低速晶振注意事项

Freescale Semiconductor Application Note

AN2606

Rev. 1, 08/2005 Table of Contents

1Introduction

Many devices use a 32768 Hz crystal; following are many reasons for choosing this frequency. First of all, it is a power of two, i.e., 32768 is the fifteenth power of two. If an oscillator driven by a 32768 Hz crystal is followed by a divide by 215 circuit, one-second ticks are obtained. Manufacturing techniques have been developed to build these crystals with very high precision. Thus, oscillators can employ these crystals to develop “one-second ticks” with the same percentage accuracy of the crystal frequency. This is why 32768 Hz crystals are so widely used in digital watches and other time-keeping functions.

The 32768 Hz crystals are also used as a clock source for internal PLL/VCO clock circuits on microcontrollers. In addition to the time-keeping functions, another desirable attribute of using a low-speed oscillator in microcontrollers is that they run on batteries. A 32768 Hz CMOS oscillator draws very little power compared to oscillators running at higher frequencies. Power consumption in oscillators is essentially a linear function of frequency. Therefore, using a low-speed oscillator to feed a PLL/VCO clock generation circuit has significant 1Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2Selecting External Circuit Components . . . . . . . . 4 3Oscillator Start-up Time . . . . . . . . . . . . . . . . . . . . 5 4Power Dissipation. . . . . . . . . . . . . . . . . . . . . . . . . 6 5Layout Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6Grit and Grime . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7Diagnosing Oscillator Problems . . . . . . . . . . . . . . 8 8Practical Methods for Evaluating Oscillator Problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

8.1Evaluating Oscillator Start-up Time. . . . . . . . 9

8.2Observing Symptoms of Overdriving the

Crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

8.3Choosing the Proper Crystal . . . . . . . . . . . . 10

8.4Assembly Techniques When Soldering Crystals

to Printed Circuit Boards10

9Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Practical Considerations for Working With Low-Frequency Oscillators

by: Meera Sridharan, Charles Melear

Introduction

implications on power saving in automotive applications, where the microcontroller must run from the power supplied by the car battery, regardless of the time interval during which the car remains parked. When the automobile is not running, the electronic circuit, by using a 32.768 KHz oscillator and driving a PLL/VCO, can turn off the PLL/VCO, so that only the oscillator runs. When the car is started, the PLL/VCO is powered on, and the entire circuit is brought from a low-power state to a fully functional mode.

Using a low-frequency crystal (32768 Hz crystals, generally) presents issues that are not normally major problems with crystals operating in the megahertz range. In general, crystals of this frequency range tend to start much more slowly than high-frequency crystals. Also, 32.768 KHz crystals may attempt to oscillate at overtones or harmonics of the fundamental frequency when they start from a power-on harmonic of the fundamental frequency of the crystal. Physical properties of the crystal determine the frequency of the overtone. A harmonic is an overtone that is a multiple of the fundamental frequency. Hence, the nth harmonic is n times the fundamental. Layout issues, proximity of high-speed conductors near and around the crystal oscillator, and component placement issues also affect the crystal’s behavior as the oscillator powers up.

As a practical note, it should be firmly understood that 32768 Hz crystals are neither created equal, nor close. Crystal parameters vary widely between different models produced by the same manufacturer, or between manufacturers and styles of crystals. Designers must be aware that crystal oscillator circuits that require “tweaking” or that must have very high tolerance components to make the oscillator work are going to be marginal, at best. Most oscillators work reliably with components that vary over a reasonable range of values. If a crystal is used that requires high-precision components, the circuit designer should give very special consideration to using a different brand or style of crystal.

Also, as a practical note, when investigating problems with oscillator circuits, great care must be taken with measurement equipment. A typical oscilloscope probe has roughly 10 M ohms of impedance. While this may not sound like very much, it is significant when probing an oscillator circuit. Simply touching the EXTAL (input) pin of an oscillator circuit with a scope probe will significantly alter the characteristics of the circuit and will introduce new problems when trying to analyze existing problems associated with the oscillator circuit.

There are numerous papers written about the mathematical models of oscillators that offer rigorous mathematical treatments. This paper is not intended to duplicate that information. However, it is the business of crystal manufacturers to understand these mathematical models and the physical parameters of their particular crystal, so that they can make recommendations as to what external components should be used with their crystal to build reliable oscillator circuits. The major parameters that crystal manufacturers must know about the microprocessor with regard to specifying external components for the 32768 Hz oscillator circuit are the input and output capacitance of the oscillator pins, EXTAL, and XTAL, respectively, and that the microcontroller has sufficient drive capability for the crystal.

Introduction

microcontrollers.

This circuit consists of two parts: an inverting amplifier that supplies a voltage gain and a 180 degree phase shift and a frequency selective feedback path. The crystal combined with C1 and C2 form a PI network that tends to stabilize the frequency and supply a 180 degree phase-shifted feedback path. In steady state, this circuit has an overall loop gain of one and an overall phase shift that is an integer multiple of 360 degrees.

The inverter for the oscillator is contained within the microcontroller. In actual practice, this is usually a chain of three inverters. However, for the purposes of designing a working oscillator, the inverter can be treated as a single stage. The inverter provides the 180 degrees of phase shift necessary for oscillation. The crystal is made up of a piezoelectric quartz. A crystal must be chosen that has good self-suppression characteristics for harmonics and overtones. Also, it must lock into the fundamental frequency in an appropriate amount of time. It cannot be emphasized enough that no amount of circuit design can ever compensate for a bad crystal. Crystals are modeled as series R-L-C networks. In the model, R is the equivalent series resistance (ESR). Motional capacitance, C represents the elasticity of the quartz and motional inductance; L represents the vibrating mass of the crystal unit. The Co, or shunt capacitance, is the capacitance formed by the metal electrodes deposited on each side of the quartz blank, plus the stray capacitance associated with the holder and leads of the component. Co is generally specified as 7pF maximum. Once again, as a practical matter, the crystal manufacturer should take into consideration these parameters and recommend values for external components.

A property of the inverter is its input capacitance, Ci, and output capacitance, Co. Sometimes, there will be an internal feedback resistor in the microcontroller. This will definitely affect the external components. If an internal feedback resistor exists, an external feedback resistor, R f, may not be needed.

The feedback resistor, R f, is usually 10 - 20 Megohms. This resistor is used to bias the input to the inverter. Because only leakage current flows into the input of the inverter, there should be very little voltage drop across the feedback resistor. This causes the input of the inverter to be pulled toward the voltage on the

Selecting External Circuit Components

output. This, of course, creates an unstable condition, and that is what is needed to form an oscillator. R f affects the loop gain of the amplifier. Lower values of R f lower loop gain, while higher values increase loop gain.The series resistor, R s , is used to limit the amount of drive current to the crystal supplied from the XTAL pin of the inverter. It is very easy to overdrive the crystal. Therefore, the series resistor must be picked with manufacturer of the crystal is probably the best source of information for picking a value for Rs. Rs must be large enough to limit the current to the crystal but, at the same time, be small enough to provide enough current to start oscillation quickly. If Rs is too small, the crystal will start up in unpredictable modes or dissipate too much power. This can cause heating problems and, in extreme cases, even damage the crystal. On the other hand, if Rs is too large, the oscillator will start slowly or not start at all.

The bypass capacitors, C1 and C2, are used, in part, to create a low-pass filter. The capacitive load on one

side of the crystal should be roughly equal to the capacitive load on the other side of the crystal. In many circuits, designers make C1 equal to C2. Because the output capacitance of the inverter is usually a few picofarads greater than the input capacitance, the bypass capacitor on the EXTAL pin can be a little larger than the bypass capacitor on the XTAL pin.

The circuit and layout capacitances also add to the values of C1 and C2. The correct choices of C1 and C2 are very important to oscillator start-up and steady-state conditions. Imbalances in capacitive and inductive impedance can cause phase and amplitude problems in the feedback loop.

As a practical matter, if a few picofarads of capacitance make a difference between whether or not the oscillator works, the oscillator circuit definitely has significant design problems that should be

investigated.

2The external oscillator circuit components are chosen with respect to two major goals. First, the power supplied to the crystal must be limited to prevent overdriving the crystal, as well as limited to the extent that the crystal will not start, and, second, to form a low-pass filter that suppresses frequencies above the crystal’s fundamental frequency.

The frequency vs. amplitude characteristics of a typical low-pass filter are presented in Figure 2.

Oscillator Start-up Time

Figure2. Frequency vs. Amplitude Characteristics of a Typical Low-pass Filter

The goal is to set the three db point of the filter roughly halfway between the fundamental crystal frequency and the crystal’s first harmonic. In the present case, the three db point should be approximately 50 Khz. Ci, Co, C1, C2, Rf, Rs, and the impedance of the crystal all figure into the calculation of the low-pass filter characteristics. Because of the difficulty in determining the characteristics of the crystal, it is always a wise practice to obtain recommendations from the crystal’s manufacturer in selecting the external oscillator-circuit components. Recall that crystal parameters vary over a wide range between manufacturer and styles of crystals. Design engineers must realize that the external component values for one type/brand of crystal may be totally different for another type/brand of crystal.

