Hello,
In one of our new PCBs we use a STM32L476VET6. For the RTC we use an external Oscillator (shown in the Picture)
The drive power is set to the highest level.
In our setup, the RTC only works at room temperature and with no probe connected. As soon as the oscillator is heated up to 65°C, it stops working. Same behaviour with the freezer spray. When a Probe for the oscilloscope is connectet to OSC_32_IN, the Oscillator also stops.
We measured the frequency of the oscillator using the MCO-Output on PA8. This way we found out, that the frequency is within the specified range of the oscillator. When using a slightly higher load capacity the frequency drops out of the specified range. When the load capacitence is lowered (we tried 15pF) , the frequency rises higher than the specifiaction of the oscillator. This makes us certain, that the right load-capacitence is used.
We also tested the oscillation-safety-factor by adding a resistor, three times the ESR of the oscillator in series to it. This also works but only at room temperature.
We also repeated the tests on a new, untreated PCB with the same results.
This is the Layout:
Note that there is a gound plane underneath the oscillator and the capacitors.
In an other project we used a STM32L4R5ZGT6 with the same oscillator and capacitor, the highest drive-level and a similar layout. The only difference is the processor. But with this setup, the oscillator can be heated up to 80°C and cooled with the freezer spray without the oscillator stopping.
Please help us find the problem.
Thanks in advance.
