To wich parameter refers HSE oscillator characteristic Gm in STM32L443 datasheet on page 123? Does it refer to AN2867 parameter gm or to Gm_crit_max?

[ChrisK1]

I'm in doubt about the transconductance parameter of the HSE oscillator in STM32L443.

To select an appropriate cristal I studied AN2867.

If the datasheet of STM32L443 which notes "Gm = 1.5mA/V" refers to gm the transconductance of the passive elements should stay below 0.3mA/V, which is really low. Selecting a crystal with appropriate characteristics will result in one with a low load capacitance, so I have trouble to even place load capacitors beside the cristal. If I take the PCB and I/O-stray capacitance to have 10pF the load capacitors should then have negative values (obviously this is impossible).

But if "Gm = 1.5mA/V" refers to Gm_crit_max then the transconductance of the passive components must only be less than this (not 5 times smaller).

Can please someone give advice here?

May there should be a naming convention about characteristics?!

Thanks a lot

Christoph

[ChrisK1]

I received this answer from the application team by e-mail:

First:

Gm=1.5mA/V means the max. allowed transconductance of the crystal which corresponds to Gm_crit_max in the AN2867 document.

Second:

“The Cs use a little assumption which more depends on layout with the PCB.

We usually use 5pF as Cs.

A good approach is to select the crystal in case of lower driving level it will be probably 6pF crystal and the usual C range used are CL1,2 as 4.7 to 6.8pF. You can then test the circuit to verify what is the LSE output frequency. By changing the CL1,2 you can slightly affect the LSE frequency to tune it into desired values.

In AN2867 you will find recommended designs and also tested crystals.�?

Gives me the courage to try it with 4.7 to 6.8pF CL1,2 with the already chosen crystals.

Thanks a lot!

Christoph