3Oscillator Start-up Time

As previously stated, the start-up time for low-speed oscillators (under 100 KHz) is relatively slow when compared to high-speed oscillators (several megahertz and greater). While high-speed oscillators may reliably start in a few hundred microseconds, low-speed oscillators typically start in a few milliseconds. Start-up times for 32.768 KHz crystals of 200 - 400 milliseconds are very common. When measuring start-up time, it is important not to disturb the oscillator circuit. Putting a typical scope probe on the EXTAL or XTAL pin usually constitutes a significant loading factor on the oscillator circuit and will affect observed values. A reasonably accurate start-up time can be obtained by measuring the time difference between the application of power and observing some type of buffered form of the oscillator output. Often, there will be an “oscillator out” signal from the microcontroller. In other cases, the 32.768 KHz oscillator may be driving a VCO, which is internal to the microcontroller, and the output of the VCO will appear on a package pin. Another way to indirectly measure the output of the oscillator is to apply the XTAL output pin to a logic gate or buffer and then measure the output of the buffer with an oscilloscope. The value of the bypass capacitor in the oscillator circuit will need to be decreased by the value of the input capacitance of the buffer, so that it does not change the oscillator circuit loading parameters. By observing the output of the buffer with an oscilloscope instead of the XTAL pin, a reliable measurement can be obtained to determine when the oscillator has started with respect to the application of power.

Power Dissipation

The time between the application of power to the oscillator circuit and observing a stable, buffered form of the 32.768 KHz signal should be in the range of 200 - 400 milliseconds. If the start-up time for the crystal is less than 200 milliseconds, the circuit should be carefully evaluated for the presence of unusual oscillator modes, overtone frequencies, and various harmonics. Recall that crystals may attempt to start at the first overtone and that overdriving the crystal will aggravate this condition. Also, crystals can start at the first overtone and then quickly switch to the fundamental crystal frequency. This can be particularly troublesome if the output of the 32.768 KHz oscillator is driving a VCO with a relatively high multiplication factor.

Another problem that can result in extreme cases of being overdriven is unpredictable oscillation modes. When using low-frequency “tuning fork” elements, the frequency of oscillation is determined by the length of the “fork.” However, if the element is grossly overdriven, just the very tips of the “fork” may oscillate. This is usually at a frequency greater than ten times the fundamental. When this happens, the “tuning fork” probably will not be able to recover until power is removed from the circuit.

Although the case just described is extreme, one must recall the wide range of parameters found in crystals.

A series resister of 50 K ohms may be optimal for one type or brand of crystal but result in a case of extreme overdriving for another type or brand of crystal. Once again, it is very important to consult the applications information supplied by the crystal manufacturer rather than the integrated circuit manufacturer for proper applications information.

At the other end of the spectrum is under-driving the crystal. These problems are much easier to diagnose because the symptoms are that the crystal will not start at all or it takes a very long time (greater than one second) to start. In this case, a designer can use successively lower (fewer ohms) series resistor values until reliable starting times of approximately 200 to 400 milliseconds are observed.

Because of the transient nature of oscillator start-up phenomena, i.e., temporary oscillation modes, it is sometimes difficult to determine that this is the root cause of system failures. Therefore, it is important to fully evaluate the start-up characteristics of crystal oscillators before committing to production.

4Power Dissipation

This topic is related to the reliability of operation over time than with getting the crystal to oscillate. While a crystal is an “open circuit” to a DC voltage, it definitely presents an impedance to an AC voltage. In other words, the crystal will dissipate energy, just like a resistor, inductor, or capacitor, when stimulated by an alternating pattern. Crystal manufacturers generally evaluate the permissible levels of power needed to insure proper crystal operation. Exceeding those levels will certainly reduce crystal life expectancy and, in some cases, will actually damage the crystal.

Some designers overdrive the crystal to get shorter oscillator start-up times. It is imperative that crystal parameters with respect to power dissipation be strictly observed since overdriving the crystal creates many more problems than it solves.

5Layout Issues

In general, low-frequency oscillator components, i.e., the crystal, bypass capacitors, series resistor, and feedback resistor should be as physically close to the microcontroller’s oscillator pins as possible. It has

Grit and Grime been previously stated in this paper that properly designed oscillator circuits employed with a high quality crystal can tolerate a reasonably wide range of component values and still yield acceptable operation. However, there are definitely limits to the range of component values. For instance, 20 pf bypass capacitors may be optimal while 50 pf bypass capacitors may be so large that they will not allow the circuit to oscillate.

Improper component layout techniques can result in unintended consequences, which add to capacitance values. For instance, if the bypass capacitors are placed several inches away from the microcontroller, lead capacitance can easily add 10 to 30 pf to the value of the bypass capacitors. Causing a differential of 30 pf between the bypass capacitors has a significant potential for causing start-up problems with the oscillator.

A second layout issue concerns conductors carrying high-frequency signals being routed near any of the external oscillator components. Because of the high impedance of many of the circuit components, capacitive coupling can become quite problematic. If a noise spike is coupled into the input of the microcontroller’s internal inverter, the spike will be reflected in the inverter’s output which in turn, will cause a significant perturbation in the amplifier. That is, the noise spike may induce the crystal to oscillate in an unpredictable mode. Since low-frequency crystals are often used with VCOs that have large multiplication factors, any noise introduced into the oscillator circuit will be multiplied by the VCO.

It cannot be stated with enough emphasis that all the external oscillator components must be located very close to the EXTAL and XTAL pins of the microcontroller. Also conductors carrying high-frequency signals or any signals with very sharp edges must be kept away from the oscillator components. If the 32.768 KHz signal is being used to drive a Real Time Clock and noise spikes get coupled into the oscillator output, the Real Time Clock will “gain” time. Since these spikes will probably occur at a relatively low rate compared to 32.768 KHz, the “creep” in the actual time will not show up very fast. What is often observed with noise coupling is that the real time clock will gain a few seconds per hour or per day. Even a few seconds per day is a very significant error for a real time clock.

If the 32.768 KHz oscillator is driving the input of a PLL/VCO, a noise spike in the oscillator output will look like an instantaneous doubling of the input frequency to the PLL/VCO. This, in turn, can cause the VCO to take a very large frequency swing. If the output of the PLL/VCO is supplying the system clock for a microcontroller, the output of the PLL/VCO can easily swing well outside the operating frequency of the microcontroller and cause the entire system to become inoperative.

6Grit and Grime

Oscillators are quite sensitive to dirt, solder flux, grease, condensation due to high humidity, and other conducting materials on the circuit board. These materials can allow a high-resistance leakage path from one of the amplifier pins to either the ground or positive terminal of the power supply. When the oscillator has power applied but has not started to oscillate, the crystal and the bypass capacitors appear as DC open circuits. An oscillator in a DC condition would appear as shown in Figure3.

The resistor, Rd, represents a high-resistance leakage path, within the range of five to 20 megohms. The feedback resistor, Rf, is also in this range. Assuming that Rd and Rf are both ten megohms, the voltage at point A is one-half the voltage difference between points B and C. Thus, if the XTAL pin is at a logic one (4.5 volts), and point C is at ground, the voltage at point A (EXTAL pin) will be 2.25 volts. If point B is at a logic zero (0.5 volts), and point C is at ground, the voltage at point A is 0.25 volts. Thus, the voltage

Diagnosing Oscillator Problems

at point A may be interpreted as a logic zero regardless of whether the XTAL pin is a logic one or a logic zero. This depends on the threshold of the inverter whose input is connected to point A. Likewise, if point C is connected to 5 volts, point A may be interpreted as a logic one regardless of the state of the XTAL

Figure3. Oscillator Configuration In a DC Condition

The only way to diagnose this problem is to remove the external circuit components, as well as the MCU from the board, and use an ohm meter to check the resistance from points A and B to ground and five volts. Anything other than a completely open circuit is a sign of trouble. Leakage paths of several megohms will definitely cause trouble. The obvious solution is to clean the printed circuit board. If the dirt, grime, or other conducting material that forms the high-resistance path is on an inner layer of the printed circuit board, the board will most likely be unusable. If the leakage is due to condensation on the board, spray the oscillator circuit with a protective coating.

7Diagnosing Oscillator Problems

Oscillator problems present a particular challenge in finding errant conditions. One reason that oscillator problems are hard to diagnose is that the problems are often transient in nature. The second challenge is that almost all measurement equipment will perturbate the system to a sufficient degree that problems caused by the test equipment cannot be separated from errant component or layout issues. Looking at the diagram shown in Figure3,it was shown that a leakage path caused by some type of conductive film could form a high resistance “sneak” path to either Vdd or Vss. A typical oscilloscope probe will often have a 10 Megohm input resistance. There is no practical difference between a leakage path created by “grit and grime” and that caused by an oscilloscope probe. If at all possible, look at a buffered form of the oscillator output, so that perturbations are not created by the test equipment.

