For community awareness:
Unless otherwise stated, STM32 GPIOs do not have partial-power-down / Ioff / back-drive protection.
Example: If VDD=0V (or VDD unpowered) and a GPIO is driven with 3.3V, then excessive current may flow into the GPIO.
This includes TT and FT GPIOs.
The behavior of a back-powered GPIO is nondeterministic.
Some back-powered GPIOs behave High-Z. Some draw excessive current. Some pull down after ~100ms (GPIO becomes an output). The behavior varies with temperature. The behavior varies chip-to-chip even with same part marking / date code. Measuring different TT and FT GPIOs on a standalone IC with multimeter in diode mode has inconsistent results: sometimes measuring OL, 0.8V, or 1.5V and the measurement changes either due to measuring other pins or due to temperature.
ST GPIO documentation may cause confusion regarding FT GPIO back-drive behavior. There are also forum posts which suggest FT GPIOs might have back-drive protection.
However, FT pins do not have back-drive protection.
Example documentation which may cause confusion:
Datasheet:
- "General Operating Conditions: VIN (All I/O except TT_xx and FT_c): MIN(MIN(VDD,VDDA)+3.6 V,5.5 V)"
- "Positive injection (when VIN > VDD) is not possible on these I/Os and does not occur for input voltages lower than the specified maximum value"
AN4899:
- "When VDD = 0 V, the input voltage on the GPIO cannot exceed 3.6 V. In case of a multi-supplied and multiplexed GPIO (VDD, VDDUSB, VLCD, VDDA), the GPIO is tolerant to 3.6 V augmented of the minimum supply voltage among VDD, VDDUSB, VLCD, and VDDA.”
- “If VDD is not present, for example grounded, the maximum voltage must not exceed 3.6 V (the exact limit value is provided in the STM32 datasheet).”
- “A TT or FT GPIO pin has no internal protection diode connected to supply (VDD). There is no physical limitation against over-voltage.”
However, ST Support clarified that there is no back-drive protection.
This response relates to a specific part number, but it seems applicable to all STM32 MCUs unless otherwise stated:
"
For you questions, it is stated in the [reference manual]:
"Voltage protection (for example five-volt tolerance for inputs) is guaranteed only if the STM32 is supplied."
This implies only if there is a valid supply voltage placed on VDD. The operation of the unpowered micro is nondeterministic.
Same for other statements in the datasheet you mentioned, they all have a prerequisite:
...
"Unless otherwise specified, typical data are based on TA = 25 °C, VDD = VDDA = 3 V. They are given only as design guidelines and are not tested."
So, in summary and in short, there's no protection when VDD=0V.
I hope this clarifies your question.
Best regards,
ST MCU Support Team
"
