我在运行TI的例程Example_2803xAdcSoc，试图使用外部中断来触发AD采样，但是无论我怎么配置触发，都是只能在外部上升沿的时候才能启动AD采样，请问这是DSP本身就是这么设置，还是我忽略了什么？
EALLOW;
AdcRegs.ADCCTL1.bit.INTPULSEPOS = 1;
AdcRegs.INTSEL1N2.bit.INT1E = 1;
AdcRegs.INTSEL1N2.bit.INT1CONT = 0;
AdcRegs.INTSEL1N2.bit.INT1SEL = 2;
AdcRegs.ADCSOC0CTL.bit.CHSEL = 4;
AdcRegs.ADCSOC1CTL.bit.CHSEL = 4;
AdcRegs.ADCSOC2CTL.bit.CHSEL = 4;
AdcRegs.ADCSOC0CTL.bit.TRIGSEL = 5;
AdcRegs.ADCSOC1CTL.bit.TRIGSEL = 5;
AdcRegs.ADCSOC2CTL.bit.TRIGSEL = 4;
AdcRegs.ADCSOC0CTL.bit.ACQPS = 9;
AdcRegs.ADCSOC1CTL.bit.ACQPS = 9;
AdcRegs.ADCSOC2CTL.bit.ACQPS = 9;
EDIS;
EALLOW;
GpioCtrlRegs.GPAMUX2.bit.GPIO21 = 0; // GPIO
GpioCtrlRegs.GPADIR.bit.GPIO21 = 0; // input
EDIS;
EALLOW;
GpioIntRegs.GPIOXINT2SEL.bit.GPIOSEL = 21; // XINT2 is GPIO21
EDIS;
XIntruptRegs.XINT2CR.bit.POLARITY = 3;
