C18, PIC18F252: Setting timer0 with an interrupt ?

Hi all...

I'm trying to set up timer0 to time about 10ms and inc a global variable
that gets displayed on an LCD. Everything is pretty much working except that
I can't change the time out value for timer0. I init the timer before  a
while loop that display the variable "time". The isr inc the value on every
time out of timer0. It also toggles RB4 so I can check the freq (for
testing). The freq is always about 4 Hz.
TIME_TIMEOUT is defined as 5000. I've tried to change this value but nothing
happens. What am I missing ?

Thanks

BTW it runs at 2MHz. There is only one isr.


//------------- code begins --------------------------
 OpenTimer0(TIMER_INT_ON & T0_16BIT & T0_SOURCE_INT & T0_PS_1_1);
 WriteTimer0(TIME_TIMEOUT);

// INTCON=0;    //make sure interrupts are disable
// INTCONbits.GIE=1;  //enable global interrupts
// INTCONbits.PEIE=1;  //enable peripheral interrupts
// INTCONbits.TMR0IE=1; //enable TMR0 overflow interrupt enable bit
 INTCON=0xe0;   //all 4 lines above combined

 while(1)
 {
  itoa(time,timeStr);
  SetDDRamAddr(LCD_LINE2); // Display value of time
  while(BusyXLCD());   // Wait if LCD busy
  putsXLCD(timeStr);

 };
//--------------- code ends ----------------------

The isr is here:

//------------- code begins ----------------------
#pragma code
#pragma interrupt InterruptHandlerLow
void InterruptHandlerLow()
{
 if (INTCONbits.TMR0IF==1)
 {
  time++;
  PORTBbits.RB4=~PORTBbits.RB4;
  WriteTimer0(TIME_TIMEOUT);
   INTCONbits.TMR0IF=0;         //clear interrupt flag
 }

}
//--------------- code ends ----------------------

What does your WriteTimer0 routine look like?

"Rodo" <dsp1024@yahoo.com> wrote in message
news:iwmQb.2682$QY5.1056@nwrddc03.gnilink.net...
> Hi all...
>
> I'm trying to set up timer0 to time about 10ms and inc a global variable
> that gets displayed on an LCD. Everything is pretty much working except
that
> I can't change the time out value for timer0. I init the timer before  a
> while loop that display the variable "time". The isr inc the value on
every
> time out of timer0. It also toggles RB4 so I can check the freq (for
> testing). The freq is always about 4 Hz.
> TIME_TIMEOUT is defined as 5000. I've tried to change this value but
nothing
> happens. What am I missing ?
>
> Thanks
>
> BTW it runs at 2MHz. There is only one isr.
>

Hmmm something smells.......
2Mhz(Xtal) / 4(CLK0) / 4(Prescaler) / 16384(16bit count) / 2(Port bit
toggle)
=  "about 4Hz"

I like Gary Kato's question.

Greg the Grog

Here is the source code:

/* $Id: t0write.c,v 1.2 2000/08/04 20:58:59 ConnerJ Exp $ */

#include <p18cxxx.h>

#include <timers.h>

/********************************************************************

* Function Name: WriteTimer0 *

* Return Value: void *

* Parameters: int: value to write to Timer0 *

* Description: This routine writes a 16-bit value to Timer0 *

* Timer0. *

********************************************************************/

void WriteTimer0(unsigned int timer0)

{

union Timers timer;

timer.lt = timer0; // Copy timer value into union

TMR0H = timer.bt[1]; // Write low byte to Timer0

TMR0L = timer.bt[0]; // Write high byte to Timer0

}


Thanks



"Gary Kato" <garykato@aol.com> wrote in message
news:20040124021433.11422.00000636@mb-m05.aol.com...
> What does your WriteTimer0 routine look like?

> OpenTimer0(TIMER_INT_ON & T0_16BIT & T0_SOURCE_INT & T0_PS_1_1);
>

Shouldn't you be using | instead of & here?

>>OpenTimer0(TIMER_INT_ON & T0_16BIT & T0_SOURCE_INT & T0_PS_1_1);
> Shouldn't you be using | instead of & here?

Nope, that's Microchip's C18 way of expressing constant flags.
They are defined accordingly so you can combine them with '&'.
A bit confusing, I agree.

I found my error. This micro triggers an interrupt when the count goes from
ffff to 0000. The delay of 10ms would need :

(2MHz/4(sclk))^-1=2us now

10ms/2us=5000 so ...

the timer should count 5000 times before it triggers the int. So :

16bit counter -1 = 65535. I need 5000, so the value should have been 60535
(and not 5000). It works :-)! I was used to the zilog z8 where the counters
count down.

Thanks



"Rodo" <dsp1024@yahoo.com> wrote in message
news:iwmQb.2682$QY5.1056@nwrddc03.gnilink.net...
> Hi all...
>
> I'm trying to set up timer0 to time about 10ms and inc a global variable
> that gets displayed on an LCD. Everything is pretty much working except
that
> I can't change the time out value for timer0. I init the timer before  a
> while loop that display the variable "time". The isr inc the value on
every
> time out of timer0. It also toggles RB4 so I can check the freq (for
> testing). The freq is always about 4 Hz.
> TIME_TIMEOUT is defined as 5000. I've tried to change this value but
nothing
> happens. What am I missing ?
>
> Thanks
>
> BTW it runs at 2MHz. There is only one isr.
>
>
> //------------- code begins --------------------------
>  OpenTimer0(TIMER_INT_ON & T0_16BIT & T0_SOURCE_INT & T0_PS_1_1);
>  WriteTimer0(TIME_TIMEOUT);
>
> // INTCON=0;    //make sure interrupts are disable
> // INTCONbits.GIE=1;  //enable global interrupts
> // INTCONbits.PEIE=1;  //enable peripheral interrupts
> // INTCONbits.TMR0IE=1; //enable TMR0 overflow interrupt enable bit
>  INTCON=0xe0;   //all 4 lines above combined
>
>  while(1)
>  {
>   itoa(time,timeStr);
>   SetDDRamAddr(LCD_LINE2); // Display value of time
>   while(BusyXLCD());   // Wait if LCD busy
>   putsXLCD(timeStr);
>
>  };
> //--------------- code ends ----------------------
>
> The isr is here:
>
> //------------- code begins ----------------------
> #pragma code
> #pragma interrupt InterruptHandlerLow
> void InterruptHandlerLow()
> {
>  if (INTCONbits.TMR0IF==1)
>  {
>   time++;
>   PORTBbits.RB4=~PORTBbits.RB4;
>   WriteTimer0(TIME_TIMEOUT);
>    INTCONbits.TMR0IF=0;         //clear interrupt flag
>  }
>
> }
> //--------------- code ends ----------------------
>
>