MSP430F5438A: 定时器的计数时钟选用外部时钟的问题

///
#include <msp430.h>
#define HZ  1000000UL
int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;   // stop watchdog timer
    P1OUT  &= ~BIT0;            // Launchpad LED, initially off
    P1DIR  |= BIT0;
    //  P1.2 is an ordinary GPIO, patched to P1.6
    P1OUT  &= ~BIT2;            // P1.2 output, initially low
    P1DIR  |= BIT2;             //  (patched to P1.6)
    //  P1.6 is TA1CLK, fed from P1.2
    P1SEL  |= BIT6;             // P1.6 as TA1CLK
    TA1CTL = TASSEL_0 | ID_0 | MC_2 | TACLR; // TA1CLK/1, Continuous [,clear]
    __delay_cycles(HZ/2);       // Dawdle for a half-second
    P1OUT  |= BIT2;             // Rising edge to TA1CLK
    __delay_cycles(10);         // Allow for wire delay
    if (TA1R != 0u)             // If it ticked,
        P1OUT |= BIT0;          //  light LED
    while (1)
    {
        LPM0;
    }
    /*NOTREACHED*/
    return 0;
}