> I'm trying to grasp the functionality of tmr0 interrupt.
>
> I have written a basic tmr0 interrupt based c code. (I used hi-tech picc
> compiler in my testings)
>
> /////////////////////////////////////////////////////////////////////////////////////////////////
>
> #include <htc.h>
>
> __CONFIG(XT & WDTDIS);
>
> void interrupt tmr0_isr (void) {
>     RB0 ^= 1;
>     T0IF = 0;
>     TMR0 = 255;
> }
>
> void main (void) {
>
>     PSA = 1;            // prescalar assigned to the WDT
>     T0CS = 0;            // select internal clock
>     T0IE = 1;            // enable timer interrupt
>     GIE = 1;            // enable global interrupts
>
>     TRISB0 = 0;
>     RB0 = 0;
>
>     TMR0 = 255;       // Load the tmr0 for the minimum interrupt rate
>
>     while(1);
>
> }
>
> // All the calculations are based on 4 MHZ XT.
>
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
//////////////////////////////
>
> Above i would expecting the toggle rate of RB0 is about 1 us  + 2us (synch
> cycles) + a few more for interrupt latency which is totally about 5 us. But
> when i tested the rate with MPLAB stopwatch i get an interrupt at each 18us.
> So the tmr0_init_value = 256 - (delay_time * clock_freq/4) dont work with my
> tmr0 calculations. (assumed no prescaler).
>
> So where am i misunderstanding, is this the problem of c compiler or me?

There's over-head for bank switching, moving things around, and GIE will
not be re-enabled until exit from your interrupt on RETFIE.

In MPLAB click View, Program Memory to see the assembler generated
by your C compiler.

  0000              CLRF STATUS
  0001              MOVLW 0
  0002              MOVWF PCLATH
  0003              GOTO 0x14

  0004              MOVWF 0x70               ;<-- int vector
  0005              MOVF STATUS, W
  0006              CLRF STATUS
  0007              MOVWF 0x20
  0008  tmr0_isr    BCF STATUS, 0x5
  0009              MOVLW 0x1
  000A              XORWF PORTB, F
  000B              BCF INTCON, 0x2
  000C              MOVLW 0xff
  000D              MOVWF TMR0
  000E              BCF STATUS, 0x5
  000F              MOVF 0x20, W
  0010              MOVWF STATUS
  0011              SWAPF 0x70, F
  0012              SWAPF 0x70, W
  0013              RETFIE  '<-- interrupts disabled until here

  0014              CLRF STATUS
  0015              GOTO main

  03F5  main    BSF STATUS, 0x5
  03F6              BSF TMR0, 0x3
  03F7              BCF TMR0, 0x5
  03F8              BSF INTCON, 0x5
  03F9              BSF INTCON, 0x7
  03FA              BCF PORTB, 0
  03FB              BCF STATUS, 0x5
  03FC              BCF PORTB, 0
  03FD              MOVLW 0xfe
  03FE              MOVWF TMR0
  03FF              GOTO 0x3ff

Regards,

Bruce

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