Practical Methods for Evaluating Oscillator Problems 8Practical Methods for Evaluating Oscillator Problems

8.1Evaluating Oscillator Start-up Time

One method for evaluating oscillator start-up time is to observe a buffered form of the oscillator with respect to the application of power to the circuit. A dual trace scope can be employed for this purpose. One probe is placed on the Vdd pin of the oscillator circuit, and the other scope probe is placed on the “clock output” pin of the microcontroller. Most microcontrollers have some form of clock output pin. The oscilloscope is set to trigger on the rising edge of Vdd, i.e., trigger upon the application of power. Two characteristics of the system can be observed. One is the rise time of the power supply, and the other is the delay between the time that Vdd reaches its proper operating range and the appearance of a clock signal at the proper frequency on the “clock out” pin of the microcontroller. Crystals require a certain amount of power to start into a stable oscillation pattern. Since the power supplied to the crystal is going to be a function of the power supply, oscillator start-up times are going are going to be strongly affected by the rise time of the power supply. Another factor is that a power supply with very sharp rise times will act like an impulse to the crystal, causing it to start faster when compared to using a power supply with a very slow rise time. By picking a specific voltage, i.e., the minimum operating voltage for the microcontroller, a standard measuring point can be used for timing measurements.

In general, an oscillator start-up time for a 32.768 KHz crystal should be within the range of 200 - 400 milliseconds. (Note: Start-up times exceeding 700 milliseconds are probably so long that they indicate the crystal is having trouble starting at all, and the value of the series resistor should be reduced.) If the microcontroller does not have a buffered form of the oscillator, another method can be employed to evaluate start-up time. A buffer can be connected to the XTAL pin of the microcontroller circuit, shown in Figure1. The load capacitor on the XTAL pin should be reduced by the amount of the input capacitance of the buffer. This keeps the capacitive load on the XTAL pin the same as before the buffer was added. The measurement point for the oscillator start up is now the output of the buffer connected to XTAL. This method is used, of course, to keep the measurement equipment from perturbating the system being evaluated.

8.2Observing Symptoms of Overdriving the Crystal

Once again, it is important that the measuring equipment does not perturbate the system. Therefore, a buffered form of the oscillator signal should be observed, or measuring equipment with extremely high impedance should be used. This means that scope probes should have an input impedance which is much greater than the feedback resistor shown in Figure3.

When a crystal is overdriven, the crystal will often attempt to start at an overtone or the first harmonic. In extreme cases, the crystal will enter some undefined oscillation mechanism that may not be a harmonic or overtone frequency at all. Observing this behavior is particularly difficult because it can be transient in nature and the duration of the transient phenomena can be less than a few milliseconds. However, these transient perturbations of the crystal can result in high-frequency signals that can cause the microcontroller to enter an unrecoverable state.

Practical Methods for Evaluating Oscillator Problems

One method to observe these transient states is to take an oscilloscope with a high repetition rate (as opposed to sampling rate). Analog oscilloscopes and digital scopes, when used with low sampling rates, generally have fairly high repetition rates. By carefully observing the scope as power is applied to the circuit, transient phenomena can often be seen visually when they are present. This method of observation is somewhat coarse in nature and will not yield accurate results in determining the exact nature of the transient phenomena. However, it can provide an indication that the oscillator is starting in an abnormal manner. If an abnormal pattern is observed, a memory scope can be employed for further evaluation. Recall that crystals can start at the correct frequency, jump to some type of abnormal mode, and then return to a normal pattern.

If any type of abnormal, transient phenomena are observed, two basic solutions can be employed. The first solution is to increase (a greater number of ohms) the size of the series resistor of the oscillator circuit. If this does not solve the problem, use a different brand or style of crystal. Different styles of crystals, even those produced by the same manufacturer, can have significantly different characteristics. The same is true for different manufacturers of the same type of crystal. As a practical consideration, when circuit-board layout issues are not causing problems, and the series and feedback resistors and bypass capacitors of the crystal oscillator circuit are required to be within extremely tight limits to insure proper crystal operation, the crystal itself is probably marginal, and consideration should be given to selecting a different brand / type of crystal. If a crystal has poor overtone or first harmonic suppression, no amount of external circuit design will compensate for this. The same is true for drive currents. If the crystal requires a very narrow range of series resistor values for proper start-up characteristics, it is a wise idea to consider a different brand / style of crystal.

8.3Choosing the Proper Crystal

Low-frequency tuning elements tend to be much larger than their high-frequency counterparts. In general, low-frequency tuning elements are more susceptible to physical damage due to stressful operating environments. Automotive operating environments are among the most challenging because of their high vibration and temperature extremes. When picking a 32.768 KHz crystal, it is important to obtain qualification data from the manufacturer regarding the type of environment the particular crystal is qualified to operate.

8.4Assembly Techniques When Soldering Crystals to

Printed Circuit Boards

Low-frequency tuning elements are more sensitive to temperature than their high-frequency counterparts. In general, low-frequency crystals can be damaged more easily than high-frequency elements in soldering processes. Particular attention must be paid to keeping soldering operations within temperature ranges that will not damage the crystal. While high-temperature damage can manifest itself in many ways, the most common symptom is the failure of the crystal to operate. Another way for temperature damage to express itself is for the crystal to display bizarre or unexplainable start-up characteristics. If problems with a crystal suddenly appear in a product line that previously had no problems, assembly soldering temperatures are a good place to investigate.

Summary 9Summary

Low-frequency oscillator circuits can be reliably produced and made to perform with accuracy and stability. It has been stated several times in this paper that crystals, themselves, vary widely in many characteristics, including power dissipation, harmonic and overtone suppression, temperature drift, and accuracy. It is difficult, and of questionable design practice, to attempt circuit designs that must compensate for a crystal with poor operating characteristics. Careful attention must be given to selecting external circuit components that allow the crystal to operate within its proper range of power dissipation, temperature, and voltage requirements. Good quality, low-frequency crystals generally do not require high-tolerance components. Having to use high precision capacitors and resistors in the oscillator circuit are often a very clear indication that the crystal being used does not have good internal characteristics. While this paper has dealt with problems associated with low-frequency oscillators, it must be stated that many brands and styles of 32.768 KHz crystals have been used for many years. They have served reliably in their intended applications. By observing a few basic precautions and being aware of simple measurement techniques, reliable low-frequency oscillators can be properly designed and used for microcontroller applications.

How to Reach Us:

Home Page:

https://www.wendangku.net/doc/156274634.html,

E-mail:

support@https://www.wendangku.net/doc/156274634.html,

USA/Europe or Locations Not Listed: Freescale Semiconductor

T echnical Information Center, CH370

1300 N. Alma School Road

Chandler, Arizona 85224

+1-800-521-6274 or +1-480-768-2130

support@https://www.wendangku.net/doc/156274634.html,

Europe, Middle East, and Africa:

Freescale Halbleiter Deutschland GmbH

T echnical Information Center

Schatzbogen 7

81829 Muenchen, Germany

+44 1296 380 456 (English)

+46 8 52200080 (English)

+49 89 92103 559 (German)

+33 1 69 35 48 48 (French)

support@https://www.wendangku.net/doc/156274634.html,

Japan:

Freescale Semiconductor Japan Ltd.

Headquarters

ARCO T ower 15F

1-8-1, Shimo-Meguro, Meguro-ku,

T okyo 153-0064

Japan

0120 191014 or +81 3 5437 9125

support.japan@https://www.wendangku.net/doc/156274634.html,

Asia/Pacific:

Freescale Semiconductor Hong Kong Ltd.

T echnical Information Center

2 Dai King Street

T ai Po Industrial Estate

T ai Po, N.T., Hong Kong

+800 2666 8080

https://www.wendangku.net/doc/156274634.html,@https://www.wendangku.net/doc/156274634.html,

For Literature Requests Only:

Freescale Semiconductor Literature Distribution Center P.O. Box 5405

Denver, Colorado 80217

1-800-441-2447 or 303-675-2140

Fax: 303-675-2150 LDCForFreescaleSemiconductor@https://www.wendangku.net/doc/156274634.html, Information in this document is provided solely to enable system and software implementers to use Freescale Semiconductor products. There are no express or implied copyright licenses granted hereunder to design or fabricate any integrated circuits or integrated circuits based on the information in this document.

Freescale Semiconductor reserves the right to make changes without further notice to any products herein. Freescale Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Freescale Semiconductor assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters that may be provided in Freescale Semiconductor data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”, must be validated for each customer application by customer’s technical experts. Freescale Semiconductor does not convey any license under its patent rights nor the rights of others. Freescale Semiconductor products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Freescale Semiconductor product could create a situation where personal injury or death may occur. Should Buyer purchase or use Freescale Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify and hold Freescale Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that Freescale Semiconductor was negligent regarding the design or manufacture of the part.

Freescale? and the Freescale logo are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners.

? Freescale Semiconductor, Inc. 2004, 2005. All rights reserved.

