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error using 2 interrupts

 
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weg22



Joined: 08 Jul 2005
Posts: 91

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error using 2 interrupts
PostPosted: Tue Aug 09, 2005 1:38 pm     Reply with quote
Hi all,

I am trying to generate 2 PWM signals using 2 interrupts (timer0 and timer1). The PWM signal on PIN_B0 works fine (timer0), but the PWM signal on PIN_B1 (timer1) has the correct period but with a 500 mV amplitude (should be 5V). I don't know why this is happening because it's the same exact code (see below).

Any help is much appreciated,
-weg

Code:

#include <16F87.H>
#include <stdlib.h>
#include <math.h>

#define B0 PIN_B0
#define B1 PIN_B1

#fuses HS,NOWDT,NOPROTECT, PUT
#use delay(clock=10000000)

int rud_pwm, elev_pwm;

//Timer interrupt to generate PWM signals
#INT_TIMER0
void pwm_signal()
{
   int i;

   //One of the parameters used to cause the interrupt to occur every 22.5 msec
   set_timer0(37); // 61 for 20 msec

   /*--------RUDDER 0 (right), 255 (left) --------*/
   output_high(B0); // generates initial high time ie 1 msec
   delay_us(1000);
   i=0;
   for(i=0;i<rud_pwm;i++) // generates remaining high time, varied by variable "pwm"
   {
      //use this to tune duty cycle
      delay_us(1);
   }
   output_low(B0);
}

//Timer interrupt to read PWM signals
#INT_TIMER1
void pwm_signal2()
{
   int j;
   //One of the parameters used to cause the interrupt to occur every 22.5 msec
   set_timer1(37); // 61 for 20 msec

   /*--------RUDDER 0 (right), 255 (left) --------*/
   output_high(B1); // generates initial high time ie 1 msec
   delay_us(1000);
   j=0;
   for(j=0;j<elev_pwm;j++) // generates remaining high time, varied by variable "pwm"
   {
      //use this to tune duty cycle
      delay_us(1);
   }
   output_low(B1);
}




main()
{
   long m;

   delay_ms(1500);

   while(1)
   {
      // enable timer interrupts
      enable_interrupts(GLOBAL);
      enable_interrupts(INT_TIMER0);
      enable_interrupts(INT_TIMER1);

      //the other parameter used to cause interrupt to occur every 20 msec
      setup_timer_0(RTCC_INTERNAL|RTCC_DIV_256);
      setup_timer_1(RTCC_INTERNAL|RTCC_DIV_256);

      rud_pwm = 128;
      elev_pwm = 128;

   }
} // end of main

kender



Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley

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PostPosted: Tue Aug 09, 2005 2:09 pm     Reply with quote
It could be pure hardware problem rather then a firmware problem. Try the following:

* Make sure that your scope works as you expect. For example, you can have a 1x probe, while the scope expects a 10x probe.
* If you can, output_high(PIN_B1) for a short time. If it will still be 500mV instead of +5V, then you are probably grounding B1 somewhere. By the way, what is B1 connected to? Does your PIC get warm?
weg22



Joined: 08 Jul 2005
Posts: 91

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PostPosted: Tue Aug 09, 2005 2:38 pm     Reply with quote
I don't think its hardware...here's why. If I change the timer0 loop to output the PWM signal on B1 and timer1 to B0 (just switching them), I see a signal on B1 and not B0. Whereas before, I saw a PWM signal on B0 and not B1. This comes back to the 2nd timer loop (timer1)...which I believe to be a software problem. Any suggestions?

Thanks,
-weg
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Tue Aug 09, 2005 2:49 pm     Reply with quote
There are several things about your code that don't look very good,
but here, you've got an 8-bit counter (Timer0) and a 16-bit counter
(Timer1), and you're loading them with the same number and expecting
them to both interrupt every 22.5 ms.
Code:
set_timer0(37);

set_timer1(37);
Ttelmah
Guest
PostPosted: Tue Aug 09, 2005 2:56 pm     Reply with quote
It is never actually reaching the second interrupt.
Your 'correct' signal, is noise being picked up because of poor grounding, or a problem with the scope probe, from the other line.
Timer1, is a 16bit counter. It does not support a 256 prescale. The values give no error, because they are legitimate values for timer0. You are sending timer1, the initialisation value for timer0, which probably corresponds (from a quick glance on the data sheet), to 'system clock from another source' (not the internal oscillator), prescale 1:1, do not synchronise external clock, use external clock on RB6, enable timer.
Without the external clock, this timer will never trigger.

Best
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