lpc24xx PWM capture timer readout problem

[lpcware]

Content originally posted in LPCWare by sns22 on Fri Feb 07 05:39:04 MST 2014
Hello everyone.

I am a newbie in lpc devices. I have been trying to interface the PWM based max6672 sensor with the PWM capture pin. I want to read the both the low time and high time of the PWM pulse. I am recieving an interrupt when the PWm input is received. But some how I am not able to calculate the rise and fall time correctly. I need this info to calculate the temperature of the sensor.

Steps in my code:
1. set up the PWM capture pin.
2. set the PWM capture registers.
3. Wait for interrupt on rise time. save the PWM0CR0 rise time value 
4. set the register to low time interrupt.
5. wait for interrupt on low time. save the PWM0CR0 low time value  = PWM0CR0 - high time.
6. calculate the temp using the high and low time of the pulse.

I am putting up my code here.

void Capture0IntrHandler(void)
{
  if ((PWM0IR == 0x10)){
    if((PWM0CCR_bit.CAP0RE  == 1)){  //low to high transition    rising edge
      period = PWM0CR0 - rising_edg;
      rising_edg = PWM0CR0;
      PWM0CCR = 0x6;
    }else{
       duty = PWM0CR0 - rising_edg;       
       PWM0CCR = 0x5;
       PWM0_Done =1;
    }
    }
  PWM0IR=0xFF; //Reset the interrupt*/
  VICADDRESS = 0;
}

void init_timer(void)   // Timer 0 used as a counter
{ 
  // PWM PCAP0.0 Pin: P1.12
//  pclk = cclk/2 = 30MHz (cclk=ARM7 core clock)

    unsigned int IRQ_priority= 0x1;   // '0' = highest, '0xF' = lowest IRQ priority
   
    PINSEL2   = 0x03000000;             //PWM PCAP0.0 Pin: P1.12                            //1 = pclk = 60 MHz or 72MHz
    PCONP_bit.PCPWM0 |=1;
    PCLKSEL0_bit.PCLK_PWM0 = 1; //pclk is set to  2 = pclk/2 = 36 MHz or 30MHz
  
 
    //RISING EDGE CAPTURE
   
    PWM0TCR_bit.CR = 0; //disable PWM counter reset
    PWM0TCR_bit.CR = 1; //Reset PWM counter
    PWM0TCR_bit.CR = 0; //disable PWM counter reset
   
    PWM0CCR_bit.CAP0RE  = 1;   //capture of rising edge
    PWM0CCR_bit.CAP0FE  = 0;   //disable falling edge capture
    PWM0CCR_bit.CAP0INT = 1; //enable interrupt rising edge
   
    PWM0CTCR_bit.CM = 0;        //PWM0 is set in timer mode
    PWM0CTCR_bit.CIS = 0;
   
    PWM0PCR =0;              //no external interrupt  
    PWM0PR = 59;           //resolution of 1 us for a 60MHz cclk = pclk
    PWM0IR = 0xFF;         //reset the interrupts
  
  VIC_SetVectoredIRQ(Capture0IntrHandler, IRQ_priority, VIC_PWM01);  // setup PWM IRQ
  PWM0TCR_bit.CE =1; //enable the PWM Capture timer 
  
}


int main (void)
{
LPC2468_Init();
  ConfigurePLL();
  LPC2468GPIOInit();
  __disable_interrupt();
  LPC2468VICInit();
  __enable_interrupt();
   
  period = 0;
  falling_edg = 0;
  rising_edg =0;
  duty = 0;
 
  init_timer();
  PWM0_Done =0; 
  while (PWM0_Done == 0){};
  y = 3;
  t1 = duty;
  t2 = rising_edg;
  k = (double)t1/t2;
  x11 = (temp_coeff - k);
  x1 = (double)powf(x11,y);
  x2 = ((double)(temp_coeff2 * k)) - temp_coeff3;
  temp_deg = ((double)((-temp_coeff1)* x1)) + x2 ;
  peri = rising_edg + duty; 
 
  return 0;
}


Thanks
Sneha