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Boot process:
The normal boot is to press boot key which is that the PWRKEY is being dragged down.
Once the boot key is pressed, all LDO are open except VSIM. After VCORE opening RESET
timer and timer out, RESET is being pitched up to start the digital baseband chip, that is,
UMT6253 starts to run and roll polling its ROWX pin, pitch up its PWRONIN pin, then you can
release the boot key. This is the initial boot process.
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