STM32L4P5 FT pins tolerant up to 5V even if supplied power to STM is 1.8V?

[triedgetech]

I have STM32L4P5CGU6 (48 pin).

Pins PB5, PB4, PB3, PA15 are used as SPI1 with another IC which has 3.3V levels.

As well as PB6, PB8, PB9 as custom input GPIOs with voltage levels of 3.3V.

VDD (pin #9, pin#24, #48) -> 1.8V

They all seem to work fine, however, I was wondering about long-term effects on such FT gpios, and to get a clarification on VDD_FT and how it works.

Reference manual RM0432 p. 334, fig. 26 shows 5V tolerant pins, there's a protection diode to VDD_FT.

AN4899, on p.9 says: "The voltage level called VDD_FT in some datasheets and reference manuals is inside the ESD protection block"

On p.17:

"STM32 devices embed five-volt tolerant GPIOs. These GPIOs are actually tolerant to

VDD + 3.6 V. It means that the I/O pins can accept such voltages without causing leakage

current and damages on the GPIOs."

Regardless of the supply voltage, VIN cannot exceed 5.5 V."

So FT Pins have no direct connection to STM's VDD, correct? But there's a protection diode from such a pin to VDD_FT? Where does VDD_FT connect to? How does schematic associated with it looks like?

[Uwe Bonnes]

Look at the datasheet. "Absolute maximum ratings" Input voltage on FT_*** pins may not be higher than 4 Volt above the minimum of the supply voltage. So if 1.8 Volt is the minimum, 5 Volt should not do harm to 5-Volt tolerant pins.

[triedgetech]

Thanks, but that's not what I quite asked. Yes, they're tolerant and supposedly there will be no harm.

How does it work though? I asked above about VDD_FT, what is it connected to in my case?

[Uwe Bonnes]

VDD_FT is only internal, not connected to any accessible pin

[triedgetech]

How does it look like though? How does it manage to be "tolerant" to "VDD + 3.6 V"?

[AScha.3]

usually on cmos circuits you have protection diodes to vss and vcc on the chip, to keep the mosfet inputs alive . so this is the limit for input voltage.

now if you want some "tolerance" to higher input voltage, just leave away the protection diode to vcc (+) .

and to get some minimal protection against ...maybe static voltage, replace the diode to vss with a zener-diode ; so you limit the input to (vss - (diode forward)0.5V) and (vss + (diode reverse break) 8V ) .

now you got a "5V tolerant" input. not so difficult... :)