STM32L 32.7680Khz Clock Problem

[GJaeg.1]

Hello,

after several Years of using the 32L475RET6 with a 12pF 32.7680KHz Crystal and 8.2pF Capacitors we need to change the PCB to a 32L486QG. As to read in the Datasheet both have 5pF Pin Input Capacity.

But the new Design (Chrystal about 1.5mm away from IC ) wont start. The Crystal (changed several Times also against a 9pF ) wont start. When i put a Finger on the Capacitors he will start and the Device is running as long my Finger is on the PCB.

The STM32L4 is switching between run mode and sleep mode several Times in his Application.

Until now i have no Idea whats going on..

Any Idea ??

Thanks and Greetings

*****

[ONadr.1]

Sometimes it helped me to put a large value resistor (1M-3M) parallel to the crystal.

[GJaeg.1]

Hello, always done...no Result.

[ONadr.1]

And few hundreds kiloohms serial rezistor (with paralel rezistor too)?

https://en-support.renesas.com/knowledgeBase/18887754

[GJaeg.1]

100/220/470K tested. Also no help. only a Finger on one of the Chrystals solved the Problem.

Unchanged PCB, just changed the Controller.

[waclawek.jan]

Have you tried changing LSE drive setting?

​

JW​

[GJaeg.1]

Hi, default was Low because its a Batterie Powerd Device and every µA counts.

Tried mid and high...also no Result

[ONadr.1]

I would try unsymetrical values of capacitors with paralel rezistor. Or, probbably better way, from known parameters of crystal (are known?) recalculate exact values rezistors and capacitors (using equation).

[GJaeg.1]

Hello, unsymetrical Values also dont help. Just ordered an Abracon Crystal which is listet in ST´s Application Not. Need some Days...but cant believe that´s the reason because the other ST which is also from the "L4" Family works with the Crystal i used over Years.