Unexpected oscillation from a GPIO pin STM32L562QEI6

[MFail.11]

Hello,

please see the schematic attached.

I observed 2 MHz oscillation (with 1V amplitude) on pin M1 (PA5) when I send a square wave with a period of 2 seconds.

The GPIO is connected to that simple circuit, used to generate heat in the resistor.

I used this config so many times with different STM32 mcu, but with this BGA mcu I found this issue.

What could be the problem?

Thanks in advance.

[waclawek.jan]

And how is this pin used in the program? How is it set in GPIO? Do you have DAC enabled?

JW

[TDK]

In addition to JWs questions, do you have any 2MHz clocks operating in your program?

Does it happen if you hold NRST low?

[MFail.11]

GPIO is set to push pull, no pull up and no pull down. DAC is not enabled.

I have not part of the program running at 2 MHz.

I have not checked with NRTS low, I assumed no signal from MCU, no oscillation.

[waclawek.jan]

> GPIO is set to push pull, no pull up and no pull down. DAC is not enabled.

OK and what else is in that program? How do you "send square wave"? Isn't your timing off?

> I have not checked with NRST low

Then check it.

JW

[MFail.11]

We found that oscillation is also in the 3.3V supply rail.

But there is no issue with that LDO.

I use 2 LDO, one for MCU and one for analog components. So they are the same, same electric connections, but that used for the MCU has oscillations.

Moving away this LDO and connecting the first with the output pad of the second the oscillation goes away and MCU runs good.

It's a mystery at the moment, because I have few PCB with same hardware and only a different STM32 and no problem so far with LDOs.