选择晶振时要考虑哪些参数

选择晶振时要考虑哪些参数？ 2011-7-19 14:26 提问者：rinkeigun|浏览次数：2555次 谢谢好心人。我想知道的是： 1. 晶振之身的参数（频率等） 2. 与周围的器件（51单片机）有什么关联，影响 3. 构成晶振的元件是什么（如C，Y） 4.哪里有最简单的电路图 我来帮他解答 2011-7-25 14:05 满意回答 1、 晶体谐振器的等效电路 图1是一个在谐振频率附近有与晶体谐振器具有相同阻抗特性的简化电路。 其中：C1为动态电容也称等效串联电容；L1为动态电感也称等效串联电感； R1为动态电阻也称等效串联电阻；C0为静态电容也称等效并联电容。 这个等效电路中有两个最有用的零相位频率，其中一个是谐振频率(Fr)，另一个是反谐振频率(Fa)。当晶体元件实际应用于振荡电路中时，它一般还会与一负载电容相联接，共同作用使晶体工作于Fr和Fa之间的某个频率，这个频率由振荡电路的相位和有效电抗确定，通过改变电路的电抗条件，就可以在有限的范围内调节晶体频率。 2、晶体的频率 晶体在应用的电路中，其电气特性表现较复杂，与其相关的频率指标也有多个，主要的是： a）标称频率（F0） 指晶体元件规范中所指定的频率，也即用户在电路设计和元件选购时所希望的理想工作频率。 b）谐振频率（Fr） 指在规定条件下，晶体元件电气阻抗为电阻性的两个频率中较低的一个频率。根据图1的等效电路，当不考虑C0的作用，Fr由C1和L1决定，近似等于所谓串联（支路）谐振频率（Fs）。 这一频率是晶体的自然谐振频率，它在高稳晶振的设计中，是作为使晶振稳定工作于标称频率、确定频率调整范围、设置频率微调装置等要求时的设计参数。c）负载谐振频率（FL） 指在规定条件下，晶体元件与一负载电容串联或并联，其组合阻抗呈现为电阻性时两个频率中的一个频率。在串联负载电容时，FL是两个频率中较低的那个频

晶振FM发射电路

晶振FM发射电路 此晶振FM发射电路经过一晚上的折腾将音质差音量小的问题显著改善，特将成果分享给爱玩的你，此电路工作非常稳定、手怎么摸电路板怎么移动电路板都不会飘频，不要和电容三点式振荡电路混为一谈 晶振找了20多个只有26.601712Mhz这个晶振音质做好、频率落在收音机的106.4频段上，变容二极管2个串联、1~5uh电感用色环电感，大家做的时候10k和两个5.1k电阻不要偏差太大、会影响音量和音质的、供电电压低于10V音质会变差，所以说供电不要低于12V。变容二极管可用V06G整流二极管代替 自我感觉经此发射电路发射出去的信号收音机接收后高音清晰低音浑厚、接收音量也已经做到可以让自己接受的量度了 最新电路图做了如下改动，将石英晶振改为陶瓷晶振、增加了一个47K电阻、减少了1个变容二极管、供电电压由12V降低为4.2V 可正常工作不影响音质。其它无改动

频率很稳定的FM发射电路图 许多无线电爱好者都希望制作一台调频发射器，特别是在８７～１０８ＭＨｚ的调频波段，可利用现成的ＦＭ收音机来接收，因而受到大家的青睐。 在许多刊物中都介绍有调频发射器的实例，但大多数采用电容三点式电路和克拉泼振荡电路。这种电路虽简单，但它的频率稳定度不高，特别是在业余条件下，稍微动动电路板或天线位置，频率就改变了。在此笔者介绍一款用晶振稳频的调频发射器。 如图１所示，由Ｖ１及相关阻容元件组成一级音频放大电路，为调制级提供足够强度的音频信号。Ｄ１是变容二极管，其等效电容量随着两极所加的反向电压变化而变化，从而使晶振及外围电路组成的振荡器中心频率随之变化，达到调频目的。振荡器输出的信号经Ｖ３倍频、放大，再由调谐变压器完成匹配与滤波后输出。 该电路用了调谐变压器，因而在制作完后要调整其磁心，使之匹配。其方法是制作一个简易场强电路（如图２所示），接至变压器的输出端，调整磁心，直到电流表指示值最大为止。电路中所用元器件尽量使用高频特性好的元器件。晶振选用标称值为２９～３６ＭＨｚ之间的晶振，Ｄ１可用MV2105，变压器需自制，可选用电视中周作骨架，去掉屏蔽罩，用∮０．２ｍｍ左右的漆包线在骨架上初级绕３匝，次级绕１匝。天线可用１／４波长的软导线代用。 成本低于10元的FM发射器 目前市场上具备FM发射功能的MP3备受消费者关注。这种功能看起来挺新奇，也可以为MP3播放器增加卖点，其实实现起来并不难。我们也可以自己动手做一个小型的FM发射机。在这里介绍一种新型发射机，该机制作简便、音质优良，适合高保真无线音响之用。

晶振电路原理介绍

晶体振荡器，简称晶振。在电气上它可以等效成一个电容和一个电阻并联再串联一个电容的二端网络，电工学上这个网络有两个谐振点，以频率的高低分其中较低的频率是串联谐振，较高的频率是并联谐振。由于晶体自身的特性致使这两个频率的距离相当的接近，在这个极窄的频率范围内，晶振等效为一个电感，所以只要晶振的两端并联上合适的电容它就会组成并联谐振电路。这个并联谐振电路加到一个负反馈电路中就可以构成正弦波振荡电路，由于晶振等效为电感的频率范围很窄，所以即使其他元件的参数变化很大，这个振荡器的频率也不会有很大的变化。 晶振有一个重要的参数，那就是负载电容值，选择与负载电容值相等的并联电容，就可以得到晶振标称的谐振频率。 一般的晶振振荡电路都是在一个反相放大器（注意是放大器不是反相器）的两端接入晶振，再有两个电容分别接到晶振的两端，每个电容的另一端再接到地，这两个电容串联的容量值就应该等于负载电容，请注意一般IC的引脚都有等效输入电容，这个不能忽略。 一般的晶振的负载电容为15p或12.5p ，如果再考虑元件引脚的等效输入电容，则两个22p的电容构成晶振的振荡电路就是比较好的选择。 晶体振荡器也分为无源晶振和有源晶振两种类型。无源晶振与有源晶振（谐振）的英文名称不同，无源晶振为crystal（晶体），而有源晶振则叫做oscillator（振荡器）。无源晶振需要借助于时钟电路才能产生振荡信号，自身无法振荡起来，所以“无源晶振”这个说法并不准确；有源晶振是一个完整的谐振振荡器。 谐振振荡器包括石英（或其晶体材料）晶体谐振器，陶瓷谐振器，LC谐振器等。

晶振与谐振振荡器有其共同的交集有源晶体谐振振荡器。 石英晶片所以能做振荡电路（谐振）是基于它的压电效应，从物理学中知道，若在晶片的两个极板间加一电场，会使晶体产生机械变形；反之，若在极板间施加机械力，又会在相应的方向上产生电场，这种现象称为压电效应。如在极板间所加的是交变电压，就会产生机械变形振动，同时机械变形振动又会产生交变电场。一般来说，这种机械振动的振幅是比较小的，其振动频率则是很稳定的。但当外加交变电压的频率与晶片的固有频率（决定于晶片的尺寸）相等时，机械振动的幅度将急剧增加，这种现象称为压电谐振，因此石英晶体又称为石英晶体谐振器。其特点是频率稳定度很高。 石英晶体振荡器与石英晶体谐振器都是提供稳定电路频率的一种电子器件。石英晶体振荡器是利用石英晶体的压电效应来起振，而石英晶体谐振器是利用石英晶体和内置IC来共同作用来工作的。振荡器直接应用于电路中，谐振器工作时一般需要提供3.3V电压来维持工作。振荡器比谐振器多了一个重要技术参数为：谐振电阻（RR），谐振器没有电阻要求。RR 的大小直接影响电路的性能，也是各商家竞争的一个重要参数。 概述 微控制器的时钟源可以分为两类：基于机械谐振器件的时钟源，如晶振、陶瓷谐振槽路；基于相移电路的时钟源，如：RC (电阻、电容)振荡器。硅振荡器通常是完全集成的RC振荡器，为了提高稳定性，包含有时钟源、匹配电阻和电容、温度补偿等。图1给出了两种时钟源。图1给出了两个分立的振荡器电路，其中图1a为皮尔斯振荡器配置，用于机械式谐振器件，如晶振和陶瓷谐振槽路。图1b为简单的RC反馈振荡器。 机械式谐振器与RC振荡器的主要区别 基于晶振与陶瓷谐振槽路(机械式)的振荡器通常能提供非常高的初始精度和较低的温 度系数。相对而言，RC振荡器能够快速启动，成本也比较低，但通常在整个温度和工作电源电压范围内精度较差，会在标称输出频率的5%至50%范围内变化。图1所示的电路能产生可靠的时钟信号，但其性能受环境条件和电路元件选择以及振荡器电路布局的影响。需认真对待振荡器电路的元件选择和线路板布局。在使用时，陶瓷谐振槽路和相应的负载电容必须根据特定的逻辑系列进行优化。具有高Q值的晶振对放大器的选择并不敏感，但在过驱动时很容易产生频率漂移(甚至可能损坏)。影响振荡器工作的环境因素有：电磁干扰(EMI)、机械震动与冲击、湿度和温度。这些因素会增大输出频率的变化，增加不稳定性，并且在有些情况下，还会造成振荡器停振。 振荡器模块 上述大部分问题都可以通过使用振荡器模块避免。这些模块自带振荡器、提供低阻方波

