incap_freqmeasure.c - Uses IC1 to measure period, uses averaging mode.

//Measures square wave frequency using input capture and Timer2

#include "pic24_all.h"
#include <stdio.h>


uint8_t getPeriodAdjust (uint8_t ICMbits) {
  if (ICMbits == IC_EVERY_16_RISE_EDGE) return 16;
  else if (ICMbits == IC_EVERY_4_RISE_EDGE) return 4;
  else return 1;
}

volatile uint8_t u8_captureFlag = 0;
volatile uint16_t u16_lastCapture;
volatile uint16_t u16_thisCapture;
volatile uint32_t u32_period;

void _ISRFAST _IC1Interrupt() {
  _IC1IF = 0;
  u16_thisCapture = IC1BUF;  //always read the buffer to prevent overflow
  if (u8_captureFlag == 0) {
    u32_period = (uint32_t) computeDeltaTicks(u16_lastCapture,u16_thisCapture,PR2);
    u32_period = ticksToNs (u32_period, getTimerPrescale(T2CONbits));
    //adjust period if necessary
#if (defined(__dsPIC33E__) || defined(__PIC24E__))
    u32_period = u32_period/getPeriodAdjust(IC1CON1bits.ICM);
#else
    u32_period = u32_period/getPeriodAdjust(IC1CONbits.ICM);
#endif
    u8_captureFlag = 1;
  }
  u16_lastCapture = u16_thisCapture;
}

void configInputCapture1(void) {
  CONFIG_RB13_AS_DIG_INPUT();
  CONFIG_IC1_TO_RP(RB13_RP);     //map IC1 to RP13/RB13
#ifdef IC1CON1
  IC1CON1 = IC_TIMER2_SRC |     //Timer2 source
            IC_INT_1CAPTURE |   //Interrupt every capture
            IC_EVERY_16_RISE_EDGE;   //capture every 16th edge

cascade off, sync mode, sync to timer 2

  IC1CON2 = IC_IC32_OFF| IC_SYNC_MODE | IC_SYNCSEL_TIMER2;
#endif
#ifdef IC1CON
  IC1CON = IC_TIMER2_SRC |     //Timer2 source
           IC_INT_1CAPTURE |        //Interrupt every capture
           IC_EVERY_16_RISE_EDGE;   //capture every 16th edge
#endif
  _IC1IF = 0;
  _IC1IP = 1;   //pick a priority
  _IC1IE = 1;   //enable
}

void  configTimer2(void) {
  T2CON = T2_OFF | T2_IDLE_CON | T2_GATE_OFF
          | T2_32BIT_MODE_OFF
          | T2_SOURCE_INT
          | T2_PS_1_64 ;  //1 tick = 1.6 us at FCY=40 MHz
  PR2 = 0xFFFF;                    //maximum period
  TMR2  = 0;                       //clear timer2 value
  _T2IF = 0;                       //clear interrupt flag
  T2CONbits.TON = 1;               //turn on the timer
}

int main (void) {
  uint32_t u32_maxPeriodNs;
  float f_freqkhz;
  configBasic(HELLO_MSG);
  configTimer2();
  configInputCapture1();
#ifdef IC1CON1
  u32_maxPeriodNs = ticksToNs (65536, getTimerPrescale(T2CONbits))/getPeriodAdjust(IC1CON1bits.ICM);
#else
  u32_maxPeriodNs = ticksToNs (65536, getTimerPrescale(T2CONbits))/getPeriodAdjust(IC1CONbits.ICM);
#endif
  printf("Maximum period is %ld ns\n",u32_maxPeriodNs);
  while (1) {
    outString("Capturing\n");
    while (!u8_captureFlag) doHeartbeat();
    f_freqkhz = (1.0/(((double)u32_period) *1.0e-9))/1000.0;
    printf("Period: %ld ns, Freq: %6.2f kHz\n",u32_period,(double)f_freqkhz);
    u8_captureFlag = 0;
    DELAY_MS(500); //do not flood uart
  }
}