For the more literal people: in some weird scenarios you can actually power an MCU via its actual GPIO pins! See [0] and [1] for example. The trick here is tha

For the more literal people: in some weird scenarios you can actually power an MCU via its actual GPIO pins! See [0] and [1] for example. The trick here is that most MCUs have integrated ESD protection diodes on all IO pins: there's basically a diode from the pin to the VCC rail, and a diode from GND to the pin. During normal operations this limits the voltage the gpio circuitry is exposed to within the 0V - Vf to VCC + Vf range, where Vf is the diode's forward voltage - usually about 0.7V or so. But if the VCC rail is floating, this means a high GPIO pin will now feed VCC - Vf to it, and similarly a low GPIO pin will drain a floating GND rail to GND + Vf. Got 5V external IO signals? Your "unpowered" MCU is now getting 3.6V, which is plenty to run it! The pins aren't going to be very happy if you try to draw nontrivial current through it, but it will work. [0]: https://electronics.stackexchange.com/questions/300866/mcu-getting-powered-up-through-gpios https://electronics.stackexchange.com/questions/300866/mcu-g... [1]: https://community.st.com/stm32-mcus-products-25/what-might-happen-if-gpio-pin-is-drived-high-level-before-stm32f407-is-powered-on-19691 https://community.st.com/stm32-mcus-products-25/what-might-h...