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The crystal oscillator has voltage but no voltage difference. Is it a short circuit?
In hardware debugging, many engineers find that there is voltage at both ends of the crystal oscillator, such as 1.6V, but there is no obvious voltage difference when measuring the crystal oscillator. The first reaction may be to suspect a short circuit.
The crystal oscillator circuit is essentially an AC oscillation circuit. When the crystal oscillator is not oscillating, the two ends will be stationary at an intermediate potential, usually close to half of the power supply voltage. The multimeter measures a stable DC voltage, so there is no voltage difference. This situation is generally: the crystal oscillator is not oscillating, not short circuit.
If a short circuit is suspected, it is recommended to use the resistance range of the multimeter for measurement. Measure the resistance at both ends of the crystal oscillator after power off:
Close to 0Ω: There may be a short circuit, such as solder bridge, internal breakdown of the crystal oscillator;
More than a few thousand ohms: It means that the crystal oscillator circuit is unobstructed, but it is not oscillating.
Reasons for the crystal oscillator not oscillating
Inappropriate load capacitance:
Crystal oscillators usually require two external capacitors. If the capacitance values do not match, it may not oscillate. It is recommended to consult the crystal oscillator specification to calculate the appropriate load capacitance value.
Feedback resistor missing:
Generally, a feedback resistor of about 1MΩ is connected in parallel at both ends of the crystal oscillator to help start the oscillation. If the value is not appropriate, the oscillation circuit may not be able to start in a closed loop.
Driver chip configuration error:
Master chips such as STM32 and ESP32 need to be set to "external crystal mode" by software, otherwise, even if the circuit is connected correctly, it will not start.
Crystal oscillator itself is damaged:
Although some crystal oscillators do not have any problems on the outside, the internal quartz chip may be broken or aged.
PCB wiring problem:
If the wiring near the crystal oscillator is too long, too thin or seriously interfered, it will also affect the oscillation effect.
How to correctly check if the crystal oscillator does not oscillate?
1. Turn off the power and measure the resistance → eliminate short circuit
2. Check the capacitance/resistance value → ensure it is reasonable
3. Replace the crystal oscillator → eliminate device problems
4. Observe the waveform with an oscilloscope → confirm whether it is oscillating
5. Check the MCU register/startup configuration → ensure the setting is correct