如何选取正确的晶振

一个号的晶体振荡器可以被泛应用到军、民用通信电台，微波通信设备，程控电话交换机，无线电综合测试仪，BP机、移动电话发射台，高档频率计数器、GPS、卫星通信、遥控移动设备等。它具有多种封装类型，最主要的特点是电气性能规范多种多样。它有以下几种不同的类型：电压控制晶体振荡器(VCXO)、温度补偿晶体振荡器(TCXO)、恒温晶体振荡器(OCXO)，以及数字补偿晶体振荡器(MCXO或DTCXO),每种类型都有自己的独特性能。 如果你的设备需要即开即用，您就必须选用VCXO或温补晶振，如果你的要求稳定度在0.5ppm以上，凯越翔建议你选择数字温补晶振(MCXO)。而模拟温补晶振则适用于稳定度要求在5ppm～0.5ppm之间的需求。VCXO只适合于稳定度要求在5ppm以下的产品。如果你的设备在不需要即开即用的环境下，如果需要信号稳定度超过0.1ppm的，可选用OCXO。 从频率稳定性方面考虑：晶体振荡器的主要特性之一是工作温度内的稳定性，它是决定振荡器价格的重要因素。稳定性愈高或温度范围愈宽，器件的价格亦愈高。工业级标准规定的-40～+75℃这个范围往往只是出于设计者们的习惯，倘若-30～+70℃已经够用，那么就不必去追求更宽的温度范围。所以设计工程师要慎密决定特定应用的实际需要，然后规定振荡器的稳定度。指标过高意味着花钱愈多。 晶体老化：造成频率变化的又一重要因素。根据目标产品的预期寿命不同，有多种方法可以减弱这种影响。晶体老化会使输出频率按照对数曲线发生变化，也就是说在产品使用的第一年，这种现象才最为显著。例如，使用10年以上的晶体，其老化速度大约是第一年的3倍。采用特殊的晶体加工工艺可以改善这种情况，也可以采用调节的办法解决，比如，可以在控制引脚上施加电压(即增加电压控制功能)等。 与稳定度有关的其他因素还包括电源电压、负载变化、相位噪声和抖动，这些指标应该规定出来。对于工业产品，有时还需要提出振动、冲击方面的指标，军用品和宇航设备的要求往往更多，比如压力变化时的容差、受辐射时的容差，等等。 输出：必须考虑的其它参数是输出类型、相位噪声、抖动、电压特性、负载特性、功耗、封装形式，对于工业产品，有时还要考虑冲击和振动、以及电磁干扰(EMI)。晶体振荡器可HCMOS/TTL兼容、ACMOS兼容、ECL和正弦波输出。每种输出类型都有它的独特波形特性和用途。应该关注三态或互补输出的要求。对称性、上升和下降时间以及逻辑电平对某些应用来说也要作出规定。许多DSP和通信芯片组往往需要严格的对称性(45%至55%)和快速的上升和下降时间(小于 5ns)。 相位噪声和抖动：在频域测量获得的相位噪声是短期稳定度的真实量度。它可测量到中心频率的1Hz之内和通常测量到1MHz。晶体振荡器的相位噪声在远离中心频率的频率下有所改善。TCXO和OCXO振荡器以及其它利用基波或谐波方式的晶体振荡器具有最好的相位噪声性能。采用锁相环合成器产生输出频率的振荡器比采用非锁相环技术的振荡器一般呈现较差的相位噪声性能。 抖动与相位噪声相关，但是它在时域下测量。以微微秒表示的抖动可用有效值或峰—峰值测出。许多应用，例如通信网络、无线数据传输、ATM和SONET要

晶振的工作原理

晶振的工作原理 一、什么是晶振? 晶振是石英振荡器的简称，英文名为Crystal，它是时钟电路中最重要的部件，它的主要作用是向显卡、网卡、主板等配件的各部分提供基准频率，它就像个标尺，工作频率不稳定会造成相关设备工作频率不稳定，自然容易出现问题。 晶振还有个作用是在电路产生震荡电流,发出时钟信号. 晶振是晶体振荡器的简称。它用一种能把电能和机械能相互转化的晶体在共振的状态下工作，以提供稳定，精确的单频振荡。在通常工作条件下，普通的晶振频率绝对精度可达百万分之五十。高级的精度更高。有些晶振还可以由外加电压在一定范围内调整频率，称为压控振荡器（VCO）。 晶振在数字电路的基本作用是提供一个时序控制的标准时刻。数字电路的工作是根据电路设计，在某个时刻专门完成特定的任务，如果没有一个时序控制的标准时刻，整个数字电路就会成为“聋子”，不知道什么时刻该做什么事情了。 晶振的作用是为系统提供基本的时钟信号。通常一个系统共用一个晶振，便于各部分保持同步。有些通讯系统的基频和射频使用不同的晶振，而通过电子调整频率的方法保持同步。 晶振通常与锁相环电路配合使用，以提供系统所需的时钟频率。如果不同子系统需要不同频率的时钟信号，可以用与同一个晶振相连的不同锁相环来提供。

电路中，为了得到交流信号，可以用RC、LC谐振电路取得，但这些电路的振荡频率并不稳定。在要求得到高稳定频率的电路中，必须使用石英晶体振荡电路。石英晶体具有高品质因数，振荡电路采用了恒温、稳压等方式以后，振荡频率稳定度可以达到10^(-9)至10 ^(-11)。广泛应用在通讯、时钟、手表、计算机……需要高稳定信号的场合。 石英晶振不分正负极, 外壳是地线，其两条不分正负 二、晶振的使用 晶振，在电气上它可以等效成一个电容和一个电阻并联再串联一个电容的二端网络，电工学上这个网络有两个谐振点，以频率的高低分其中较低的频率是串联谐振，较高的频率是并联谐振。由于晶体自身的特性致使这两个频率的距离相当的接近，在这个极窄的频率范围内，晶振等效为一个电感，所以只要晶振的两端并联上合适的电容它就会组成并联谐振电路。这个并联谐振电路加到一个负反馈电路中就可以构成正弦波振荡电路，由于晶振等效为电感的频率范围很窄，所以即使其他元件的参数变化很大，这个振荡器的频率也不会有很大的变化。 晶振有一个重要的参数，那就是负载电容值，选择与负载电容值相等的并联电容，就可以得到晶振标称的谐振频率。 一般的晶振振荡电路都是在一个反相放大器（注意是放大器不是反相器）的两端接入晶振，再有两个电容分别接到晶振的两端，每个电容的另一端再接到地，这两个电容串联的容

石英晶振设计电路,Oscillation Circuit Design Overview

Oscillation Circuit Design Overview Oscillation Circuit Design Key Parameters DRIVE LEVEL (DL), OSCILLATION FREQUENCY AND LOAD CAPACITANCE (CL), OSCILLATION ALLOWANCE, FREQUENCY-TEMPERATURE CURVE DRIVE LEVEL (DL) The drive level of a crystal unit is shown by the level of the operating power or the current consumption (see Figures 9,10, and 11). Operating the crystal unit at an excessive power level will result in the degradation of its characteristics, which may cause frequency instability or physical failure of the crystal chip. Design your circuit within absolute maximum drive level. OSCILLATION FREQUENCY AND LOAD CAPACITANCE (CL) The load capacitance (CL) is a parameter for determining the frequency of the oscillation circuit. The CL is represented by an effective equivalent capacitance that is loaded from the oscillation circuit to both ends of the crystal unit (see Figure 12). The oscillation frequency varies depending upon the load capacitance of the oscillation circuit. In order to obtain the desirable frequency accuracy, matching between the load capacitances of the oscillation circuit and the crystal unit is required. For the use of the crystal unit, match the load capacitances of the oscillation circuit with the load capacitances of the crystal

好晶振的选择方法

好晶振的选择方法 晶振选型时关心的技术指标： 1.频率：基本参数，选型必须知道的参数。 频率越高一般价格越高。但频率越高，频差越大，从综合角度考虑，一般工程师会选用频率低但稳定的晶振，自己做倍频电路。总之频率的选择是根据需要选择，并不是频率越大就越好。要看具体需求。比如基站中一般用10MHz的OCXO，但由于很好的频率稳定性，属于高端晶振。至于范围，晶振的频率做的太高的话，就会失去意义，因为有其他更好的频率产品代替。 KVG的产品频率范围是：25kHz-1.3G。基本上所有应用中的晶振都可以在KVG产品种找到。 2.频率稳定度：关键参数，KVG的高端晶振可以达到10-9级别。 指在规定的工作温度范围内，与标称频率允许的偏差。用PPm（百万分之一）表示。一般来说，稳定度越高或温度范围越宽，价格越高。对于频率稳定度要求±20ppm 或以上的应用，可使用普通无补偿的晶体振荡器。对于介于±1 至±20ppm 的稳定度，应该考虑TCXO 。对于低于±1ppm 的稳定度，应该考虑OCXO。比如OCXO-3000SC，频稳为+/- 2x10^-9。如果客户有十分特别的频稳要求，KVG可以定制。 3.电源电压： 常用的有3.3V、5V、2.8V等。 KVG的产品2。8V 3。3V 5V都有。其中3.3V应用最广。 4.输出： 根据需要采用不同输出。（HCMOS,SINE,TTL,PECL,LVPECL,LVDS,LVHCMOS等）每种输出类型都有它的独特波形特性和用途。应该关注三态或互补输出的要求。对称性、上升和下降时间以及逻辑电平对某些应用来说也要作出规定。 KVG产品有些系列有HCMOS/TTL，有些系列有LVPECL/LVDS输出。根据客户需要我们可以帮助客户选型。 5.工作温度范围： 工业级标准规定的-40～+85℃这个范围往往只是出于设计者们的习惯，倘若-30～+70℃已经够用，那么就不必去追求更宽的温度范围。对于某些特殊场合如航天军用等，对温度有更苛刻的要求。 KVG的产品都用普通和工业级标准，对于军工极KVG也有。军工级一般需要定制，KVG在定制方面有优势。 6.相位噪声和抖动： 相位噪声和抖动是对同一种现象的两种不同的定量方式，是对短期稳定度的真实度量。振荡器以及其它利用基波或谐波方式的晶体振荡器具有最好的相位噪声性能。采用锁相环合成器产生输出频率的振荡器比采用非锁相环技术的振荡器一般呈现较差的相位噪声性能。但相对的，拥有好的相位噪声和抖动的同时振荡器的设计复杂，体积大，频率低，造价高。 KVG的晶振系列涵盖了各种设计技术，可根据客户要求进行选择。例如V-850采用倍频器（过滤谐波）技术，具有高频，低抖动(<0.1ps rms 12kHz-20MHz)。实际上相位噪声和抖动是短期频率稳定度的度量，所以一般越高端的晶振，即频稳越好的晶振，这些指标也相应越好。KVG可以提供各种档次相位噪声的晶振，

