MSP430F5系列时钟问题

在MSP430F5的手册中有
16-Bit RISC Architecture
– Extended Memory Operations
– Up to 18-MHz System Clock
MSP430F543x, MSP430F541x
– Up to 25-MHz System Clock
MSP430F543xA, MSP430F541xA

Unified Clock System
– FLL Control Loop for Frequency
Stabilization
– Low-Power/Low-Frequency Internal Clock
Source (VLO)
– Low-Frequency Trimmed Internal Reference
Source (REFO)
– 32-kHz Crystals
– High-Frequency Crystals up to 32 MHz

在MSP430F5的user's guide中有
XT1CLK: Low-frequency/high-frequency oscillator that can be used either with low-frequency 32768-Hz watch crystals, standard crystals, sonators, or external clock sources in the 4-MHz to 32-MHz range.

XT2CLK: Optional high-frequency oscillator that can be used with standard crystals, resonators, or external clock sources in the 4-MHz to 40-MHz range.

请问怎样理解上面提到的几个时钟或晶振？如果将系统主时钟配置为XT2CLK且外接的40MHz的晶振，那系统的时钟还是25MHz 吗？

430一般有三个时钟源，DCO，XT1，XT2，F5好像没DCO，它有三个时钟，这三个时钟的信号来源即是这三个时钟源，这三个时钟源通个寄存器设置会倍频（pll）混频啊就产生这三个时钟，所以外接40M时还要看你选的系统时钟是哪个，并且看寄存器是怎样设置的

将板子的外围芯片都卸下后，再次测试功耗，在时钟为8m时进入lpm3电流大概130μa左右， 默认dco时钟时进入lpm3则只有3μa左右，板子上电源芯片耗电约2μa左右，看来8m时钟时lpm3还是比较耗电，只是不知道原因为何！