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No requirements are placed on the power up/down sequence of the various power pins to ensure the correct reset state for all the modules. However, if the 3.3-V transistors in the level shifting output buffers of the I/O pins are powered prior to the 1.8-V transistors, it is possible for the output buffers to turn on, causing a glitch to occur on the pin during power up. To avoid this behavior, power the VDD (core voltage) pins prior to or simultaneously with the VDDIO (input/output voltage) pins, ensuring that the VDD pins have reached 0.7 V before the VDDIO pins reach 0.7 V.
Additionally it is recommended that no voltage larger than a diode drop (0.7 V) should be applied to any pin prior to powering up the device. Voltages applied to pins on an unpowered device can bias internal p-n junctions in unintended ways and produce unpredictable results.