单片机最小系统原理图

单片机最小系统 单片机最小系统,或者称为最小应用系统,是指用最少的元件组成的单片机可以工作的 系统. 对51系列单片机来说,最小系统一般应该包括:单片机、晶振电路、复位电路. 下面给出一个51单片机的最小系统电路图. 说明

复位电路:由电容串联电阻构成,由图并结合"电容电压不能突变"的性质,可以知道,当系统一上电,RST脚将会出现高电平,并且,这个高电平持续的时间由电路的RC值来决定.典型的51单片机当RST脚的高电平持续两个机器周期以上就将复位,所以,适当组合RC的取值就可以保证可靠的复位.一般教科书推荐C 取10u,R取8.2K.当然也有其他取法的,原则就是要让R C组合可以在RST脚上产生不少于2个机周期的高电平.至于如何具体定量计算,可以参考电路分析相关书籍. 晶振电路:典型的晶振取11.0592MHz(因为可以准确地得到9600波特率和19200波特率,用于有串口通讯的场合)/12MHz(产生精确的uS级时歇,方便定时操作) 单片机:一片AT89S51/52或其他51系列兼容单片机 特别注意:对于31脚(EA/Vpp),当接高电平时,单片机在复位后从内部ROM的0000H开始执行;当接低电平时,复位后直接从外部ROM的0000H开始执行.这一点是初学者容易忽略的. 复位电路： 一、复位电路的用途 单片机复位电路就好比电脑的重启部分，当电脑在使用中出现死机，按下重启按钮电脑内部的程序从头开始执行。单片机也一样，当单片机系统在运行中，受到环境干扰出现程序跑飞的时候，按下复位按钮内部的程序自动从头开始执行。 单片机复位电路如下图：

二、复位电路的工作原理 在书本上有介绍，51单片机要复位只需要在第9引脚接个高电平持续2US就可以实现，那这个过程是如何实现的呢？ 在单片机系统中，系统上电启动的时候复位一次，当按键按下的时候系统再次复位，如果释放后再按下，系统还会复位。所以可以通过按键的断开和闭合在运行的系统中控制其复位。 开机的时候为什么为复位 在电路图中，电容的的大小是10uF，电阻的大小是10k。所以根据公式，可以算出电容充

晶振的基本原理及特性

晶振的基本原理及特性 晶振一般采用如图1a的电容三端式(考毕兹) 交流等效振荡电路；实际的晶振交流等效电路如图1b，其中Cv是用来调节振荡频率，一般用变容二极管加上不同的反偏电压来实现，这也是压控作用的机理；把晶体的等效电路代替晶体后如图1c。其中Co，C1，L1，RR是晶体的等效电路。 分析整个振荡槽路可知，利用Cv来改变频率是有限的：决定振荡频率的整个槽路电容C=Cbe,Cce,Cv 三个电容串联后和Co并联再和C1串联。可以看出：C1越小，Co越大，Cv变化时对整个槽路电容的作用就越小。因而能“压控”的频率范围也越小。实际上，由于C1很小(1E-15量级)，Co不能忽略(1E-12量级，几PF)。所以，Cv变大时，降低槽路频率的作用越来越小，Cv变小时，升高槽路频率的作用却越来越大。这一方面引起压控特性的非线性，压控范围越大，非线性就越厉害；另一方面，分给振荡的反馈电压(Cbe上的电压)却越来越小，最后导致停振。 采用泛音次数越高的晶振，其等效电容C1就越小；因此频率的变化范围也就越小。 晶振的指标 总频差：在规定的时间内，由于规定的工作和非工作参数全部组合而引起的晶体振荡器频率与给定标称频率的最大偏差。 说明：总频差包括频率温度稳定度、频率老化率造成的偏差、频率电压特性和频率负载特性等共同造成的最大频差。一般只在对短期频率稳定度关心，而对其他频率稳定度指标不严格要求的场合采用。例如：精密制导雷达。 频率稳定度：任何晶振，频率不稳定是绝对的，程度不同而已。一个晶振的输出频率随时间变化的曲线如图2。图中表现出频率不稳定的三种因素：老化、飘移和短稳。

图2 晶振输出频率随时间变化的示意图 曲线1是用0.1秒测量一次的情况，表现了晶振的短稳；曲线3是用100秒测量一次的情况，表现了晶振的漂移；曲线4 是用1天一次测量的情况。表现了晶振的老化。 频率温度稳定度：在标称电源和负载下，工作在规定温度范围内的不带隐含基准温度或带隐含基准温度的最大允许频偏。 ft=±(f max-fmin)/(fmax+fmin) ftref ＝±MAX[｜(fmax-fref)/fref｜，｜(fmin-fref)/fref｜] ft：频率温度稳定度(不带隐含基准温度) ftref：频率温度稳定度(带隐含基准温度) fmax ：规定温度范围内测得的最高频率 fmin：规定温度范围内测得的最低频率 fref：规定基准温度测得的频率 说明：采用ftref指标的晶体振荡器其生产难度要高于采用ft指标的晶体振荡器,故ftref指标的晶体振荡器售价较高。 开机特性（频率稳定预热时间）：指开机后一段时间(如5分钟)的频率到开机后另一段时间(如1小时)的频率的变化率。表示了晶振达到稳定的速度。这指标对经常开关的仪器如频率计等很有用。 说明：在多数应用中，晶体振荡器是长期加电的，然而在某些应用中晶体振荡器需要频繁的开机和关机，这时频率稳定预热时间指标需要被考虑到（尤其是对于在苛刻环境中使用的军用通讯电台，当要求频率温度稳定度≤±0.3ppm(-45℃～85℃)，采用OCXO作为本振，频率稳定预热时间将不少于5分钟，而采用MCXO只需要十几秒钟)。 频率老化率：在恒定的环境条件下测量振荡器频率时，振荡器频率和时间之间的关系。这种长期频率

有源晶振电路及工作原理简述

有源晶振电路及工作原理简述 有源晶振是由石英晶体组成的,石英晶片之所以能当为振荡器使用，是基于它的压电效应：在晶片的两个极上加一电场，会使晶体产生机械变形；在石英晶片上加上交变电压，晶体就会产生机械振动，同时机械变形振动又会产生交变电场，虽然这种交变电场的电压极其微弱，但其振动频率是十分稳定的。当外加交变电压的频率与晶片的固有频率(由晶片的尺寸和形状决定)相等时，机械振动的幅度将急剧增加，这种现象称为“压电谐振”。 压电谐振状态的建立和维持都必须借助于振荡器电路才能实现。图3是一个串联型振荡器，晶体管T1和T2构成的两级放大器，石英晶体XT与电容C2构成LC电路。在这个电路中，石英晶体相当于一个电感，C2为可变电容器，调节其容量即可使电路进入谐振状态。该振荡器供电电压为5V，输出波形为方波。 有源晶振引脚排列: 有源晶振引脚识别，实物图如上图(b)所示. 有个点标记的为1脚，按逆时针（管脚向下）分别为2、3、4。 方形有源晶振引脚分布： 1、正方的，使用DIP-8封装，打点的是1脚。 1-NC；4-GND；5-Output；8-VCC 2、长方的，使用DIP-14封装，打点的是1脚。 1-NC；7-GND；8-Output；14-VCC

注:有源晶振型号众多，而且每一种型号的引脚定义都有所不同，接法也有所不同，上述介绍仅供参考,实际使用中要确认其管脚列方式. 有源晶振通常的接法： 一脚悬空，二脚接地，三脚接输出，四脚接电压。 有源晶振与无源晶振的联系与区别 无源晶振与有源晶振的英文名称不同，无源晶振为crystal（晶体），而有源晶振则叫做oscillator（振荡器）。无源晶振是有2个引脚的无极性元件，需要借助于时钟电路才能产生振荡信号，自身无法振荡起来，所以“无源晶振”这个说法并不准确；有源晶振有4只引脚，是一个完整的振荡器，其中除了石英晶体外，还有晶体管和阻容元件，因此体积较大。 石英晶体振荡器的频率稳定度可达10^-9/日，甚至10^-11。例如10MHz的振荡器，频率在一日之内的变化一般不大于0.1Hz。因此，完全可以将晶体振荡器视为恒定的基准频率源(石英表、电子表中都是利用石英晶体来做计时的基准频率)。从PC诞生至现在，主板上一直都使用一颗14.318MHz的石英晶体振荡器作为基准频率源。 有源晶振不需要DSP的内部振荡器，信号质量好，比较稳定，而且连接方式相对简单（主要是做好电源滤波，通常使用一个电容和电感构成的PI型滤波网络，输出端用一个小阻值的电阻过滤信号即可），不需要复杂的配置电路。相对于无源晶体，有源晶振的缺陷是其信号电平是固定的，需要选择好合适输出电平，灵活性较差，而且价格高。 下图为晶体及晶振实特图,左边两个是晶振,右边14.38MHz的为晶体.

