I have a question regarding the operation of STM32F407 five volt tolerant I/O pins.
In our application we have a situation where the MCU might be powered down, but the peripherals connected to I2C bus should still remain functional. In a typical MCU environment the input protection diodes at the I/O pins would make this kind of operation impossible, but with STM32 devices there is a separate VDD_FT suppy feeding the protection diodes at the five volt tolerant I/O pins.
On page 77 of STM32F407 datasheet there is a following statement:
"Positive injection is not possible on these I/Os. A negative injection is induced by VIN<VSS. IINJ(PIN) must never be exceeded. Refer to Table 11 for the values of the maximum allowed input voltage."
Does this mean that we can feed the I/O pin with a pull-up resistor (connected to another +3.3V supply) in an I2C application even though the MCU is powered down without a risk that the internal protection diode clamps the input voltage? We must of course make sure that the injected current specification and the maximum input voltage specification (VDD+4V) are not exceeded.
Many thanks for the reply in advance!