5V-tolerant input pulls 5V down to 4V

[Diez.R.]

Hi all:

I am using an STM32407 running at 3.3V, and I have 2 inputs connected to 5V signals.

One of the signals comes from an outside sensor (a data line), and the other one is connected to a 5V optocoupler populated on the same board.

I have measured the voltage at the inputs, and it is 4 V. In both cases, it looks like the (stable) 5 V signal is pulled down to 4 V as soon as it is connected to the STM32 input.

I do not know much about hardware, and I am not sure how to interpret this. Is that normal?

I have read somewhere that: "STM32 devices embed five-volt tolerant GPIOs. It means that the I/O pins can accept such voltages without causing leakage current and damages on the GPIOs."

I would have thought that there would be (almost) no current flowing through an input, so the voltage should stay at 5V. Or is that a misconception on my part?

Or maybe it is impossible, and I am not measuring or connecting correctly.

Thanks in advance,

 rdiez

[Tomas DRESLER]

Hi Diez,

please see the datasheet of STM32F407, look up the pins used as 5V input in your design in the table 7, and check if they are marked in I/O structure column as FT or TTa.

TTa pins are 3.3V compatible only. Beware note (5) at some FT pins and make sure the signals are not used in analog mode, but as digital input.

Tomas

[Diez.R.]

They are marked as FT, and they a configured as digital inputs.

[Tomas DRESLER]

What are the pin markings and precise sales type, please?

[Diez.R.]

One of them is PE1. There is no note (5) next to it.

[Tomas DRESLER]

And rest of the answer, please?

[Diez.R.]

I am not sure what you mean by "pin markings". The chip model is STM32F407VGT6.

[Tomas DRESLER]

Haven't you turned on pull up resistor or output mode accidentally on this pin? Or, don't you inject some high frequency or voltage on any other FT pin?

[Diez.R.]

Thanks for your hints. I'll take a look when I'm back in the office.