Uart使用Receive-Start Edge检测模式的程序疑问

#include  <msp430x14x.h>

void Set_DCO (void);

void main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  P3SEL |= 0x30;                            // P3.4,5 = USART0 TXD/RXD
  BCSCTL1 |= DIVA_3;                        // ACLK= LFXT1CLK/8
  Set_DCO();                                // Set DCO

  ME1 |= UTXE0 + URXE0;                     // Enabled USART0 TXD/RXD
  UCTL0 |= CHAR;                            // 8-bit character, SWRST = 1
  UTCTL0 |= SSEL1 + SSEL0 + URXSE;          // UCLK = SMCLK, start edge detect
  UBR00 = 0xD0;                             // 9600 from 2Mhz
  UBR10 = 0x00;
  UMCTL0 = 0x00;                            // No modulation
  UCTL0 &= ~SWRST;                          // Initialize USART state machine
  IE1 |= URXIE0;                            // Enable USART0 RX interrupt

  for (;;)
  {
    while (!(UTCTL0 & TXEPT));              // Confirm no TXing before --> LPM3
    _DINT();                                // Disable interrupts for flag test
    _NOP();
    if (!(UTCTL0 & SSEL0))
       _BIS_SR(LPM0_bits + GIE);            // RX'ing char, enter LPM0, int's active
    else
    _BIS_SR(LPM3_bits + GIE);               // Enter LPM3, int's active
  }
}

#pragma vector=UART0RX_VECTOR
__interrupt void usart0_rx (void)
{
  if ((IFG1 & URXIFG0))                     // Test URXIFG0
  {
    while (!(IFG1 & UTXIFG0));              // USART0 TX buffer ready?
    TXBUF0 = RXBUF0;                        // RXBUF0 to TXBUF0
    _BIC_SR_IRQ(LPM3_bits);                 // Exit LPM3 after reti
    UTCTL0 |= SSEL0;                        // SSEL0 = 1, no RX activity  为什么这里要重新选择时钟呢？
  }
  else                                      // Start edge
  {
    UTCTL0 &= ~URXSE;                       // Clear URXS signal
    UTCTL0 |= URXSE;                        // Re-enable edge detect
    _BIC_SR_IRQ(SCG1 + SCG0);               // DCO reamins on after reti
    UTCTL0 &= ~SSEL0;                       // SSEL0= 0, RX activity  为什么这里要重新选择时钟呢？

  }
}