When no power is supplied to the device, that is, the DC/DC Converter output and VDDIO are OFF (at ground potential), a voltage cannot be applied to the device pins because each pin contains an ESD diode from the pin to supply. This diode turns on when a voltage higher than one diode-drop is supplied to the pin.
If a voltage must be applied to the signal pads while the chip is in a low-power state, the VDDIO supply must be on so the SLEEP or Power_Down state can be used.
Similarly, to prevent the pin-to-ground diode from turning on, do not apply a voltage that is more than one diode-drop below the ground to any pin.