Load capacitance problem of microcontroller

The two capacitors to ground next to the microcontroller crystal are called the load capacitors of the crystal , which are connected to the two legs of the crystal and the capacitance to ground, respectively. They are usually in the tens of pico hairs. They will affect the resonant frequency and output amplitude of the crystal. Generally, when you order a crystal, the supplier will ask you what the load capacitor is.

Generally, the crystal oscillator of a single-chip microcomputer works in a parallel resonance state, which can also be understood as a part of the resonant capacitor. It is selected according to the load capacitance required by the crystal oscillator provided by the crystal oscillator manufacturer. In other words, the frequency of the crystal oscillator is measured under the load capacitance it provides, which can maximize the error of the frequency value. It can also guarantee errors such as temperature drift. The values ​​of the two capacitors are the same, or the difference is not large. If the difference is too large, it is easy to cause an imbalance in the resonance, which is easy to cause the oscillation to stop or simply not oscillate. The crystal oscillator load capacitance value refers to the load capacitance value that participates in the oscillation in the AC circuit of the crystal oscillator and is connected in series or in parallel with the crystal oscillator. The circuit frequency of the crystal oscillator is mainly determined by the crystal oscillator itself. Since the load capacitance participates in the circuit oscillation, it will definitely play a role in fine-tuning the frequency. The smaller the load capacitance value, the higher the oscillation circuit will be.

The crystal pins of various logic chips can be equivalent to a capacitor three-point oscillator. The inside of the crystal pin is usually an inverter, or an odd number of inverters connected in series. A resistor is connected between the crystal output pin XO and the crystal input pin XI. For CMOS chips, it is usually between several megahertz and tens of megahertz. Many chip pins already include this resistor inside, so there is no need to connect it outside the pin. This resistor is to make the inverter in a linear state at the beginning of oscillation. The inverter is like an amplifier with a large gain, which is easy to start. The quartz crystal is also connected between the input and output of the crystal pin, which is equivalent to a parallel resonant circuit. The oscillation frequency should be the parallel resonant frequency of the quartz crystal. The two capacitors next to the crystal are grounded, which are actually the voltage-dividing capacitors of the three-point capacitor circuit, and the grounding point is the voltage-dividing point. Taking the grounding point, i.e., the voltage-dividing point, as the reference point, the input and output of the oscillation pin are inverted, but from the perspective of the parallel resonant circuit, i.e., the two ends of the quartz crystal, a positive feedback is formed to ensure that the circuit continues to oscillate. When the chip is designed, these two capacitors have been formed. Generally, the two capacities are equal. The size of the capacity varies depending on the process and layout, but it is relatively small after all, and may not be suitable for a wide frequency range. When connected externally, it is about a few PF to tens of PF, depending on the frequency and the characteristics of the quartz crystal. It should be noted that the value of the two capacitors in series is parallel to the resonant circuit, which will affect the oscillation frequency. When the two capacitances are equal, the feedback coefficient is 0.5, which generally meets the oscillation conditions. However, if it is difficult to start oscillation or the oscillation is unstable, the capacitance of the input terminal to the ground can be reduced, and the value of the output terminal can be increased to increase the feedback amount.

The mismatch of the oscillation circuit causes the crystal oscillator to fail to oscillate, affecting three indicators of the oscillation circuit: frequency error, negative impedance, and excitation level.

① The frequency error is too large, causing the actual frequency to deviate from the nominal frequency, thus causing the crystal oscillator to fail to oscillate.

Solution: Choose products with appropriate PPM values.

② Negative impedance that is too large or too small will cause the crystal oscillator to stop oscillating. The crystal oscillator gradually stops oscillating during operation, and it starts to work again when you touch it with your hand or heat the crystal oscillator pin with a soldering iron.

Solution: If the negative impedance is too large, you can increase the value of the crystal oscillator's external capacitors Cd and Cg to reduce the negative impedance; if the negative impedance is too small, you can reduce the value of the crystal oscillator's external capacitors Cd and Cg to increase the negative impedance. Generally speaking, the negative impedance value should be no less than 3-5 times the nominal maximum impedance of the crystal oscillator.

③ If the excitation level is too large or too small, the crystal oscillator will not oscillate. If the excitation level is too large, the crystal oscillator may become hot during operation and gradually stop oscillating.

Solution: Adjust the excitation level of the oscillator circuit to the crystal output by adjusting the size of Rd in the circuit. Generally speaking, the smaller the excitation level, the better. In addition to low power consumption, it is also related to the stability of the oscillator circuit and the service life of the crystal.

Crystal oscillator PCB wiring: When wiring the PCB, the crystal oscillator circuit should be as short and straight as possible, and as close to the MCU as possible, to minimize the impact of stray capacitance in the oscillation circuit on the crystal oscillator; when wiring the PCB, try not to run signal lines under the crystal oscillator to avoid electromagnetic interference to the crystal oscillator, which may cause instability in the oscillation circuit. It is generally not recommended to use ultrasonic cleaning for circuit boards with crystal oscillators to avoid resonance and damage to the crystal oscillator, resulting in defects.

Position on the PCB: If your PCB is relatively large, the crystal oscillator should be positioned closer to the edge as much as possible. This is because if the crystal oscillator is designed in the middle, it will be affected by the mechanical tension caused by the deformation of the PCB, which may cause defects. If your PCB is relatively small, the crystal oscillator should be positioned closer to the middle as much as possible and should not be designed at the edge. This is because the PCB is small and generally SMT reflow soldering involves multiple panels. The mechanical tension generated during panel separation will affect the crystal oscillator and may cause defects.