Well, I don't know if this is it, but here are some comments...

I like writing interrupt service routines in assembly since the various C
functions are not guaranteed to be reentrant. Your interrupt may mess up
some intermediate value in mainline code.

Are you doing an interrupt context save? With my writing the ISR in
assembly, I know exactly which registers I mess with (typically w, bsr,
fsr0, status, etc.). I have a subroutine that saves these away and another
that restores them when I'm done.

What does the PORTB=PORTB line do for us?

I guess there's also a possibility of the PORTB bit changing during the
ISR. I'd probably copy it to a local variable, then do all the checking on
the unchanging local.

Another write wrote about having trouble with two ISRs, both as high
priority. I've generally made everything high priority and had no trouble.
Sometimes my main ISR does a context save, then calls each individual ISR,
which does a flag check, clear flag and execute if true or exit if false.
Other times I've had the flag test in the main ISR and only call the
individual ISR if the flag is set (then clear it in the individual ISR).
Even though it's a bit slower, I tend to like doing the test inside the
individual ISR since code is more compartmentalized.

Good luck!

Harold


>
> #include <p18F252.h>
> #pragma config OSC = XT, OSCS = OFF, PWRT = ON, BOR = OFF, WDT = OFF,
> STVR = OFF, LVP = OFF, DEBUG = OFF
>
> //--------------------------------------------------------------------------
> --
>
> void main (void);
> void InterruptHandlerHigh (void);
>
> union {
>   struct  {
>     unsigned Timeout:1;         //flag to indicate a TMR0 timeout
>     unsigned None:7;
>   } Bit;
>   unsigned char Byte;
> } Flags;
>
> //=========================
> union {
>         struct {
>                 unsigned char B0;
>                 unsigned char B1;
>                 unsigned char B2;
>                 unsigned char B3;
>                 } byte;
>                 unsigned long b1234;
> } pulse_count;
>
>
> //=========================
> union {
>   struct {
>     unsigned RB0:1;
>     unsigned RB1:1;
>     unsigned RB2:1;
>     unsigned RB3:1;
>     unsigned RB4:1;
>     unsigned RB5:1;
>     unsigned RB6:1;
>     unsigned RB7:1;
>         };
>         unsigned char old_portb;
> } old_portb_bits;
>
> //--------------------------------------------------------------------------
> --
> // Main routine
>
> void main () {
>         Flags.Byte = 0;
>         INTCON = 0x28; //disable global and enable TMR0 interrupt and RB
>         INTCON2 = 0x85; //TMR0 high priority
>         RCONbits.IPEN = 1; //enable priority levels
>         TMR0H = 0; //clear timer and set TMR0H
>         TMR0L = 0; //clear timer low byte and also updates high byte from
> TMR0H
>         T0CON = 0x88; //set up timer0 - no prescaler
>         INTCONbits.GIEH = 1; //enable interrupts
>         TRISB = 0x7E; //PORTB bits 0 and 7 are output, the rest are input
>         TRISC = 0; //PORTC bits are output
>         PORTC = 0;
>
>         old_portb_bits.old_portb = 0;
>         pulse_count.b1234 = 0;
>
>         while (1) {
>                 PORTC = pulse_count.byte.B0;
>         }
> }
>
> //--------------------------------------------------------------------------
> --
> // High priority interrupt vector
>
> #pragma code InterruptVectorHigh = 0x08
> void
> InterruptVectorHigh (void)
> {
>   _asm
>     goto InterruptHandlerHigh //jump to interrupt routine
>   _endasm
> }
>
> //--------------------------------------------------------------------------
> --
> // High priority interrupt routine
>
> #pragma code
> #pragma interrupt InterruptHandlerHigh
>
> void InterruptHandlerHigh () {
>
>         if (INTCONbits.TMR0IF) {                                //check
> for
> TMR0 overflow
>                 INTCONbits.TMR0IF = 0;                          //clear
> interrupt flag
>         }
>
>         if (INTCONbits.RBIF) {
>                 PORTB = PORTB;
>                 INTCONbits.RBIF = 0;
>                 if (PORTBbits.RB4 == 1 && old_portb_bits.RB4 == 0)
> (unsigned
> long)pulse_count.b1234++;
>                 old_portb_bits.old_portb = PORTB;
>         }
> }
>
> --
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