I. Definition and Principle

A quartz crystal oscillator, commonly known as a quartz crystal or crystal oscillator, is an electronic component that uses the piezoelectric effect of quartz crystals (also known as crystals) to generate high-precision oscillation frequencies and is a passive component. The component is mainly composed of quartz crystal chips, bases, shells, silver glue, silver and other components. According to the lead condition, it can be divided into two types: plug-in (with leads) and surface mount (without leads).

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Every microcontroller system has a crystal oscillator. The role of the crystal oscillator in the microcontroller system is very large. The higher the clock frequency provided by the crystal oscillator, the faster the microcontroller will run. The crystal oscillator uses a crystal that can convert electrical energy and mechanical energy into each other to work in a resonant state to provide stable and accurate single-frequency oscillation. The role of the crystal oscillator is to provide the basic clock signal for the system. Usually a system shares a crystal oscillator to keep all parts synchronized.

II. Difference between active and passive crystal oscillators

1. Passive crystal oscillators are non-polar components with 2 pins and require a clock circuit to generate an oscillating signal. They cannot oscillate on their own. Active crystal oscillators have 4 pins and are complete oscillators. In addition to quartz crystals, they also have transistors and resistance-capacitance components. If there is a clock circuit, use passive, otherwise use active.

2. Passive crystals do not have voltage problems. The signal level is variable and determined by the start-up circuit. The same crystal can be used for multiple voltages and can be used for multiple DSPs with different clock signal voltage requirements.

3. Active crystal oscillators have good signal quality, are relatively stable, and have relatively simple connection methods without complicated configuration. The circuit has a point marked as pin 1, which is 2, 3, and 4 counterclockwise (pin down). The usual usage of active crystal oscillators: one foot is suspended, two feet are grounded, three feet are connected to output, and four feet are connected to voltage. Compared with passive crystals, the disadvantage of active crystal oscillators is that their signal level is fixed and they need to choose a suitable output level with poor flexibility and high price.

III. Crystal Oscillator Measurement Method

1. Power on the device to make it in normal working state and use a multimeter for measurement. Set the multimeter to R×10k gear and test the resistance value between the two ends of the crystal oscillator. If it is infinite, it means that there is no short circuit or leakage in the crystal oscillator.

2. Insert the test pen into the test socket, pinch any pin of the crystal oscillator with your finger, touch the metal part at the top of the test pen with another pin, if the neon bubble of the test pen turns red, it means that the crystal oscillator is in normal state . On the contrary, it means that the crystal oscillator is damaged.

3. Measure its capacitance with a digital multimeter’s capacitance gear. Generally speaking, a good crystal oscillator has a capacitance of several tens of pf. Generally damaged crystal oscillators have significantly reduced capacity.

4. Use a multimeter to measure whether the voltage between the two pins of the crystal oscillator is half of the chip operating voltage. For example, if the operating voltage is 5V, whether the voltage of the crystal oscillator pin is about 2.5V. In addition, if you touch another foot of the crystal with tweezers, this voltage will change significantly, indicating that it has started up. Generally speaking, when a crystal oscillator oscillates, use a multimeter’s DC voltage gear to measure its pin voltage. Different circuit voltages are different.

Note: A 1.5V battery connected to both ends of a crystal oscillator puts it close to your ear and listens carefully when you hear a click sound that means it has started up and it’s okay.

IV. Reasons for Crystal Oscillator Not Starting

1. PCB board wiring error;

2. The quality of single-chip microcomputer has problems;

3. The quality of crystals has problems;

4. The load capacitor or matching capacitor does not match or there are problems with capacitor quality;

5. The PCB board gets damp and causes impedance mismatch so it cannot start;

6. The wiring of the crystal circuit is too long;

7. There are wires between two feet of crystals;

8. Influence of peripheral circuits;

V.Solutions for Crystal Oscillator Not Starting:

1.Exclude possible circuit errors so you can compare with recommended circuits for corresponding model microcontrollers;

2.Exclude the possibility of poor peripheral components, because peripheral components are nothing more than resistors and capacitors, and you can easily identify whether they are good products;

3.Exclude the possibility that the crystal oscillator is a stop-vibration product, because you will not only try one or two crystal oscillators;

4.Try to change the capacitance at both ends of the crystal, maybe the crystal oscillator can start up, please refer to the use instructions of the crystal oscillator for the size of the capacitance;

5.When wiring PCBs, the wiring of the crystal circuit should be as short as possible and as close to IC as possible to prevent wiring between two feet of crystals;

VI.Development Trend of Crystal Oscillator

1.Miniaturization, thinning, and chipization.

2.High precision and high stability: The total accuracy of uncompensated crystal oscillators can also reach 25ppm.

3.Low noise and high frequency: Phase noise is an important parameter characterizing the frequency jitter of an oscillator.

4.Low power consumption and fast start-up: low voltage operation, low level drive and low current consumption.