a stopwatch is built with a resolution of tenths of conds and the longest interval of interest is two min-utes,then an accuracy of 1part in 1200(2minutes x 60conds/minute x 10tenth/cond)may be acceptable sin
ce any error is less than the resolution of the device.2.When logic elements are operated near their specified
limits.It may be necessary to maintain clock frequency accuracy within very tight limits in order to avoid exceed-ing the limits of the logic family being ud,or in which the timing relationships of clock signals in dynamic MOS memory or shift register systems must be prerved.3.Baud rate generators for communications equipment.4.Any system that must interface with other tightly speci-fied systems.Particularly tho that u a “handshake”technique in which Request or Acknowledge puls must be of specific widths.LOGICAL OSCILLATORS
Before describing any specific circuits,a few words about logical oscillators may clear up some recurring confusion.Any odd number of inverting logic gates will oscillate if they are tied together in a ring as shown in Figure 1.Many begin-ning logic designers have discovered this (to their chagrin)by i
nadvertently providing such a path in their designs.How-ever,some people are confud by the circuit in Figure 1be-cau they are accustomed to eing sinewave oscillators implemented with positive feedback,or amplifiers with non-inverting gain.Since the concept of pha shift be-comes a little strained when the inverters remain in their lin-ear region for such a short period,it is far more straightfor-ward to analyze the circuit from the standpoint of ideal
switches with finite propagation delays rather than as ampli-fiers with 180˚pha shift.It then becomes obvious that a “1”chas itlf around the ring and the network oscillates.
The frequency of oscillation will be determined by the total propagation delay through the ring and is given by the follow-ing equation.
Where:
f =frequency of oscillation Tp =Propagation delay per gate n =number of gates
This is not a practical oscillator,of cour,but it does illus-trate the maximum frequency at which such an oscillator will run.All that must be done to make this a uful oscillator is to slow it down to the desired frequency.Methods of doing this are described later.
100percent
To determine the frequency of oscillation,it is necessary to examine the propagation delay of the inverters.CMOS propagation delay depends on supply voltage and load ca-pacitance.Several curves for propagation delay for Fair-child’s 74C line of CMOS gates are reproduced in Figure 3.From the,the natural frequency of oscillation of an odd number of gates can be determined.An example may be instructive.
Assume the supply voltage is 10V.Since only one input is driven by each inverter,the load capacitance on each in-verter is at most about 8pF.Examine the curve in Figure 3c that is drawn for V CC =10V and extrapolate it down to 8pF.We e that the curve predicts a propagation delay of about 17ns.We can then calculate the frequency of oscillation for three inverters using the expression mentioned above.Thus:
Lab work indicates this is low and that something clor to 16MHz can be expected.This reflects the conrvative na-ture of the curves in Figure 3.
Since this frequency is directly controlled by propagation de-lays,it will vary a great deal with temperature,supply volt-age,and any external loading,as indicated by the graphs in Figure 3.In order to build a ufully stable oscillator it is nec-
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FIGURE 1.Odd Number of Inverters
Will Always Oscillate
Fairchild Semiconductor Application Note 118October 1974
CMOS Oscillators
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©1998Fairchild Semiconductor Corporation
essary to add passive elements that determine oscillation frequency and minimize the effect of CMOS characteristics.STABLE RC OSCILLATOR
Figure 2illustrates a uful oscillator made with three invert-ers.Actually,any inverting CMOS gate or combination of gates could be ud.This means left over portions of gate packages can be often ud.The duty cycle will be clo to 50%and will oscillate at a frequency that is given by the fol-lowin
g expression.
The following three special cas may be uful.
hunt
Figure 4illustrates the approximate output waveform and the voltage V 1at the charging node.
