Standby power consumption is too high, touching the UART pins with hand fixes it?

[Waller.George]

Hi all,

Just looking for some thoughts to what might be happening here.

I'm using an STM32L052C8T microcontroller in a low power application. I send the MCU to sleep in standby mode however the MCU power consumption is too high at about 615uA. It may start off at an acceptable 15uA however handling of the PCB will cause something to happen for the current to fluctuate from 200uA up to 650uA almost randomly oscillating. I can fix the issue by holding the edge of the PCB with one hand (not connected to earth or the PCB GND) and touching some exposed pads for the UART TX/RX pins. I can also fix the issue by forcing the PCB into my antistatic mat (the signals are exposed on the bottom). I can touch either one and the consumption instantly drops to 15uA and stays there. I did not have any pull up/downs on the UART so I tried adding 10k or 1M on both to either 3V3 or GND but these did not do anything. I have many signals exposed on the PCB for the programming jig, I tried going through all of them to see if the same effect occurred by touching them, they did not. These signals consisted of the SWD, reset and BOOT0 pins. I do not deinit the pins when going into sleep as I thought it was unnecessary, I have tried deiniting and setting them as GPIO inputs, outputs and analogue types before sleeping but this does nothing.

If anyone has any advice, that would great!

Thanks

George

[waclawek.jan]

Digital input pins left floating?

JW

[S.Ma]

Are you using ADC and/or DAC and/or Vref/Temp measurements?

Are all pins and regulators not included in the current consumption?

Are the clock sources all stopped? (external generators if any)

[Waller.George]

I use the ADC in the application.

I'm measuring the power consumption at the input of my 3.3V regulator.

I only have the MCU and external RTC powered in the low power state.

Hmm I don't think the problem is to do with the MCU config, I'm not sure. I think it might be EMC related.

When I have the board placed on the ESD mat just on it's edge (no electrical connections touching the mat) the power draw settles to about 50uA, stable. If I move it 10cm to the right, off of the mat, the consumption sits around 650uA. If I touch the UART pins (only works with the UART pins) the consumption goes down to 15uA.

This is very bizarre and something I haven't come across before.

[Waller.George]

So with the PCB powered with a battery, one finger on a UART pin and another on the ESD mat I can reduce the current consumption to 15uA. Thing is I have no idea what this means. Seems very weird to me. I have also found any other non connected MCU will work the same as the UART pin.

[Waller.George]

I've been reading a little more on what to do with pins that are unconnected. Section 6.6 of this manual says to connect the pins to GND which could be the reason I see extra power consumption. This theory does not explain touching one pin with a hand working!