STM32G474 WKUP Pin needs to be pulled down?

[Ryan Coleman]

Hello,

I’m using an STM32G474 in standby mode and have found some weird behavior. I know the MCU is entering standby mode as the flag is set upon wake up, however the MCU is still drawing ~120μA in standby. 

The WKUP pin is floating when not pulled up. To wake, it’s connected to a 10k pull up resistor via a switch. I’ve been poking around and found that connecting the WKUP pin to GND while in standby drops the current down to about 2μA, in line with the standby current spec. I soldered a 1MΩ resistor between the pin and GND and this seems to do the trick while allowing my pull up to still operate and wake the MCU. 

Any idea why this is happening? Perhaps someone could point me to a reference document that explains it? 

Thanks in advance! 

[MSing.1713]

Non-expert opinion.
It is unwise to let an input pin float when powered down.  Many of the STM32 MCUs have a register where you can enable a pull-up/down when in the lowest power modes.  This should be well documented in the RM for the part.

An input that isn't in a defined state may drift in these low power states.  This causes the input FETs to switch enough to draw power.  We generally use the internal pull-ups/downs when possible because it sometimes means we can reduce overall power more efficiently.  For example, if we need the pull-down in standby, but want to let the pin remain high most of the time when running.  In your case, a soldered pull-down would introduce a static draw.