晶振的匹配电容选择修订稿

晶振的匹配电容选择 WEIHUA system office room 【WEIHUA 16H-WEIHUA WEIHUA8Q8-

匹配电容是指晶振要正常震荡所需要的电容，一外接电容是为了使晶振两端的等效电容等于或接近于负载电容（晶体的负载电容是已知的，在出厂的时候已经定下来了，一般是几十PF，）。应用时一般在给出负载电容值附近调整可以得到精确频率，此电容的大小主要影响负载谐振频率，一般情况下，增大电容会使振荡频率下降，而减小电容会使振荡频率升高， 晶振的负载电容=[(Cd*Cg)/(Cd+Cg)]+Cic+△C] 式中Cd,Cg为分别接在晶振的两个脚上和对地的电容,Cic（集成电路内部电容）+△C（PCB上电容，一般情况下，Cd、Cg取相同的值并联后等于负载电容是可以满足振荡条件的, 在许可的范围内Cd和Ｃg的值越小越好，电容值偏大会虽然有利于震荡的稳定，但是电容过大会增加起振的时间。如果不易起振或振荡不稳定可以减小输入端对地电容量, 而增加输出端的值以提高反馈量。 在电路中输出端和输入端之间接了一个大的电阻，这是由于连接晶振的芯片端内部是一个线性运算放大器，将输入进行反向180度输出，晶振处的负载电容电阻组成的网络提供另外180度的相移，整个环路的相移360度，满足振荡的相位条件，同时还要求闭环增益大于等于1，晶体才正常工作。晶振输入输出连接的电阻作用是产生负反馈，保证放大器工作在高增益的线性区，一般在M欧级，输出端的电阻与负载电容组成网络，提供180度相移，同时起到限流的作用，防止反向器输出对晶振过驱动，损坏晶振，有的晶振不需要是因为把这个电阻已经集成到了晶振里面。 设计是注意事项： 1.使晶振、外部电容器（如果有）与 IC之间的信号线尽可能保持最短。当非常低的电流通过IC晶振振荡器时，如果线路太长，会使它对 EMC、ESD 与串扰产生非常敏感的影响。而且长线路还会给振荡器增加寄生电容； 2.尽可能将其它时钟线路与频繁切换的信号线路布置在远离晶振连接的位置； 3.当心晶振和地的走线； 4.将晶振外壳接地。

晶振选型与应用知识

石英晶振选型与应用知识 石英晶体是压电晶体的一种，沿着特定的方向挤压或拉伸，它的两端会产生正负电荷，这种效应称为正压电效应；相反，对晶体施加电场导致晶体形变的效应，称为逆压电效应。所以在石英晶片两面施加交变电场，晶片就会产生形变，而形变又会产生电场，这是一个周期转换的过程。对于特定的晶片，这个周期是固定的，我们利用这个周期来产生稳定的基准时钟信号。 石英晶体元器件，是利用石英晶体的压电效应实现频率控制、稳定或选择的关键电子元器件。包括石英晶体谐振器、石英晶体振荡器和石英晶体滤波器。在石英晶片的两面镀上电极，经过装架、调频、封装等工序后制成石英晶体元件。石英晶体元件与集成电路等其它电子元件组合成石英晶体器件。本文主要介绍石英晶振：即所谓石英晶体谐振器（无源晶振）和石英晶体振荡器（有源晶振）的统称。一般的概念中把晶振就等同于谐振器理解了，振荡器就是通常所指钟振。石英晶振是一种用于稳定频率和选择频率的电子元件，已被广泛地使用在无线电话、载波通讯、广播电视、卫星通讯、仪器仪表等各种电子设备中. 一、石英晶振的型号命名方法 1.国产石英晶体谐振器的型号由三部分组成： –第一部分：表示外壳形状和材料， B表示玻璃壳，J表示金属壳，S表示塑料封型； –第二部分：表示晶片切型，与切型符号的第一个字母相同， A表示AT切型、B表示BT切型， –第三部分：表示主要性能及外形尺寸等， 一般用数字表示，也有最后再加英文字母的。 JA5为金属壳AT切型晶振元件，BA3为玻壳AT切型晶振元件。 2石英晶体振荡器的型号命名有四部分组成： .

–第一部分：主称 用大写字母Z表示石英晶体振荡器； –第二部：类别 用大写字母表示，其意义见下表： –第三部分：频率稳定度等级 用大写字母表示，其意义见下表： –第四部分：序号 用数字表示，以示产品结构性能参数的区别

无源晶振有源晶振工作原理

无源晶振（晶体谐振器）工作原理：在石英水晶片的两边镀上电极，通过在两电极上加一定的电压，因为石英有压电效应，电压形成了，自然就会产生形变，从而给IC提供一个正弦波形。通过IC的内部整形和PLL电路后产生方波，然后输入给下级电路。有源晶振就是把频率部分和驱动PLL电路集成在IC外部，自成一体，直接输出方波供下级电路使用。 无源晶振（晶体谐振器）有插件和贴片之分，贴片又分为两脚和四脚，四脚贴片其对脚为有效脚，剩下两脚可以作为接地，也可以悬空不起太大作用。而有源晶振（晶体振荡器）均为四脚：1脚为使能端，2脚为接地端，3脚为输出端，4脚为电源端。不过振荡器的种类很多，英文缩写为OSC或XO。还有特殊功能的振荡器，例如压控振荡器（VCXO）、温度补偿振荡器（TCXO）、压控带温补偿振荡器（VC-TCXO）、恒温振荡器（OCXO)等。 无源晶振是一种无极性元件，需要借助于时钟电路才能产生振荡信号，自身无法振荡起来。无源晶振没有电压的要求，信号电平是可变的，也就是说是根据起振电路来决定的。同样的晶振可以适用于多种电压，可用于多种不同时钟信号电压要求的DSP，而且价格通常也较低，因此对于一般的应用如果条件许可建议用晶体，这尤其适合于产品线丰富批量大的生产者。无源晶振相对于晶振而言其缺陷是信号质量较差，通常需要精确匹配外围电路（用于信号匹配的电容、电感、电阻等），更换不同频率的晶体时周边配置电路需要做相应的调整。使用时建议采用精度较高的石英晶体，尽可能不要采用精度低的陶瓷晶体。 有源晶振是一个完整的振荡器，里面除了石英晶体外，还有晶体管和阻容元件。有源晶振不需要DSP 的内部振荡器，信号质量好，比较稳定，而且连接方式相对简单（主要是做好电源滤波，通常使用一个电容和电感构成的PI型滤波网络，输出端用一个小阻值的电阻过滤信号即可），不需要复杂的配置电路。相对于无源晶体，有源晶振的缺陷是其信号电平是固定的，需要选择好合适输出电平，灵活性较差，价格相对较高。对于时序要求敏感的应用，有源晶振是个更好的选择。因为可以选用比较精密的晶振，甚至是高档的温度补偿晶振。有些DSP内部没有起振电路，只能使用有源晶振。有源晶振相比于无源晶体通常体积较大，但现在许多有源晶振是表贴的，体积和无源晶振相当，有的甚至比无源晶振还要小。 在电子学上，通常将含有晶体管元件的电路称作“有源电路”，而仅由阻容元件组成的电路称作“无源电路”。无源晶振与有源晶振的英文名称不同，无源晶振为crystal（晶体），而有源晶振则叫做晶体振荡器（oscillator）。 有源晶振是有石英晶体组成的,石英晶片之所以能当为振荡器使用，是基于它的压电效应：在晶片的两个极上加一电场，会使晶体产生机械变形；在石英晶片上加上交变电压，晶体就会产生机械振动，同时机械变形振动又会产生交变电场，虽然这种交变电场的电压极其微弱，但其振动频率是十分稳定的。当外加交变电压的频率与晶片的固有频率(由晶片的尺寸和形状决定)相等时，机械振动的幅度将急剧增加，这种现象称为“压电谐振”。压电谐振状态的建立和维持都必须借助于振荡器电路才能实现。

晶振选型指南(精)

