LSE Not Working After Replacing STM32L4R9ZIT6

[Roland Walkner]

Hello,

I am using an STM32L4R9ZIT6 in a new project. After replacing the MCU with a new one, the LSE oscillator stopped working. And the MCu doesn’t start until one activates the LSI.

To troubleshoot the issue, we have:

• Replaced the crystal
• Tried different capacitor values
• Verified that the crystal works normally in another setup

Since the only change was the MCU replacement, we are wondering what could be causing this problem. Do you have any suggestions on possible causes or additional checks we should perform?

Thank you in advance for your help.

Best regards,

Roland

[mƎALLEm]

Hello ​@Roland Walkner and welcome to the ST community,

Most probably that’s a solder joint issue. Verify your soldering.

Apply flux and reheat/resolder the MCU.

[Roland Walkner]

Hello ​@mƎALLEm , 

we have the problem with 2 different PCBs and we already checked the solder joints and resolder them.

[mƎALLEm]

As you replaced the MCU (I suppose that it worked before), I don’t think the issue comes from the MCU but to your board/ the soldering or PCB trace problem ..

[AScha.3]

Hi,

Check for the LSE drive, if this CPU has different drive level, set it to high.

The drive low setting is standard for new parts, but might not work with all crystals or if there is some remains from soldering, flux etc.

The drive level can be set by registers rcd_bdcr setting, or easy in Cube;

Set it to high or medium high, then it should run always fine.

[Roland Walkner]

Hi ​@AScha.3,

thanks for your reply we tested all possible levels but unfortunately it didn’t change anything. The quarz is still not running.

[Ozone]

Did you consider the possibility that your design (quartz, components, PCB) might be marginal ?
ST specifies ranges for electrical parameters, including clock pins. Individual MCUs might differ slightly.
 

[AScha.3]

Hi,

So if crystal LSE not working, at hi drive setting, you simply have a contact problem.

As you have a lqfp144 , you can access the pins and check continuity pin to crystal and no short between pins and adjacent pins.

As it was working before, there only possible:

- CPU defective 

- crystal defective 

- no contact or shorted 

- not cleaned, soldering flux still there 

Should not be so difficult to find out.

[waclawek.jan]

Why did you replace the mcu?

How do you know it worked before?

Check all GND/VCC connections, including 

Try some absolute minimal code with direct register writes (or not even code, do it in debugger): enable PWR in RCC, enable backup domain access in PWR, perform explicit backup domain reset in RCC_BDCR and enable LSE in RCC_BDCR, observe through LSERDY bit in RCC_BDCR, or set up a MCO pin.

JW