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FIGURE 2.Three Gate Oscillator
Propagation Delay vs Ambient Temperature MM54C00/MM74C00,MM54C02/MM74C02,MM54C04/MM74C04
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(a)
Propagation Delay vs Ambient Temperature MM54C00/MM74C00,MM54C02/MM74C02,MM54C04/MM74C04
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(b)
Propagation Delay Time vs
Load Capacitance MM54C00/MM74C00,MM54C02/MM74C02,MM54C04/MM74C04
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(c)
FIGURE 3.Propagation Delay for 74C Gates
2
Note that the voltage V 2will be clamped by input diodes when V 1is greater than V CC or more negative than ground.During this portion of the cycle current will flow through R2.At all other times the only current through R2is a very mini-mal leakage term.Note also that as soon as V 1pass through threshold (about 50%of supply)and the input to the last inverter begins to change,V 1will also change in a direc-tion that reinforces the switching ,providing posi-tive feedback.This further enhances the stability and predict-ability of the network.
This oscillator is fairly innsitive to power supply variations due largely to the threshold tracking clo to 50%of the sup-ply voltage.Just how stable it is will be determined by the fre-quency of oscillation;the lower the frequency the more sta-bility and vice versa.This is becau propagation delay and the effect of threshold shifts compri a smaller portion of the overall period.Stability will also be enhanced if R1is made large enough to swamp any variations in the CMOS output resistance.
TWO GATE OSCILLATOR WILL NOT NECESSARILY OSCILLATE
A popular oscillator is shown in Figure 5a .The only undesir-able feature of this oscillator is that it may not oscillate.This is readily demonstrated by letting the value of C go to zero.The network then d
egenerates into Figure 5b ,which obvi-ously will not oscillate.This illustrates that there is some value of C1that will not force the network to oscillate.The real difference between this two gate oscillator and the three gate oscillator is that the former must be forced to oscillate by the capacitor while the three gate network will always os-cillate willingly and is simply slowed down by the capacitor.The three gate network will always oscillate,regardless of the value of C1but the two gate oscillator will not oscillate when C1is small.
The only advantage the two gate oscillator has over the three gate oscillator is that it us one less inverter.This may or may not be a real concern,depending on the gate count in each ur’s specific application.However,the next ction offers a real minimum parts count oscillator.A SINGLE SCHMITT TRIGGER MAKES AN OSCILLATOR
Figure 6illustrates an oscillator made from a single Schmitt trigger.Since the MM74C14is a hex Schmitt trigger,this os-cillator consumes only one sixth of a package.The remain-ing 5gates can be ud either as ordinary inverters like the MM74C04or their Schmitt trigger characteristics can be ud to advantage in the normal manner.Assuming the five inverters can be ud elwhere in the system,Figure 6must reprent the ultimate in low gate count oscillators.
Voltage V 1is depicted in Figure 7and changes between the two thresholds of the Schmitt trigger.If the thresholds were constant percentages of V CC over the supply voltage range,the oscillator would be innsitive to variations in V CC .However,this is not the ca.The thresholds of the Schmitt trigger vary enough to make the oscillator exhibit a good deal of nsitivity to V CC .
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FIGURE 4.Waveforms for Oscillator in Figure 2AN006022-7
(a)
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(b)
FIGURE 5.Less Than Perfect Oscillator
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FIGURE 6.Schmitt Trigger Oscillator
Applications that do not require extreme stability or that have access to well regulated supplies should not be bothered by this nsitivity to V CC .Variations in threshold can be ex-pected to run as high as four or five percent when V CC varies from 5V to 15V.
A CMOS CRYSTAL OSCILLATOR
Figure 8illustrates a crystal oscillator that us only one CMOS inverter as the active element.Any odd number of in-verters may be ud,but the total propagation delay through the ring limits the highest frequency that can be obtained.Obviously,the fewer inverters that are ud,the higher the maximum possible frequency.CONCLUSIONS
A large number of oscillator applications can be imple-mented with the extremely simple,reliable,inexpensive and versatile CMOS oscillators described in this note.The os-cillators consume very little power compared to most other approaches.Each of the oscillators requires less than one full package of CMOS inverters of the MM74C04variety.Frequently such an oscillator can be built using leftover gates of the MM74C00,MM74C02,MM74C10variety.Sta-bility superior to that easily attainable with TTL oscillators is readily attained,particularly at lower frequencies.The os-cillators are so versatile,easy to build,and inexpensive that they should find their way into many diver designs.
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diplomaticFIGURE 7.Waveforms for Schmitt Trigger Oscillator in Figure 6
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FIGURE 8.Crystal Oscillator
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C M O S O s c i l l a t o r s
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