incap_switch_pulse_measure.c - Uses 16-bit input capture to measure pulse width

Measures the pulse width of pushbutton switching using IC1 input capture and Timer2 Timer overflow tracking is used to measure long pulse widths. This example assumes a debounced switch. To configure this example to run with an external 8 MHz crystal for for a FCY=40MHz, define the C preprocessor macro: CLOCK_CONFIG=PRIPLL_8MHzCrystal_40MHzFCY and have an external crysal + 2 capacitors on the OSC1/OSC2 pins. Typical crystal accuracy for through hole is +/-20 pmm, so for a 100000 us pulse width measurement this is +/- 2 us.

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


volatile uint16_t u16_oflowCount = 0;

//Interrupt Service Routine for Timer2
void _ISRFAST _T2Interrupt (void) {
  u16_oflowCount++;
  _T2IF = 0;                 //clear the timer interrupt bit
}

typedef enum  {
  STATE_WAIT_FOR_FALL_EDGE = 0,
  STATE_WAIT_FOR_RISE_EDGE,
} ICSTATE;

ICSTATE e_isrICState = STATE_WAIT_FOR_FALL_EDGE;
volatile uint8_t u8_captureFlag = 0;
volatile uint16_t u16_lastCapture;
volatile uint16_t u16_thisCapture;
volatile uint32_t u32_pulseWidth;

void _ISRFAST _IC1Interrupt() {
  _IC1IF = 0;
  u16_thisCapture = IC1BUF;  //always read the buffer to prevent overflow
  switch (e_isrICState) {
    case STATE_WAIT_FOR_FALL_EDGE:
      if (u8_captureFlag == 0) {
        if (u16_thisCapture == 0 && _T2IF) u16_oflowCount = 0 -1; //simultaneous timer with capture
        else u16_oflowCount = 0;
        u16_lastCapture = u16_thisCapture;
        e_isrICState = STATE_WAIT_FOR_RISE_EDGE;
      }
      break;
    case STATE_WAIT_FOR_RISE_EDGE:
      if (u16_thisCapture == 0 && _T2IF) u16_oflowCount++; //simultaneous interrupt, increment oflow count
      u32_pulseWidth = computeDeltaTicksLong(u16_lastCapture,u16_thisCapture,PR2, u16_oflowCount); //get delta ticks
      u32_pulseWidth = ticksToUs(u32_pulseWidth,getTimerPrescale(T2CONbits)); //convert to microseconds
      u8_captureFlag = 1;
      e_isrICState = STATE_WAIT_FOR_FALL_EDGE;
      break;
    default:
      e_isrICState = STATE_WAIT_FOR_FALL_EDGE;
  }
}

/// Switch1 configuration
inline void CONFIG_SW1()  {
  CONFIG_RB13_AS_DIG_INPUT();     //use RB13 for switch input
  ENABLE_RB13_PULLUP();           //enable the pull-up
  DELAY_US(1);            //delay for pull-up
}

void configInputCapture1(void) {
  CONFIG_IC1_TO_RP(RB13_RP);      //map IC1 to RB13
#ifdef IC1CON                  //older familes
  IC1CON = IC_TIMER2_SRC |     //Timer2 source
           IC_INT_1CAPTURE |   //Interrupt every capture
           IC_EVERY_EDGE;      //Capture every edge
#endif
#ifdef IC1CON1                  //PIC24E/dsPIC33E
  IC1CON1 = IC_TIMER2_SRC |     //Timer2 source
            IC_INT_1CAPTURE |   //Interrupt every capture
            IC_EVERY_EDGE;      //Capture every edge

cascade off, sync mode, sync to timer 2

  IC1CON2 = IC_IC32_OFF| IC_SYNC_MODE | IC_SYNCSEL_TIMER2;
#endif
  _IC1IF = 0;
  _IC1IP = 2;   //higher than Timer2 so that Timer2 does not interrupt IC1
  _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_8 ;  //1 tick = 0.2 us at FCY=40 MHz
  PR2 = 0xFFFF;                    //maximum period
  TMR2  = 0;                       //clear timer2 value
  _T2IF = 0;                       //clear interrupt flag
  _T2IP = 1;                       //choose a priority
  _T2IE = 1;                       //enable the interrupt
  T2CONbits.TON = 1;               //turn on the timer
}

int main (void) {
  configBasic(HELLO_MSG);
  CONFIG_SW1();    //use RB13
  configInputCapture1();
  configTimer2();


  while (1) {
    outString("Press button...");
    while (!u8_captureFlag) doHeartbeat();
    printf(" %ld us\n",u32_pulseWidth);
    u8_captureFlag = 0;
  }
}