STM32WL: TCXO series impedance calculation

[AJ_1]

Hello,

I'm planning to use an STM32WL5X microcontroller in an upcoming project, and I found myself confused on the recommended impedance for external 32MHz oscillators. Please see the attached image for a screenshot from the datasheet.

The datasheet says that the incoming clock signal needs a minimum of 0.4 Vpp, and an absolute max of 1.2 Vpp. Note 1 below the table says that oscillators outputting a 0.8 Vpp signal should have a 220 ohm resistor and 10 pF capacitor put in series before the oscillator input pin. The oscillator input has a minimum input capacitance of about 11 pF.

If the input signal is 0.8 Vpp, and you make a voltage divider as described, the input signal is going to be way less than 0.4 Vpp. How is that supposed to work??

@Peter BENSCHand @HamiltonDC, I found other forum responses from you two on this issue, but I still don't understand the math behind this. How is the input signal wave supposed to be greater than 0.4 Vpp?

[AJ_1]

The datasheet shows the resistor and capacitor in series with the HSE32 input pin.

 

But when I simulate that circuit with a 0.8V amplitude AC source, there's only 366mV on the OSC_IN pin at 32MHz.

 

[STTwo-32]

Hello @AJ_1 

I think you may find really helpful to analyze the NUCLEO-WL55JC1. It should help you to understand how it works and how to select the values.

Best Regards.

STTwo-32

[LCE · Accepted solution]

@AJ_1 your analysis looks good, there might be a "documentation bug".

As always, check some stuff that works, so as STTwo-32 said, simply do it as on the Nucleo-board (or some other evaluation board with the same controller).

But very smart that you check that detail before schematics and PCB layout, too many people just "throw stuff" onto their boards and wonder afterwards...

good engineering = attention to detail