恒温晶振、温补晶振选用指南 晶体振荡器被广泛应用到军、民用通信电台,微波通信设备,程控电话交换机,无线电综合测试仪, BP 机、移动电话发射台,高档频率计数器、 GPS 、卫星通信、遥控移动设备等。它有多种封装,特点是电气性能规范多种多样。它有好几种不同的类型:电压控制晶体振荡器(VCXO 、温度补偿晶体振荡器(TCXO 、恒温晶体振荡器(OCXO ,以及数字补偿晶体振荡器(MCXO 或 DTCXO , 每种类型都有自己的独特性能。如果您需要使您的设备即开即用, 您就必须选用 VCXO 或温补晶振,如果要求稳定度在 0.5ppm 以上,则需选择数字温补晶振 (MCXO 。模拟温补晶振适用于稳定度要求在 5ppm ~0.5ppm 之间的需求。 VCXO 只适合于稳定度要求在 5ppm 以下的产品。在不需要即开即用的环境下,如果需要信号稳定度超过 0.1ppm 的,可选用OCXO 。 频率稳定性的考虑 晶体振荡器的主要特性之一是工作温度内的稳定性, 它是决定振荡器价格的重要因素。稳定性愈高或温度范围愈宽,器件的价格亦愈高。工业级标准规定的 - 40~+75℃这个范围往往只是出于设计者们的习惯, 倘若 -30~+70℃已经够用, 那么就不必去追求更宽的温度范围。设计工程师要慎密决定特定应用的实际需要,然后规定振荡器的稳定度。指标过高意味着花钱愈多。晶体老化是造成频率变化的又一重要因素。根据目标产品的预期寿命不同, 有多种方法可以减弱这种影响。晶体老化会使输出频率按照对数曲线发生变化,也就是说在产品使用的第一年, 这种现象才最为显著。例如, 使用 10年以上的晶体, 其老化速度大约是第一年的 3倍。采用特殊的晶体加工工艺可以改善这种情况,也可以采用调节的办法解决,比如, 可以在控制引脚上施加电压 (即增加电压控制功能等。与稳定度有关的其他因素还包括电源电压、负载变化、相位噪声和抖动,这些指标应该规定出来。对于工业产品,有时还需要提出振动、冲击方面的指标,军用品和宇航设备的要求往往更多,比如压力变化时的容差、受辐射时的容差,等等。输出必须考虑的其它参数是输出类型、相位噪声、抖动、电压特性、负载特性、功耗、封装形式,对于工业产品,有时还要考虑冲击和振动、以及电磁干扰 (EMI 。晶体振荡器可 HCMOS/TTL兼容、 ACMOS

晶体振荡器电路设计指南

AN2867 应用笔记 ST微控制器振荡器电路 设计指南 前言 大多数设计者都熟悉基于Pierce(皮尔斯)栅拓扑结构的振荡器，但很少有人真正了解它是如何工 作的，更遑论如何正确的设计。我们经常看到，在振荡器工作不正常之前，多数人是不愿付出 太多精力来关注振荡器的设计的，而此时产品通常已经量产；许多系统或项目因为它们的晶振 无法正常工作而被推迟部署或运行。情况不应该是如此。在设计阶段，以及产品量产前的阶 段，振荡器应该得到适当的关注。设计者应当避免一场恶梦般的情景：发往外地的产品被大批 量地送回来。 本应用指南介绍了Pierce振荡器的基本知识，并提供一些指导作法来帮助用户如何规划一个好的 振荡器设计，如何确定不同的外部器件的具体参数以及如何为振荡器设计一个良好的印刷电路 板。 在本应用指南的结尾处，有一个简易的晶振及外围器件选型指南，其中为STM32推荐了一些晶 振型号(针对HSE及LSE)，可以帮助用户快速上手。

目录ST微控制器振荡器电路设计指南目录 1石英晶振的特性及模型3 2振荡器原理5 3Pierce振荡器6 4Pierce振荡器设计7 4.1反馈电阻R F7 4.2负载电容C L7 4.3振荡器的增益裕量8 4.4驱动级别DL外部电阻R Ext计算8 4.4.1驱动级别DL计算8 4.4.2另一个驱动级别测量方法9 4.4.3外部电阻R Ext计算 10 4.5启动时间10 4.6晶振的牵引度(Pullability) 10 5挑选晶振及外部器件的简易指南 11 6针对STM32?微控制器的一些推荐晶振 12 6.1HSE部分12 6.1.1推荐的8MHz晶振型号 12 6.1.2推荐的8MHz陶瓷振荡器型号 12 6.2LSE部分12 7关于PCB的提示 13 8结论14

晶振的作用与原理

晶振的作用与原理 一，晶振的作用 (1)晶振是石英振荡器的简称，英文名为Crystal，它是时钟电路中最重要的部件，它的主要作用是向显卡、网卡、主板等配件的各部分提供基准频率，它就像个标尺，工作频率不稳定会造成相关设备工作频率不稳定，自然容易出现问题。 (2)晶振还有个作用是在电路产生震荡电流,发出时钟信号.晶振是晶体振荡器的简称。它用一种能把电能和机械能相互转化的晶体在共振的状态下工作，以提供稳定，精确的单频振荡。在通常工作条件下，普通的晶振频率绝对精度可达百万分之五十。高级的精度更高。有些晶振还可以由外加电压在一定范围内调整频率，称为压控振荡器（VCO）。 (3)晶振在数字电路的基本作用是提供一个时序控制的标准时刻。数字电路的工作是根据电路设计，在某个时刻专门完成特定的任务，如果没有一个时序控制的标准时刻，整个数字电路就会成为“聋子”，不知道什么时刻该做什么事情了。 (4)晶振的作用是为系统提供基本的时钟信号。通常一个系统共用一个晶振，便于各部分保持同步。有些通讯系统的基频和射频使用不同的晶振，而通过电子调整频率的方法保持同步。晶振通常与锁相环电路配合使用，以提供系统所需的时钟频率。

如果不同子系统需要不同频率的时钟信号，可以用与同一个晶振相连的不同锁相环来提供。 (5)电路中，为了得到交流信号，可以用RC、LC谐振电路取得，但这些电路的振荡频率并不稳定。在要求得到高稳定频率的电路中，必须使用石英晶体振荡电路。石英晶体具有高品质因数，振荡电路采用了恒温、稳压等方式以后，振荡频率稳定度可以达到10^(-9)至10^(-11)。广泛应用在通讯、时钟、手表、计算机……需要高稳定信号的场合。石英晶振不分正负极, 外壳是地线，其两条不分正负 二，晶振的原理； 石英晶体振荡器是利用石英晶体（二氧化硅的结晶体）的压电效应制成的一种谐振器件，它的基本结构大致是从一块石英晶体上按一定方位角切下薄片（简称为晶片，它可以是正方形、矩形或圆形等），在它的两个对应面上涂敷银层作为电极，在每个电极上各焊一根引线接到管脚上，再加上封装外壳就构成了石英晶体谐振器，简称为石英晶体或晶体、晶振。其产品一般用金属外壳封装，也有用玻璃壳、陶瓷或塑料封装的。

常用晶振型号一览表

1.8432MHz 18.432MHZ 25MHZ 4 MHZ 12 MHZ 16 MHZ 13 MHZ 21.47727 MHZ 33.8688 MHZ 3.6864 MHZ 10.245 MHZ 14.7456 MHZ 7.9296875 MHZ 24.576 MHZ 7.2 MHZ 22.1184 MHZ 21.504 MHZ 1.8432 MHZ 13.25 MHZ 24 MHZ 2 MHZ 9.8304 MHZ 20.945 MHZ 9.216 MHZ 14.31818 MHZ 76.8 MHZ 7.3728 MHZ 11.0592 MHZ 44.545 MHZ 40 MHZ 16.384 MHZ 27 MHZ 26 MHZ 48 MHZ 45 MHZ 90 MHZ 130 MHZ 112.32 MHZ 130 MHZ 45.1 MHZ 110.52 MHZ 21.4 MHZ 106.95 MHZ 128.45 MHZ 21.4 MHZ 38.85 MHZ 70 MHZ 45.1 MHZ 26.050 MHZ 8.192 MHZ 44 MHZ 15.36 MHZ 20 MHZ 125 MHZ 25 MHZ 50 MHZ 27 MHZ 65 MHZ 17.734475 MHZ 100 MHZ 32.768 KHZ 31.5 MHZ 29.5 MHZ 56 MHZ 12.288 MHZ 18.432 MHZ 33.333 MHZ 26.975 MHZ 27.145 MHZ 75 MHZ 153.6 MHZ 150 MHZ 455 KHZ 4.91 MHZ 6 MHZ 16.9344 MHZ 10 MHZ 3.64 MHZ 4.1952 MHZ 30 MHZ 8.38 MHZ 4.09 MHZ 16.8 MHZ 4.25 MHZ 9.83 MHZ 33.8688 MHZ 10.7 MHZ 10.8 MHZ 32 MHZ 5 MHZ 14 MHZ 17.28 MHZ 2.68 MHZ 3 MHZ 12.5 MHZ 3.2 MHZ 465 MHZ 446 MHZ 1960 MHZ 433.92 MHZ 225 MHZ 1842 MHZ.5 MHZ 942.5 MHZ 243.5 MHZ 85.38 MHZ 1489 MHZ 1441 MHZ 897.5 MHZ 280 MHZ 926.5 MHZ 903.5 MHZ 360 MHZ 881.5 MHZ 947.5 MHZ 340 KHZ 400 KHZ 26 MHZ 10.245 MHZ 1880 MHZ 1747.5 MHZ 1960 MHZ 1575.45 MHZ 1847 MHZ 842.5 MHZ 1842.5 MHZ 315 MHZ 310 MHZ 19.68 MHZ 13.56 MHZ