请教一个定时器捕捉模式的问题
//***********************************************************************//MSP-FET430P140 Demo - Timer_A0, Capture of ACLK/8 using SMCLK
//
//Description: Input capture of ACLK/8 on P1.1(TA0)
//Run to breakpoint at the _NOP() instruction to see 16 capture
//values and the differences.
//ACLK = 32768Hz, MCLK = SMCLK = default ~800kHz
////* An external watch crystal on XIN XOUT is required for ACLK *//
//
// MSP430F149
// -----------------
// /|\| XIN|-
// | | | 32kHz
// --|RST XOUT|-
// | |
// | P2.0/ACLK|---+
// | | |
// | P1.1/TA0|<--+
// | |
// | P1.0|--->LED
//
//H. Grewal
//Texas Instruments Inc.
//Feb 2005
//Built with IAR Embedded Workbench Version: 3.21A
//******************************************************************************
#include<msp430x14x.h>
unsigned int new_cap=0;
unsigned int old_cap=0;
unsigned int cap_diff=0;
unsigned int diff_array; // RAM array for differences
unsigned int capture_array; // RAM array for captures
unsigned char index=0;
unsigned char count = 0;
void main(void)
{
volatile unsigned int i;
WDTCTL = WDTPW + WDTHOLD; // Stop watchdog timer
for (i=0; i<20000; i++) // Delay for crystal stabilization
{
}
P1DIR = 0x01; // Set P1.0 out,1.1 input dir
P1OUT &= ~0x01; // LED off
P1SEL = 0x02; // Set P1.1 to TA0
P2DIR = 0x01; // P2.0-ACLK
P2SEL |= 0x01;
BCSCTL1 |= DIVA_3; // ACLK/8
CCTL0 = CM_1 + SCS + CCIS_0 + CAP + CCIE; // Rising edge + CCI0A (P1.1)
// + Capture Mode + Interrupt
TACTL = TASSEL_2 + MC_2; // SMCLK + Continuous Mode
_BIS_SR(LPM0_bits + GIE); // LPM0 + Enable global ints
}
#pragma vector=TIMERA0_VECTOR
__interrupt void TimerA0(void)
{
new_cap = TACCR0;
cap_diff = new_cap - old_cap;
diff_array = cap_diff; // record difference to RAM array
capture_array = new_cap;
if (index == 16)
{
index = 0;
P1OUT ^= 0x01; // Toggle P1.0 using exclusive-OR
}
old_cap = new_cap; // store this capture value
count ++;
if (count == 32)
{
count = 0;
_NOP(); // SET BREAKPOINT HERE
}
}
********************************************************************************************
今天做了这个捕捉模式的程序，使用BCSCTL1 |= DIVA_3;时，计算出来的频率是对的，是4096Hz。但我改成BCSCTL1 |= DIVA_0，就不对了，后来我打开高频晶振，主时钟MCLK设置成8MHz或者4MHz时，计算出的频率是对的，但设置成2Mhz或1Mhz时还是不对。这是为什么呢，定时器用的是子时钟SMCLK，为什么非要开主时钟呢？
请高手帮帮我，解释一下
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