Excessive power consumption when MCU is in standby mode, touching the PCB fixes.

[Boscoe]

Hi all, I have this really strange issue that I can't get my head around. I have a custom PCB with an STM32L052 MCU, an external RTC and an ST25DV NFC IC. The MCU and RTC are always powered and the NFC is powered up when a rising edge interrupt (generated by the NFC field energy) is sent to the MCU. When the MCU is in standby the power consumption for my PCB should be around 15uA however it's currently hovering around 500uA which is way too much for my application. The strange thing is I can fix the fix by touching a UART pin on the MCU or by pinching the ribbon cable going from the MCU to the NFC PCB. I don't have to make an electrical connection to fix it. Just the presence of my hand or my ESD mat is enough.

Now, I hear you say I need termination? I do have termination and I have modded the board by grounding every pin that isn't used on the microcontroller. All other signals are terminated to GND or 3V3 with a pull up resistor as necessary. There isn't any clocks, data being sent or any signal changing state at all. The MCU pins are put into analogue mode (lowest power) when slept, not that is matters as the memory is not retained in the mode I'm using. If I remove the NFC PCB and the ribbon, the power consumption goes up to 2mA.

Any pointers or previous experience would be amazing. This worries me about EMC testing, too.

[Andy Watson]

Have you got some floating digital inputs? And/or digital inputs that have an indeterminate analogue voltage on them?