Is it possible to supply IO with 3.3V
Yes, it is possible to supply VDD_HV_IO with 3.3V.
However make sure you correctly understand the voltage detectors. Especially LVD400_IM which has to be disabled in order to prevent unintended resets from LVD at 4V.
One more thing I want to add.
Maybe it is not 100% clear from reference manual.
The device could work with IO at 3.3V, however during reset phase 0 to 3 the LVD400 (4V LVD) is active by default.
So if you want to run IO at 3.3V you have to start with 5V on VDD_HV_IO_MAIN and when device reach phase 3 o reset you can configure via DCF records LVD400 (turn it off). After this point you can switch to 3.3V IO supply.
Otherwise the LVD400 will keep device in reset as 3.3V will always be treated as undervoltage event by LVD400.
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