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28335数据手册的6.8 Power Sequencing对上电顺序及其原因作了说明，对于先后上电的时间间隔没有要求。
No requirements are placed on the power up/down sequence of the various power pins to ensure the correct reset state for all the modules. However, if the 3.3-V transistors in the level shifting output buffers of the I/O pins are powered prior to the 1.9-V transistors, it is possible for the output buffers to turn on, causing a glitch to occur on the pin during power up. To avoid this behavior, power the VDD pins prior to or simultaneously with the VDDIO pins, ensuring that the VDD pins have reached 0.7 V before the VDDIO pins reach 0.7 V.
