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Positive injection on FT pin of unpowered device

n.serina
Associate III

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

4 REPLIES 4

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)

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Uwe Bonnes
Principal III

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

Andreas Bolsch
Lead II

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
Associate

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!