RAK3172 Unused pins

[IGtti (Iaran Gadotti)]

Dear Coleagues,

I started my project and I miss the details about a typical circuit for the RAK3172, with a special attention for the unused pins. What should be done with each pin, leave it floating or pulling up or down?
Additionally, what are the pins state during Boot, Firmware update and running mode?

Thank you.
Best regards

[carlrowan]

Welcome to RAK forum @IGtti ,

There is a good application note from STmicroelectronics about unused GPIO pins. The doc is actually not just about unused pins but generally about GPIO hardware design for STM32 chips.

I have to check the pin state during Boot and Firmware update. I’ll get back to you.

[carlrowan]

Hi @IGtti ,

Here’s the table of pin conditions in different states.

RAK3172 Pins State.xlsx

[IGtti (Iaran Gadotti)]

Hi @carlrowan , very good information. I confirmed in the STM32 datasheet and the “Chip Default” configuration is Analog input for all indicated pins.
That means all these pins are input and normally they should not be left floating.
It would be nice if your firmware configures the pins as output low, giving a safe state and no need for external pull-down resistors.

Have you noticed any issues when leaving these pins floating?

[Schulz (Ernst)]

I will be not shure that your proposal is the best solution to keep power consumption as low as possible!
A lot of IoT-Application have the goal to have long battery live time!

E_T

[IGtti (Iaran Gadotti)]

Follow the note from STM32 datasheet:

Caution: Any floating input pin can also settle to an intermediate voltage level or switch inadvertently,
as a result of external electromagnetic noise. To avoid current consumption related to
floating pins, they must either be configured in analog mode, or forced internally to a definite
digital value. This can be done either by using pull-up/down resistors or by configuring the
pins in output mode.

[carlrowan]

Hi @IGtti ,

I’ve worked in different industries before and I can’t recall critical issues on unused pins left floating on MCUs. Since chip default is analog input, then it seems all will be fine. But still, it depends on your application. I am not sure on what lab/QA test you will subject your device (also what qualifications) but issues sometimes our found once you subject it to various test: thermal cycle, overvoltage, HALT, EMC, etc.