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So I think I found a replacement for my U8100. I thought don’t understand if/how the absence of 1V8_S2 and 1V2_S2 could be caused by a faulty U8100 and if not so what could be the problem. Also as you pointed out U8100 has some platform-dependent pins, what exactly does that mean? I found a A2442 donor board which is the same model, but not sure if it’s a 8/14 or 10/16 cores, could that be a problem?
If is same model, is the same platfom, so you can change it.
U8100 is master PMU and must generate 3V3_S2 by itself, without an external control.
It only needs 3V8_AON to generate 1V2/1V8__AON and 3V3_S2.
Also generates 1V5_AON for its core and 1V8 for SRAM.
If 1V2/1V8_AON are both present, PP3V8_AON_MPMU_ISNS is surely present.
Then 3V3_S2 must be there too; unless is shorted to ground, or has lower resistance than normal.
Possibly bad solders.
In fact, all LDOs derived from PP3V8_AON_MPMU_ISNS, should be there.