In yesterday's article "When the MCU has no crystal oscillator...", Xiao Bian focuses on the importance of quartz crystal oscillator in the MCU. However, as a precise frequency component, the crystal oscillator in the MCU is very It is prone to problems, and a slight collision may cause damage to the crystal oscillator. Therefore, it is a common phenomenon that the crystal oscillator of the single chip does not vibrate. Xiaobian's several customers who are doing single-chip microcomputers have also consulted on this issue. Today, Xiaobian has made a simple introduction to the problems and processing methods often encountered by the single-chip crystal oscillator.
Analysis of the reason why the crystal oscillator does not vibrate
First of all, we analyze the reasons why the crystal oscillator of the single chip does not vibrate.
1 PCB wiring error, the current PCB is no longer a single function circuit (digital or analog circuit), but a mixture of digital and analog circuits. Therefore, there may be problems when the PCB is routed, causing the crystal to not vibrate;
2 quality problems of single chip or crystal oscillator;
3 The load diode or matching capacitor does not match the crystal oscillator or the capacitor quality is faulty;
4 PCB board is damp, resulting in impedance mismatch and unable to start vibration;
5 The crystal oscillator circuit is too long or there is a trace between the two legs, which causes the crystal oscillator to not vibrate. Generally, the trace of the crystal oscillator circuit should be as short as possible and as close as possible to the oscillator during PCB layout. It is strictly forbidden to walk between the crystals. line;
The 6 crystal oscillator is affected by the peripheral circuit and does not start to vibrate.
1 crystal oscillator selection, the selection of a suitable crystal oscillator is very important for the microcontroller, we must at least consider the resonance frequency, load capacitance, excitation power, long-term stability of temperature characteristics and other parameters when choosing the crystal oscillator. A suitable crystal oscillator will ensure that the microcontroller will work properly.
2 The crystal oscillator caused by the capacitor is unstable. The two capacitors C1 and C2 in the crystal oscillator circuit have a great influence on the stability of the crystal oscillator. Each crystal oscillator has its own characteristics, so we must according to the value provided by the crystal manufacturer. Select external components. Usually within the allowable range, the lower the C1 and C2 values, the better. If the C value is too large, it is beneficial to the stability of the oscillator, but it will increase the start-up time. In general, we make the C2 value greater than the C1 value, which will speed up the crystal oscillation during power-on.
3 The problem that the crystal oscillator of the single-chip microcomputer is driven excessively, the crystal oscillator being driven excessively will gradually lose the contact electroplating of the crystal oscillator and cause the crystal frequency to rise. We can use an oscilloscope to detect, OSC, output pin. If a very clear sine wave is detected and the upper and lower limits of the sine wave meet the clock input requirements, the crystal is not excessively driven. Conversely, if the sine waveform The crests, the troughs are flattened at both ends, and the waveform is square. The crystal oscillator is driven excessively. In this case, the resistor RS is needed to prevent the crystal oscillator from being overdriven. The easiest way to determine the magnitude of the resistor RS is to connect a 5k or The 10k trimming resistor is slowly turned up from 0 until the sine wave is no longer flattened. By this method, the nearest resistor RS value can be found.
4 When drawing a PCB, the crystal oscillator is required to be as close as possible to its amplifier circuit (IC pin). This is due to the limited output capability of the crystal, which only outputs electrical energy in milliwatts. Inside the IC (Integrated Circuit), this signal is amplified by a few hundred or even thousands of times by an amplifier to be used normally. The crystal oscillator and the IC are usually connected by a copper trace. This trace can be regarded as a capacitor or a number of conductors. When the conductor cuts the magnetic flux, it will generate current. The longer the conductor, the stronger the current generated.
The crystal oscillator is like the heart of a single chip microcomputer! We all know that the role of the microcontroller crystal oscillator is to provide the basic clock signal for the system. Usually a system shares a crystal to keep the parts in sync. The quartz crystal model and frequency used by different types of single-chip microcomputers may also be different. If the crystal oscillator in the MCU is out of order, the MCU will not work properly. Therefore, if you find that your MCU is not working properly, it may be caused by a crystal problem.
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