單片機(jī)畢業(yè)設(shè)計英文翻譯1

1、APPLICATION NOTEX-tal oscillators on 8-bit microcontrollersAN96103 SummaryDesigning-in 8-bit microcontrollers occasionally raises questions regarding the crystal oscillator circuit. The support groups in Eindhoven and Zuerich have gained some experience in responding to these kind of customer questi

2、ons. This report reflects some of this experience. For engineers in the field as well as development engineers involved in microcontroller based products that do not have specific oscillator knowledge, reading this report may result in some awareness of the oscillator issues making it easier to appr

3、oach questions on this subject.1. What this note IS and what it is NOT.In the early days of electronics it was quite a challenge to design a circuit that did NOT oscillate. All kinds of un-for-seen component characteristics were the main reason for this, as well as limited knowledge of the oscillati

4、on phenomena. Electronics have come a long way since those days and today components characteristics are well defined. Oscillation has become a science and integrating oscillators even more. On the subject of oscillation and of integration of crystal (X-tal)* oscillators there are many scientific pu

5、blications and courses that provide high level knowledge on this subject. This report will NOT repeat that.In many digital circuits, oscillator circuits are also integrated on the same chip just to provide the clock signal for the digital electronics. Usually only the active parts of the oscillator

6、part is embedded and not the passive frequency determining parts. These parts are usually traditional components supplied by other manufacturers. Thissituation is also valid for most currently supplied microcontrollers.“How to assure oscillation? = a very legal question from the application point of

7、 view. This report IS based on application feedback from the field, providing some background and practical knowledge to come closer to the ultimate answer to this question.*(A crystal or quartz in a circuit diagram is very often indicated as X-tal.)2.The oscillator stage.Most microcontroller device

8、s have an oscillator circuit (Xtal1,Xtal2) that will oscillate with an external crystal and external capacitors. The oscillator stage is basically an inverter type gate consisting of a N-channel and a P- channel transistor. The main difference with a digital inverter stage is an integrated bias resi

9、stor (also called:feedback resistor) connected between output and input. This (semiconductor-) resistor feeds back the output voltage to the input which will balance (bias) the stage in its analog working area. In the quiescent situation this will generate a DC- input and output level of about 1/2 V

10、dd for CMOS devices (For TTL compatible versions it is just a little less). 2.1 Transconductance.When this oscillator stage is driven with an input voltage variation then this will result in an output current variation through an external load. This relation (dVi/dIo) is defined as the transconducta

11、nce of the oscillator stage. This tranconductance, indicated with “gm is defined as the amount of current change as a result of the input voltage change. The unit is A/Volt or S (S=Siemens). 2.2 The Pierce oscillator.The standard circuit for the oscillator is given in figure 3. A Crystal is connecte

12、d between the output (Xtal2) and the input (Xtal1). Output and input have a capacitor connected to the ground. This is basically a Pierce oscillator circuit. With a proper dimensioning of the external components the circuit should generate an (almost) sine wave shape signal on the Xtal2 output pin.

13、One of the parameters related to the oscillator stage that affects the oscillation is the transconductance. A certain value of gm is needed to assure start-up. During power-on a noise signal or a transient should result in an amount of energy fed in to the X-tal to make it start and resonate.This is

14、 one of the basic requirements for any oscillator stage.3. The external components.The frequency determining element in the external components is the crystal (X-tal). Basically a crystal behaves as an LC-circuit for serial resonance. Figure 4. shows a commonly used equivalent circuit. The resonant

15、frequency is determined by the value of L and C, so this is series resonance. C0 represents the total parallel capacitance of the crystal and its value is usually much higher then the one of C. However its influence on the resonating frequency is very small.Some typical values for these equivalent c

16、omponents based on a 10 MHz crystal are:L = 0.01 H, C = 0.026 pF, Rx = 10 ohms, Co = 8.5 pF.Note that in an application the total equivalent value of Co is also highly influenced by the two external capacitors in the basic Pierce oscillator circuit figure 3. In fact the two capacitors in series shun

17、t the crystal, meaning the Co is in fact increased.4. Oscillation condition.An oscillator stage and external components are supposed the generate the clock signal. Is just connecting the external components to the oscillator stage the only condition for oscillation? Again, there are theories on the

18、oscillation condition and the Barkhausen rule covers the oscillation condition basics. In a practical situation however there are many circuit parameters that will determine whether an oscillator circuit will show reliable oscillation.Here are just some of them: - Vdd, supply voltage. - fosc, oscill

19、ator frequency. - gm, oscillator stage transconductance. - Rx, equivalent resistor value. - C0, equivalent total parallel capacity. - Closed loop gain.Only for the crystal there are more then ten parameters. For a practical evaluation it is almost impossible to include all of them. A second reason f

