Clock training board fir PIC 12F275

/*  short button push gives one output pulse
*   longer button push will trigger automatic pulses with 1 sec intervals
*   push again and it stops the automatic pulses
*/ 
// PIC12F675 Configuration Bit Settings
#pragma config FOSC = INTRCIO   // Oscillator Selection bits (INTOSC oscillator: I/O function on GP4/OSC2/CLKOUT pin, I/O function on GP5/OSC1/CLKIN)
#pragma config WDTE = OFF       // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF      // Power-Up Timer Enable bit (PWRT disabled)
#pragma config MCLRE = OFF      // GP3/MCLR pin function select (GP3/MCLR pin function is digital I/O, MCLR internally tied to VDD)
#pragma config BOREN = OFF      // Brown-out Detect Enable bit (BOD disabled)
#pragma config CP = OFF         // Code Protection bit (Program Memory code protection is disabled)
#pragma config CPD = OFF        // Data Code Protection bit (Data memory code protection is disabled)

// #pragma config statements should precede project file includes.
// Use project enums instead of #define for ON and OFF.

#include <xc.h>
#include <pic12f675.h>

#define _XTAL_FREQ 4000000


unsigned int counter = 0;
unsigned int timer = 0;
char clock_on = 0;

void __interrupt() ISR(void) {
    if (INTCONbits.T0IF == 1) {
        counter++; // 0-65535
        if (clock_on) {
             if (counter > 1000) {
                GP0 = ~GP0;   // ~ means not and toggles GP0
                counter = 0;
             }
        }
        INTCONbits.T0IF = 0; // Clear Timer0 overflow 
        TMR0 = 0;
    }
}

void main(void) {
    ANSEL = 0b00000000; // Set ports as digital I/O, no analog input
    CMCON = 0b00000111; //comparators CM2:CM0 off
    VRCON = 0x00; // Shut off the Voltage Reference
    ADCON0 = 0x00; // Shut off the A/D Converter
    WPU = 0x00;
    TRISIO = 0b00011000; // GP3, GP4 = input
    GPIO = 0x00;
    OPTION_REG = 0b00000000; // bit 7 and 6 don't matter, bit 5 CLKO, bit 4 ?, bit 3 timer0 , bit 2,1,0 = 1:2 pre-scaler => 1125Hz
    INTCON = 0b11100000; // Enables all unmasked, peripheral and Timer0 interrupt
    PIE1 = 0x00; // Peripheral Interrupt Enable 1 all zero
    T1CON =  0x00; //clear the unused timer
    TMR0 = 0;

    while (1) {
      if (GP4 == 1) {
          counter = 0;
          while (counter < 20);  // de-bouncing, not using __delay_ms()
          while (GP4 == 1) { // while pressed timer increments
              GP0= 1;
              clock_on = 0;
              timer++;
          }
      } else {    // not pressed
          if (timer > 20000) {  // about 3 seconds
               clock_on = 1;    // t0 loop     
          } 
          if (clock_on == 0) GP0 = 0; // not interfere with t0 loop
          timer = 0;
      }           
    } // while
    return;
}