G2553 Set DCO问题

P1OUT = 0x00; // Clear P1 output latches
P1SEL = 0x10; // P1.4 SMCLK output
P1DIR = 0x11; // P1.0,4 output

void Set_DCO(unsigned int Delta) // Set DCO to selected frequency
{
unsigned int Compare, Oldcapture = 0;

BCSCTL1 |= DIVA_3; // ACLK = LFXT1CLK/8
TACCTL0 = CM_1 + CCIS_1 + CAP; // CAP, ACLK
TACTL = TASSEL_2 + MC_2 + TACLR; // SMCLK, cont-mode, clear

while (1)
{
while (!(CCIFG & TACCTL0)); // Wait until capture occured
TACCTL0 &= ~CCIFG; // Capture occured, clear flag
Compare = TACCR0; // Get current captured SMCLK
Compare = Compare - Oldcapture; // SMCLK difference
Oldcapture = TACCR0; // Save current captured SMCLK

if (Delta == Compare)
break; // If equal, leave "while(1)"
else if (Delta < Compare)
{
DCOCTL--; // DCO is too fast, slow it down
if (DCOCTL == 0xFF) // Did DCO roll under?
if (BCSCTL1 & 0x0f)
BCSCTL1--; // Select lower RSEL
}
else
{
DCOCTL++; // DCO is too slow, speed it up
if (DCOCTL == 0x00) // Did DCO roll over?
if ((BCSCTL1 & 0x0f) != 0x0f)
BCSCTL1++; // Sel higher RSEL
}
}
TACCTL0 = 0; // Stop TACCR0
TACTL = 0; // Stop Timer_A
BCSCTL1 &= ~DIVA_3; // ACLK = LFXT1CLK
}

你好，

       在G2553的例程里面有set DCO的程序如上，有些没看懂。

1.SMCLK和ACLK都设置为在管脚输出了，那timer从哪里捕获的呢？CCI2B是timer的输入，又是比较器的输出，但是比较器好像没有设置输入啊？请问timer是如何捕获ACLK的呢？

2.这个程序DCOCTL--这句话，因为DCOCTL的低位都是MOD，高位才是DCO，所以一般都只能修改MOD而到不了修改DCO。这样是否合理呢？