Positive injection on FT pin of unpowered device

[n.serina]

Hello,

Thanks for the great support .

I have a doubt regarding FT pin of the STM32F407x microcontroller .

Will the microcontroller gets damaged if 5V is applied when it is unpowered ?

in the datasheet Table 12. Current characteristics of section 5.2 . Its written like below

"Positive injection is not possible on these I/Os. A negative injection is induced by VIN<VSS. IINJ(PIN) must

never be exceeded"

What i understood is that positive injection is not possible on FT pin until VDD+4V is applied . so , if VDD = 0V and i apply 5V to FT , there must be injection current flowing and it may damage the device . I didnt find any current limits though (which may help me to put series resistance to limit the current )

Can anybody please help me with this ?

Also, in the reference manual

Figure 25. Basic structure of a five-volt tolerant I/O port bit

there is a term called VDD_FT . can you please explain whats the value of this signal ? is VDD_FT > VDD ? and how VDD_FT is derived ?

Thank you

[Tesla DeLorean]

VDD_FT - Given hows this stuff works, it is most likely a voltage derived from the highest observed on all FT pins connected to the same rail/ring.

ie VDD_FT(max)

[Uwe Bonnes]

VDD +4 V is the absolute maximun for a FT pin.. See 5.2 Absolute maximum ratings in the datasheet.

[Andreas Bolsch]

AN4899 contains some more information, but still not exhaustive. If in doubt, use series resistor and Schottky diode to VDD or zener to VSS for additional protection.

[FCorm.1]

I have the exact same question as you @n.serina​  "Will the microcontroller get damaged if 5V is applied when it is unpowered?", specifically on FT pins.

Did you ever find out an answer?

Cheers!