20、or this is that many parameter values are not known to those engineersapplying the component. Does this mean that oscillation will be a matter of luck? Without going in much theoretical detail the impedance approach can be used to give an indication on the expected oscillation. The idea behind it is

21、 that the effective transconductance (1/ohm) should at least make up for the load (ohm)Figure 5. shows a characteristic that indicates a relation between the Rx,C0 and oscillation. When the coordinates of Rx and C0 are in the upper area, oscillation conditions are not met. When the coordinates cross

22、 at the characteristic or are in the lower area there will be proper oscillation.Now the difficulty arises in putting actual values on the Rx and C0 axis, as well as determining the right shapeand position of the characteristic. Practical experience however indicates that a circuit like in figure 3:

23、 with C1=C2 = 30 pF will be in the safe area, when C0 is about 10 pF and Rx is about 100 ohm or smaller.A lower value of Rx allows a higher value of C0 and visa versa.5. The drive level issue.Once we have oscillation is there something else to worry about?Most of the information so far is based on e

24、xperience with the standard 5 Volt microcontrollers. Their oscillator stage was designed to drive a crystal with about 1 mW. This means that the crystal itself should be specified to perform proper resonation when it is driven at this power level. This crystal parameter is called the drive level and

25、 it is specified with the crystal parameters.6. Oscillator circuits in practice.All the 5V microcontrollers have a Pierce oscillator circuit (Xtal1,Xtal2) that will oscillate with an external X-tal and two capacitors. This standard circuit is presented in most data sheets. In some cases there may be

26、 good reasons to modify the standard circuit e.g. to reduce interference and/or to compensate external influences.6.1 Reducing amplitudeReducing the oscillator amplitude is a frequently used approach for e.g. reducing interference. The circuit (figure5) is based on the “Recommendations to reduce the

27、 interference of 558 oscillator (see References). This circuit generates an (almost) sine wave shape signal on the Xtal2 pin.This is achieved by using asymmetrical capacitors, and using the largest capacitor on the INPUT, (XTAL1). In this situation the voltage on XTAL1 is decreased.The input voltage

28、 for the amplifier stage is lower so the stage will not go into saturation and therefore produce a signal with much less harmonics.A customer modified the circuit of figure 5. to the circuit of figure 6.Apparently the loop gain was changed by swapping the two capacitors and then compensated the by s

29、hunting aresistor to the X-tal. This circuit also works, but generates an output signal with more distortion. Increasing the output capacitor will result in more current through the output XTAL2.6.2 DC-offsetsThe main reason for modifying the circuit with a capacitor in series with the input (see ci

30、rcuit of figure 7) had to do with the influence of moisture on the oscillator circuit components. This could effect the basic circuit of figure 5.because due to the conductance of moisture a DC-offset is introduced to the XTAL1 input resulting in an asymmetric bias of the oscillator stage which may

31、affect start-up and/or event prevent the stage from oscillating. When this is due to the PCB-tracks and/or oscillator circuit external components it could be eliminated by using an extra capacitor with a non critical value when it is much larger then the value C1 and C2 (see figure 7).Another possib

32、ility of reducing the sensitivity for environmental conditions like DC-offsets is shunting the oscillator stage with a resistor rather than shunting the X-tal. This way the input sensitivity is reduced but also the loop gain. It can also be used for amplitude reduction. It may be necessary to modify

33、 the value of the C1. See figure 8:All these circuit examples were tested with a 559 device. They all start oscillating already at about 3 Volts Vdd.6.3 Driving other circuitryAnother frequently asked question rises when more microcontrollers are used in the same application and/or microcontrollers

34、are used with other ICs that require a clock signal. In many of these cases it is possible to use one crystal only.Figure 9 shows this situation when using the standard oscillator circuit. The Xtal2 output (micro 1) is directly connected to the Xtal1 input of the second microcontroller. The Xtal2 ou

35、tput from the second microcontroller remains not connected.The circuit for the microcontroller 1 may be modified as described earlier in this report. In these cases always connect the input of the microcontroller 2 to the Xtal2 output of microcontroller 1 rather than direct on the X-tal.This is to m

36、inimize the effects of the extra load on the oscillator circuit. To avoid any DC-offset for microcontroller2 (a DC offset may result in more asymmetric duty cycle of the signal on Xtal2) the connection between uC1 and 2 may be DC-decoupled by using a capacitor. Figure 10 shows the situation.Both cir

37、cuits are applicable for most microcontrollers. Some controllers however have two on-board oscillators one for a direct clock signal and one for a PLL clock. In this last case the X-tal frequency is normally 32 kHz. This oscillator stage has pull down devices on the input and output.This means that

