nRF52832 GPIO state at no input power (Vdd)

[Brosnan]

Hello.

I wanted to know what the gpio state when no power is connected to the device.

I have connected a RTC battery to one of the ADC channels via a series 100K resistor, for monitoring the cell voltage in operation. But I noticed when the device is kept off for a long duration the cell voltage drops significantly. I think the gpio is sinking current in power off state. Is this possible?

Thanks

Brosnan

[ketiljo]

There are ESD protection diodes to VDD that will start to conduct and drain the battery trough the resistor. So the GPIO should be considered pulled low when the device is not powered.

[Brosnan]

Thanks for your response Ketiljo,

What should I do to prevent this from happening. I want to monitor the battery voltage when the board is powered. And not drain when the board is unpowered.

I have increased the resistor value to 10M ohms, but was thinking if this will degrade the accuracy of the measurement.

[ketiljo]

There must be a considerable current draw to drain the battery this fast, and if the resistor is indeed 10M ohm, the current must go somewhere else. 

[Brosnan]

Is there a possibility that the cell discharge characteristics are into play here? Like the battery will discharge from 3.28V to 3.0V faster and then stay at 3.0V for most of it capacity?

[ketiljo]

Does the battery discharge at all after a while?

[Brosnan]

So what is noticed now is the battery discharges quickly to 3.0V from 3.2V and then stays at that voltage. Observing this on all units (after 1+month) with the 10M ohms resistor in series.

[ketiljo]

Sounds reasonable. If the nominal voltage of the battery is 3.0 V, is should quickly go down to that level and stay there until the battery is depleted.