If a crystal unit oscillation circuit does not oscillate, the following four conditions can be considered as causes.
- Incorrect microcontroller settings
- C-MOS inverter is not operating as an amplifier.
- Crystal unit characteristics do not meet the specifications.
- Insufficient oscillation margin
These issues occur due to mismatches in circuit conditions or component characteristics, and it is possible to identify the causes and take countermeasures by checking the settings and circuit conditions in order.
If oscillation does not occur, isolate the causes more easily by checking in the following order.
- Check the microcontroller settings.
Check if the settings for enabling the oscillation circuit and the initialization processes are correctly configured.
- Check the amplification stage of the oscillation circuit.
Check if the C-MOS inverter is operating as an amplifier, the feedback resistor (Rf) is correctly connected, or the feedback resistor is built into the microcontroller.
- Check the characteristics of the crystal unit.
Check if the crystal unit characteristics such as R1 and the load resonance frequency (fL) are within the specification range.
- Check the oscillation margin.
Check if negative resistance is sufficiently ensured in the oscillation circuit.
Causes and countermeasures
<Cause (1): Incorrect microcontroller settings>
The settings for enabling the oscillation circuit or the initialization processes have not been configured.
Depending on the IC, the settings for enabling the oscillation circuit or reset processes may be required, and the oscillation conditions may not be met due to insufficient settings.
Main countermeasures
- Check if the activation settings and initialization processes for the oscillation circuit are correctly configured
<Cause (2): C-MOS inverter is not operating as an amplifier>
The bias voltage is not correctly applied to the inverter, which is the amplification stage of the oscillation circuit.
If the feedback resistor (Rf) is not properly connected, the inverter may not operate as an inverting amplifier and may fail to oscillate.
Main countermeasures
- Check if the feedback resistor (Rf) is correctly connected or if the feedback resistor is built into the microcontroller
- Check if the feedback resistor value is appropriate
<Cause (3): Crystal unit characteristics do not meet the specifications>
The electrical characteristics of the crystal unit (R1 and resonance frequency, etc.) do not match the circuit conditions.
If the characteristics are out-of-specification, the oscillation condition may not be satisfied, which can cause the oscillation circuit to fail to operate.
Main countermeasures
- Check if the R1 and resonance frequency of the crystal unit are within the specification range
- Check if the crystal unit being used is compatible with the circuit conditions
<Cause (4): Insufficient oscillation margin>
The margin required for the oscillation circuit to oscillate in a stable manner is insufficient.
If the negative resistance of the circuit is small, the oscillation circuit may fail to start or maintain oscillation, which can cause a failure to oscillate or oscillation stoppage.
Main countermeasures
- Ensure sufficient oscillation margin (as a general guideline, 5x or more of the R1 standard value for consumer applications and 10x or more for automotive applications)
- Adjust the load capacitance to increase the negative resistance of the circuit
- Adjust the limiting resistance to improve the oscillation margin
■Specific example of improving oscillation margin (design parameter adjustment)
Reduce the value of the external load capacitance.
Reducing the external load capacitance increases the impedance of the inverter input and output, which increases the negative resistance of the oscillation circuit, and improves the oscillation margin as a result. However, reducing the external load capacitance increases the oscillation frequency. After changing the external load capacitance, confirm that the oscillation frequency falls within the desired frequency range.
Reduce the value of the limiting resistance.
The amplitude amplified by the inverter is attenuated by the limiting resistance, so reducing this value reduces the attenuation of the amplitude and increases the negative resistance of the oscillation circuit, which improves the oscillation margin. However, since the attenuation of the amplitude is reduced, the drive level increases. Check the specifications for the crystal unit being used to confirm that the drive level does not exceed the range of use.
Explanation of technical terms
What is oscillation margin?
It is an index that indicates the amount of margin required for an oscillation circuit to start and maintain oscillation.
It is generally recommended to ensure a margin of 5x or more with respect to the equivalent series resistance (R1) of the oscillator.
It is generally recommended to ensure a margin of 5x or more for consumer applications and 10x or more for automotive applications relative to the standard value of the equivalent series resistance (R1) of the oscillator.
What is a feedback resistor (Rf)?
It is a resistor that is used to operate a C-MOS inverter as an inverting amplifier.
It is an important component that determines the operating point of the oscillation circuit and can cause a failure to oscillate if the connection or resistance values are inappropriate.
For the feedback resistor, refer to “Crystal Units: Basic Knowledge of Technical Terms.”
What is load capacitance?
It is the capacitive component connected to the oscillation circuit.
Load capacitance is an important circuit parameter that impacts the oscillation frequency and the stability of the oscillation circuit (oscillation margin).
For the load capacitance, refer to “Crystal Units: Basic Knowledge of Technical Terms.”
Scenarios where this information is useful
During the design process
It serves as a reference for considering the necessary circuit conditions and component constants when designing an oscillation circuit.
During prototyping and evaluation
It can be used as a confirmation item or evaluation procedure when oscillation does not occur in a prototype circuit.
Troubleshooting
It is helpful for isolating the cause when a failure to oscillate or oscillation stoppage occurs.
Mass production defects
It can be used for analyzing the causes and preventing a recurrence of oscillation failures and unstable operation occurring in mass-produced goods.