38、transistors are added (on-chip)which are switched on under certain conditions, e.g. when the oscillator is disabled.These transistor then connect Xtal1 and Xtal2 to the ground. To avoid the flow of (too) much current into the pull down transistor it is recommended to insert a resistor in the interco

39、nnection to the Xtal4-input. Figure 11 shows this situation.7. Emulation and the X-tal oscillator.Developing microcontroller applications are usually carried out with the use of an emulator device. In a carefully configured oscillator circuit around a microcontroller the microcontroller is removed a

40、nd replaced by a connector with a cable connection to emulating device which hopefully has identical oscillator stage characteristics.Even when it has, inserting a cable can dramatically influence the oscillator behaviour especially at higher frequencies. Two low-cost emulators each have different w

41、ays to handle with this issue:7.1.PDS-51 approach.In the PDS51 system this problem is approached by not using the target oscillator hardware. The system has a separate oscillator circuit and the target X-tal has to be removed from the application and then inserted on the PDS51 daughter board.When th

42、e application requires the signal Xtal2 to be present on the target (for driving other circuitry) there is a jumper to be set. Note that the signal from the PDS51 is then usually a better square wave shape signal then the original Xtal2 output signal.When the target microcontroller is not using its

43、own oscillator but gets a signal on the Xtal1 input from an other source then this signal can be used for the emulator when it has TTL drive capability.7.2 .DS-750 /EB-51 approach.Two other popular third party low cost emulator board are the DS-750 and EB-51. Both systems have several fixed internal

44、 oscillator frequencies of which one is to be selected by the user. This will probably cover the used frequencies in most applications. When the application uses a frequencies that is not available on these board then there is an option to go external, meaning that the emulating microcontrolller on

45、the emulator board will be using the oscillator components on the target application. As stated before this means that there is a cable inserted between microcontroller oscillator stage and the external components. In this situation be aware that the induction and capacitance values of cables and co

46、nnectors can affect oscillator behaviour dramatically especially at higher frequencies as well as in those cases where the Xtal2 signal also drives other circuitry.摘自周立功單片機(jī)網(wǎng)站：基于八位微控制器AN96103的X-tal振蕩器應(yīng)用指南摘要所設(shè)計的八位微控制器有時會對晶體振蕩器電路的問題，在應(yīng)對這些客戶問題上，那些在Eindhoven和 Zuerich的支持團(tuán)隊已經(jīng)積累了許多經(jīng)驗。這篇報告就反映了一些這方面的經(jīng)驗知識。在該領(lǐng)域

47、的工程師以及參加研發(fā)微控制器的工程師缺乏具體的振蕩器知識，如果他們閱讀了這篇文章，將會對振蕩器知識有更新的認(rèn)識，解決這個領(lǐng)域上的問題將會變的更容易。 1.需要注意和防止的問題 在早期的電子業(yè)上，想要設(shè)計一個不震蕩的電路是非常困難的，因為沒有可供參考的電路構(gòu)成特性的材料和非常有限的震蕩現(xiàn)象知識是造成這種結(jié)果的主要原因。電子業(yè)的開展已經(jīng)走了漫長的道路，直到今天，這種構(gòu)成特性終于被定義出來了。震蕩現(xiàn)象以及集成振蕩器已經(jīng)開展成為了一門科學(xué)。關(guān)于震蕩電路和一體化晶體振蕩器的問題，目前有許多科學(xué)出版物和培訓(xùn)班為你提供更高層次的知識。本文章不再贅述。 在許多數(shù)字電路，振蕩器電路也集成在同一個芯片上數(shù)字電子產(chǎn)

48、品提供時鐘信號，通常只有振蕩器的靈活局部是嵌入式，但這一局部卻不是被動頻率確實(shí)定局部。這些部件的構(gòu)成局部通常是由其他制造商所提供的。這種形式通常也是目前提供微控制器的最有效途徑?！霸鯓哟_保振蕩？從應(yīng)用的觀點(diǎn)看，這是一個非常有法律性的問題。這篇文章是根據(jù)應(yīng)用領(lǐng)域的反應(yīng)，提供一些背景和實(shí)踐知識，以確保接近解決問題的最正確答案。在電路圖中，x-tal往往表示石英晶體2振蕩器電路 大多數(shù)微控制器設(shè)備都包含一個振蕩器電路(Xtal1,Xtal2)，通過部晶振和外部電容器來實(shí)現(xiàn)振蕩。振蕩器根本上是由一個N溝道和一個P溝道的晶體管構(gòu)成的逆變型門。它們的主要區(qū)別是在數(shù)字式逆變階段中連接輸入和輸出之間的是一個集

