LSE is not running reliably on custom PCB with STM32L151CBU6.

[EDeve]

I have made a customize PCB with STM32L151CBU6 and it's LSE is not running reliably.

LSE only runs when I apply some heat with my hot air station. But after some minutes, the LSE stops and again I need to apply some heat to make it run again.

Can anyone provide me any information about if there is issue with the design or selection of components?

The 32.768KHz crystal I am using is :- FC-135 32.7680KA-AC0

I have tried different values capacitors 6.8pF, 12pF, 15pF, 18pF. But there is same problem with all values.

[waclawek.jan]

Is there a ground plane? What is the route of GND to which the capacitors from crystal are connected, into the GND pins of mcu?

JW

[EDeve]

Yes, this a four layer PCB with dedicated power and ground planes.

[Tesla DeLorean]

Is this a 6-7pf crystal?​

[EDeve]

It is a 9pF crystal as mentioned on supplier's website.

[Tesla DeLorean]

Hard to cut n paste from phone. The 9 and 12pf crystal aren't suitable for F1, F2 and F4 parts. Check App Note ​for STM32 and BOM for L1 DISCO boards.

[waclawek.jan]

Please review AN2867.

While it recommends the EPSON FC135 crystal, unfortunately, the model number does not guarantee the load capacitance. According to the DS, 7 pF, 9 pF, 12.5 pF  variants are available for this model. Interestingly, AN2867 quotes this model as 6pF...

JW

[EDeve]

Alright! I have read the AN2867 and figured out that I must use 6pF or 7pF crystal for my design. I will test them and come back here again with results.

[Piranha]

Does your code reset HSEBYP bit and wait on HSERDY bit in RCC_CR register?

[EDeve]

I am not using the HSE. the HSE crystal is not mounted.