timer_OneS_int

Code Breakdown:

1. Preprocessor Directives:

• #include <pic.h>: Includes the header file pic.h, which contains definitions and declarations specific to PIC microcontrollers.
• #define _XTAL_FREQ 4000000: Defines a preprocessor constant _XTAL_FREQ with the value 4000000, representing the crystal frequency in Hertz. This value is used by the compiler to calculate delay times and other timing-related calculations.

2. Variable Declarations:

• unsigned char count;: Declares an unsigned character variable count to keep track of the number of 4ms intervals that have elapsed.

3. main() Function:

• Initialization:

  • TRISC = 0;: Configures PORTC as output pins.
  • PORTC = 0;: Clears the output pins on PORTC.
  • T0CS = 0;: Selects the internal clock source (Fosc/4) for TMR0.
  • PSA = 0;: Assigns the prescaler to TMR0.
  • PS2 = 1; PS1 = 0; PS0 = 0;: Sets the prescaler to 32, providing a timer tick every 125 microseconds.
  • count = 0;: Initializes the count variable to 0.
  • T0IF = 0;: Clears the TMR0 interrupt flag.
  • T0IE = 1;: Enables the TMR0 interrupt.
  • GIE = 1;: Enables global interrupts.
  • TMR0 = 131;: Initializes TMR0 with the value 131 to generate a 4ms delay.
  • RC0 = 1;: Sets the output on RC0.

• Main Loop:

  • while (1) { ... }: The main loop continues indefinitely.
  • RC5 = 1; __delay_ms(150); RC5 = 0; __delay_ms(150);: Toggles RC5 with a 150ms delay between states.

4. Interrupt Service Routine (ISR):

• void interrupt isr() { ... }: Defines the interrupt service routine.
  • if (T0IF == 1) { ... }: Checks if the TMR0 interrupt flag is set.
  • TMR0 = 131;: Reloads TMR0 with the value 131 to start another 4ms delay.
  • count++;: Increments the count variable.
  • if (count == 250) { ... }: Checks if 250 4ms intervals have elapsed (1 second).
    • RC0 = !RC0;: Toggles the RC0 output.
    • count = 0;: Resets the count variable to 0.
  • T0IF = 0;: Clears the TMR0 interrupt flag.

Explanation:

The code implements a simple blinking LED circuit that toggles an output pin (RC0) every second. The main components of the code are:

• Timer Initialization: TMR0 is configured as an internal timer with a prescaler of 32, providing a timer tick every 125 microseconds. The initial value of TMR0 is set to 131 to generate a 4ms delay.
• Interrupt Handling: The TMR0 interrupt is used to increment a counter and toggle the output pin after 250 4ms intervals (1 second).
• Main Loop: The main loop includes a delay loop that toggles another output pin (RC5) every 300ms, independent of the timer-based LED toggling.

#include <pic.h>
unsigned char count;
#define _XTAL_FREQ 4000000


void main()
{
TRISC=0;    // PORTC output
PORTC=0;   

T0CS=0;        // Internal Mode (Fosc/4)
PSA=0;        // Prescaler assigned to TMR0
PS2=1;
PS1=0;
PS0=0;        // Prescaler ratio 1:32
count=0;

T0IF=0;        // Clear TMR0 Overflow int. bit
T0IE=1;        // TMR0 overflow int. enable
GIE=1;        // Global int. enable

TMR0=131;    // (256-125=131) for 4mS time
RC0=1;        // output on
while(1)   
{
RC5=1;
__delay_ms(150);
RC5=0;
__delay_ms(150);

}
}

void interrupt isr()    // interrupt function
{
    if(T0IF==1){
    TMR0=131;   
    count++;       
        if(count==250){        // 0.5Sec?
        RC0=!RC0; // RC0 toggle
        count=0;
        }       
    T0IF=0;
    }

}