capturing the signal from another board by using Timer....

hi ....

I am trying to configure the MSP430G2553 Microcontroller to 250 KHz by using the following DCO configuration,

  DCOCTL = 0xC8;

  BCSCTL1 = 0x02;

  BCSCTL2 = 0;

  BCSCTL3 |= LFXT1S_2;

But the frequency generation of microcontroller ranges is around 255 KHz to 272 KHz (it will keep on  varying in each and every Microcontroller (MSP430G2553) around 5 KHz to 20kHz its not generating exact 250Khz (+1kHz/-2Khz).

My First question:

                                Is there any way to generate exact 250 KHz (with minimum tolerances) frequency?

To overcome the above issue I am trying to change the DCO setting to generate the exact 250 KHz (with minimum tolerances) frequency based on the captured 1Khz PWM signal (50% duty cycle).

How?

Assume my controller(Launch pad 1) is working at 250 KHz (with minimum tolerances) frequency, in this case if we are capture the  1Khz PWM(Launch pad 2 generating  PWM)  signal(50%  duty cycle) by using timer in capture mode the timer capture count will be,

Launch pad 1 is running 250 KHz so 1/250,000 = 4 Microsecond.

Launch pad 2 generating the 1 KHz PWM signal (50% duty cycle) ,1/1000 = 1000 Microsecond.

My captured count should be 1000/4 =250 timer counts.

Based on this calculation if  I capture the signal on every rising edge, my capture/compare register values should  be 250 than I will assume that my controller is working at 250 KHz (with minimum tolerances) frequency until I keep on  changing  my DCO settings values.

My Second question:

I cannot able to capture the signal at constant counts (250 counts(with some tolerances ) it continually varying. Can you please tell me what mistake I am doing?

We are using internal oscillator and we are not using capture/compare ISR.

Here is the my program

volatile UINT16 u16Time_Stamp = 0;

UINT16 u16Dc0Cotrlb = (UINT16)(DCO_CLK_SETTINGS_1);

volatile UINT32  TempCount;

     unsigned int Dc0Cotrlb = 0xC8;

void main()

{

      WDTCTL = WDTPW + WDTHOLD;

  DCOCTL = 0xC8;

  BCSCTL1 = 0x02;

  BCSCTL2 = 0;

  BCSCTL3 |= LFXT1S_2;

    //Configure Port Pins

    P1DIR &= ~BIT1;                        

    P1SEL |= BIT1;                          

    P1SEL2 &= ~BIT1;                          

    TA0CCTL0 = CM_1 + CCIS_0 + SCS + CAP;

    TA0CTL = TASSEL_2 + MC_2 + TACLR;

    COMH_vUARTInit(9600);

      while(1)

      {

            while(!(TA0CCTL0 & CCIFG))

            {

            u16Time_Stamp = TA0CCR0;

            if(TA0CCTL0 & COV)

            {

                  TA0CCTL0 &= ~COV;

            }

            else if((u16Time_Stamp > u16Exp_CountLow) && (u16Time_Stamp < u16Exp_CountHigh))

                  {

                        _nop();

                        _nop();

                        _nop();

                        _nop();

                  }

                  else if(u16Time_Stamp < u16Exp_CountLow)

                  {

                        DCOCTL  = Dc0Cotrlb++;

                  }

                  else if(u16Time_Stamp > u16Exp_CountLow)

                  {

                        DCOCTL  = Dc0Cotrlb--;

                  }else if(u16Time_Stamp == 250)

                  {

                        PrintF(u16Time_Stamp);

                  } 

                  TA0CCR0 = 0;

                  TA0R = 0;

                  TA0CCTL0 &= ~(CCIFG + COV + CCI);

            }

      }

}