The ADC resets to the ADC off state. When powering up the ADC, use the following sequence:
1. If external reference is desired, enable this mode using bits 15-14 in the ADCREFSEL Register. This
mode must be enabled before band gap is powered.
2. Power up the reference, bandgap, and analog circuits together by setting bits 7-5 (ADCBGRFDN[1:0],
ADCPWDN) in the ADCTRL3 register.
3. Before performing the first conversion, a delay of 5 ms is required.
跟据手册所说，若是利用外部基准，则对ADCREFSEL寄存器进行配置进行配置，但是必须在带隙上电之前配置。所以我在
InitAdc()程序里进行配置，配置如下：
AdcRegs.ADCREFSEL.bit.REF_SEL = 1; //选择外部参考电压为2.048v
AdcRegs.ADCTRL3.all = 0x00E0; // Power up bandgap/reference/ADC circuits
DELAY_US(ADC_usDELAY); // Delay before converting ADC channels
请高手指教，为什么结果跟内部3v参考时，结果一样？
