DATASHEET上: 3.3 V DC-to-DC Power Supply: This is the power supply for the on-chip DC-to-DC converter only. If the DC-to-DC is not used, these pins must be left unconnected. 应该是供内核电压(1.8V)的吧,什么时候可以不用呢?掉电模式? 还要保持"must be left unconnected"状态,怎么实现? 请高手支招?