Hi Viktor
for unused pins connection one can look at
Table 1. MMPF0200 pin connection guidelines AN4751 Schematic guidelines for the MMPF0200
https://www.nxp.com/docs/en/application-note/AN4751.pdf
yes one can use external (not pmic) power for processor, provided that
power-up sequence is followed (it is described in i.MX6SX Datasheet).
https://www.nxp.com/docs/en/data-sheet/IMX6SXCEC.pdf
Best regards
igor
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