Posted on September 19, 2013 at 10:04The STM32F20xxx Datasheet Tables 9 and 10 give the information you need. The Absolute Maximum positive voltage at a Five volt Tolerant (FT) pin is Vdd + 4V. For other pins it's +4V. If Vdd is at zero then it's +4V in both cases!
You must not allow more than plus or minus 5mA to be injected into any pin. So for ''ordinary'' pins a 3k3 series resistor would limit the inject current from a +3.3V signal to less than 1mA. (Since the input takes next to no current you could use 10k, 47k 100k etc.) For Five volt Tolerant (FT) pins, the Datasheet tells you (Table 10 note 3) that ''3. 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 9 for the values of the maximum allowed input voltage.''
So if the pin you're using is an FT one, it should tolerate a 3.3V signal even with Vdd at zero. If not then use a series resistor to limit injected current.
Alternatively, use your signal to drive a discrete N channel enhancement mode mosfet (gate) and connect the source of the mosfet to Vss and the drain to the microcontroller pin (with pull-up resistor to Vdd). The signal will be inverted but it won't drive any current into the mosfet gate or microcontroller pin.
