9/12

ACCURATE TIMEBASE FOR LOW-COST ST7 APPLICATIONS WITH INTERNAL RC...

/******************************************************************************

Enable MCC to watch the real internal RC oscillator frequency

******************************************************************************/

MCC_Init(MCC_DEFAULT);

MCC_Init(MCO_ENABLE);

/******************************************************************************

TIMER A Library called to capture the rising edge at the pin PF4

******************************************************************************/

TIMERA_Init(TIMER_FCPU_2);

/*Timer clock is fcpu/2=1 MHz at fosc=4MHz*/

TIMERA_ICAP_Mode(TIMER_ICAP_1,TIMER_EDGE_0);

TIMERA_IT_Enable(TIMER_ICAP_IT_ENABLE);

EnableInterrupts;

/******************************************************************************

Loop to capture 8 samples and to do averaging of it with using glitch

filter algorithm to protect the wrong signal capture

******************************************************************************/

for (i=0;i<8;i++)

/*averaging done for 8 samples*/

{

while (cap != 2);

/*wait loop for two captures*/

/*management the possibility of overflow of counter*/

if (capture2 < capture1)

{

capture2 = ((0xffff - capture1) + capture2);

}

else

{

capture2 = (capture2 - capture1);

}

capture2 = capture2 >> 3;

/**********************Glitch filtering Algorithm **********************/

if (i==0)

{

capture_range = capture2;

}

fraction = fraction + capture2;

if((capture2>(1.1*capture_range))||(capture2<(0.9 * capture_range)))

{

fraction =0; i=0;

}

capture1=capture2=cap=0;

}

/*fraction = Measured RC frequency/assumed internal RC Frequency (here 4 MHz)*/

fraction = ((float)fraction/0x4e20);

Nop;

/******************************************************************************

TIMER PARAMETER CALIBRATION

******************************************************************************/

#ifdef TIMERB_PWM_CAL

/********** Timer PWM generation for 5 KHz frequency*****************/

/******************************************************************************

100 value to get 100 microcesconds at 2MHz fcpu, equation is used as

per ST72F521 datasheet

******************************************************************************/

PWM_CNT = (int)(200 * fraction); /*numerator calculation*/