PIC Microcontroller PWM and TIMER

[torchd]

Hi guys
I am new to PIC microcontrollers. I am measuring relative humidity in a room by using a resistive humidity sensor. The sensor uses an ac signal. I have generated the ac signal by using the PWM signal from a PIC 4520( 1kHz 0-5V). I send this signal to an op amp and convert the signal to -1 to +1 V square wave. What I need to know is how do I sample the voltage across the sensor and know when my voltage is high(+1V) to be able to sync my ADC samplings.
(I am using CCP1 and Timer 2 )
Thanks in advance

The Code I used to generate the PWM signal is shown below:

#include <htc.h>


void main()
{
int i;
OSCCON = 0b01111110; //oscillator control register set for 8mhz

TRISC=0x00;
T2CON = 0b00000110; // prescaler + turn on TMR2;
PR2 = 0b001111111;
CCP1CON = 0b00111100;
CCPR1L=63;

//TMR2ON=1;
while(1)
{

}


}

 

[andre_luis]

Could you draw a flowchart and/or any behavioural scheme ?

+++

 

[btbass]

You can use the timer 2 interrupt.

From data sheet:

When TMR2 is equal to PR2, the following events
occur on the next increment cycle:
• TMR2 is cleared
• The CCPx pin is set.

Timer two interrupt will occur when TMR2 is equal to PR2. This is when the pwm output is set high.

 

[torchd]

Hi Guys Thanks for the reply.
After doing some research I found that I cannot use both CCP modules with pwm at different frequencies.( I am using CCP2 for 120kHz pwm signal). So I found that if I want to create a 1kHz square wave using CCP1, will I need to use the compare mode on the CCP1. Can anyone tell how I will be able to read the adc at when pin is high.
Thanks Guys

 

[btbass]

You could use a timer interrupt to generate the square wave.
Pre-load the timer for a 500uS interrupt frequency and toggle an output pin in the interrupt.
Then you would know when the line is high.

 

[andre_luis]

torchd

Why don´t you use TIMER0 and perfor what btbass sugested ?
I didn´t see it being used at above code you posted.

+++

 

[torchd]

Guys I have tried using timer 0 but I only a high signal
Code

#include <htc.h>
#include <math.h>
__CONFIG(1,XT);
int count=0;
int time=0;
unsigned char shad;

void main(void)
{
	TRISC=0x00;
	T0CON=0b11000111;
	TMR0IE=1;	//Enable TIMER0 Interrupt
	PEIE=1;		//Enable Peripheral Interrupt
	GIE=1;		//Enable INTs globally
	while(1);
	{	RC2=shad;
	}
}





//Main Interrupt Service Routine (ISR)
void interrupt ISR()
{
	//Check if it is TMR0 Overflow ISR
	if(TMR0IE && TMR0IF)
	{
		time++;
		if (time==10)
		{
	
		count++;
	    if (count%2==0)
		{
			shad=0;
		}
		if (count%2==1)
		{
			shad=1;
		}
		if (count==6000)
		{
			count=1;
		}	
		time=0;	
		}//Clear Flag
		TMR0IF=0;
	}
}

 

[btbass]

You can pre load the timer to get the interrupt frequency. You can check this by using Mplab SIM, set the correct oscillator frequency in \Debugger\Settings\, and put a break point in the interrupt routine. Use the stop watch to adjust the pre load value for 500uS interrupt.

void interrupt ISR()
  {
  static char toggle = 0;

  if(TMR0IF)
    {
    if(!toggle){
     RC2 = 1;
     toggle = 1;
     }
    else{
     RC2 = 0;
     toggle = 0;
     }

    TMR0IF = 0;

    TMR0 = 160;  /* Pre load timer for 500uS interrupt */
    }
  }

 

[bjuric]

Couldn't you set that op-amp to generate the needed square wave, and use positive transition as interrupt to start the conversion.
If I even understood the problem correctly.