49、成偏電阻也稱為：反應(yīng)電阻。該器件半導(dǎo)體將輸出電壓反應(yīng)到輸入局部，其目的是在模擬工作階段使其到達(dá)平衡。在靜態(tài)狀態(tài)下，將產(chǎn)生一個直流輸入和輸出，其數(shù)值大約是CMOS器件中電壓的1/2。2.1 跨電導(dǎo) 當(dāng)一個電壓變量驅(qū)動該振蕩器，系統(tǒng)通過一個輸出外部負(fù)載輸出一個電流變量。dVi/dIo輸入電壓/輸出電流定義為振蕩階段的跨電導(dǎo)，用符號購gm表示跨電導(dǎo)，輸入電壓的變化造成輸出電流的變化，單位是A/V或是S西門子。2.2 皮爾斯振蕩器 如圖3所示為振蕩器的標(biāo)準(zhǔn)電路。輸出Xtal2局部和輸入Xtal1局部通過一個石英晶體相連接，并且輸入和輸出通過電容器接地。這是一個根本的皮爾斯振蕩器。在Xtal2輸出引腳上

50、產(chǎn)生一個正弦波，跨電導(dǎo)是影響振蕩階段的相關(guān)參數(shù)。gm的一個定值是保證啟動的重要參數(shù)。在電源啟動噪音信號或是在瞬態(tài)期間都會把一定的能量輸入到x-tal振蕩器中，使之啟動并發(fā)生諧振。這是任何一個振蕩階段根本需求之一。3.外部組件 在外部器件中，頻率的決定性部件是石英晶體。根本上是石英晶體使得LC電路產(chǎn)生串聯(lián)諧振，如圖4所示，是一個常用的等效電路。電感值和電容值決定著諧振頻率，因此成為串聯(lián)諧振。C0代表并聯(lián)電容的晶體，其數(shù)值要比電容值大。但其影響的共振頻率非常小。晶體頻率是10 MHz時，其相應(yīng)的等效組分的值為：L = 0.01 H, C = 0.026 pF, Rx = 10，Co = 8.5 p

51、F。在圖3中根本的皮爾斯振蕩器電路中，外部的兩個電容值影響著CO等效值。事實(shí)上，兩個電容器串聯(lián)分流晶體，這意味著CO值增加了。4.振蕩條件 振蕩器和外部元件都產(chǎn)生時鐘信號。難道連接的外部件就是振蕩器震蕩的唯一條件嗎？再次，振蕩條件定理和巴克豪森規(guī)那么涵蓋了振蕩條件根底。然而在實(shí)際情況下有許多電路參數(shù)決定是否有電路展示可靠的振蕩。其電路參數(shù)如下： - Vdd, 電源電壓 - fosc, 振蕩頻率 - gm, 振蕩器跨電導(dǎo) - Rx, 等效電阻值 - C0, 總并聯(lián)電容等效值 - 閉環(huán)增益。對于晶體元件的參數(shù)要超過10個，首先是對于一個實(shí)際部件的評價不可能包括所有的參數(shù)。第二個原因是有些工程人員并

52、不知曉某些參數(shù)應(yīng)用在哪些部件上。難道這就意味著振蕩現(xiàn)象就是運(yùn)氣的現(xiàn)象嗎？沒有太多詳盡的理論中說明阻抗方法可以用來說明預(yù)期振蕩。如圖5所示，其作者的想法是有小的跨導(dǎo)至少能夠補(bǔ)償電阻，圖中說明電容值、電阻和振蕩器之間的關(guān)系，當(dāng)電阻和電容的坐標(biāo)在上面的區(qū)域時，說明不滿足振蕩條件。當(dāng)電容值和電容值在坐標(biāo)軸上或在下面的區(qū)域，那么說明此時有適宜的振蕩?，F(xiàn)在的困難是確定Rx軸和CO軸上的實(shí)際值以及確定正確的圖形和位置的特點(diǎn)。然而在實(shí)際上，如圖3中的電路圖說明：當(dāng)C0約為10 pF和Rx為100歐姆或更小時。并且C1=C2=30Pf時，設(shè)備工作在平安區(qū)域。較小的電阻值允許一個較大的電容值，反之亦然。5. 驅(qū)動問題 我們也曾擔(dān)憂振蕩會發(fā)生別的問題？到目前為止，大局部是以標(biāo)準(zhǔn)電壓為5伏的微控制器為根底的，其振蕩器階段的目的是要推動約1兆瓦的晶體。這就意味著，當(dāng)電壓值到達(dá)驅(qū)動值時晶體本身立即動作。這個晶體參數(shù)稱為驅(qū)動級并且是指定的晶體參數(shù)。6.實(shí)際的振蕩器電路 所有的5V的微控制器都由一個皮爾斯振蕩器Xtal1,Xtal2)組