// ***********************************************
// Project: Interfacing PWM to 8051
// Author: Hack Projects India
// Module description: Operate array of LED's
// ***********************************************
#include<reg51.h>
// PWM_Pin
sbit PWM_Pin = P2^0;                      // Pin P2.0 is named as PWM_Pin
// Function declarations
void cct_init(void);
void InitTimer0(void);
void InitPWM(void);
// Global variables
unsigned char PWM = 0;                 // It can have a value from 0 (0% duty cycle) to 255 (100% duty cycle)
unsigned int temp = 0;    // Used inside Timer0 ISR
// PWM frequency selector
/* PWM_Freq_Num can have values in between 1 to 257            only
 * When PWM_Freq_Num is equal to 1, then it means highest PWM frequency
 * which is approximately 1000000/(1*255) = 3.9kHz
 * When PWM_Freq_Num is equal to 257, then it means lowest PWM frequency
 * which is approximately 1000000/(257*255) = 15Hz
 *
 * So, in general you can calculate PWM frequency by using the formula
 *     PWM Frequency = 1000000/(PWM_Freq_Num*255)
 */
#define PWM_Freq_Num   1      // Highest possible PWM Frequency
// Main Function
int main(void)
{
   cct_init();                 // Make all ports zero
   InitPWM();              // Start PWM
   PWM = 127;              // Make 50% duty cycle of PWM
   while(1)                // Rest is done in Timer0 interrupt
   {}
}
// Init CCT function
void cct_init(void)
{
                P0 = 0x00;  
                P1 = 0x00;  
                P2 = 0x00;  
                P3 = 0x00; 
}
// Timer0 initialize
void InitTimer0(void)
{
                TMOD &= 0xF0;    // Clear 4bit field for timer0
                TMOD |= 0x01;    // Set timer0 in mode 1 = 16bit mode
                TH0 = 0x00;      // First time value
                TL0 = 0x00;      // Set arbitrarily zero
                ET0 = 1;         // Enable Timer0 interrupts
                EA  = 1;         // Global interrupt enable
                TR0 = 1;         // Start Timer 0
}
// PWM initialize
void InitPWM(void)
{
                PWM = 0;         // Initialize with 0% duty cycle
                InitTimer0();    // Initialize timer0 to start generating interrupts
}
// Timer0 ISR
void Timer0_ISR (void) interrupt 1  
{
                TR0 = 0;    // Stop Timer 0
                if(PWM_Pin)      // if PWM_Pin is high
                {
                                PWM_Pin = 0;
                                temp = (255-PWM)*PWM_Freq_Num;
                                TH0  = 0xFF - (temp>>8)&0xFF;
                                TL0  = 0xFF - temp&0xFF;             
                }
                else             // if PWM_Pin is low
                {
                                PWM_Pin = 1;
                                temp = PWM*PWM_Freq_Num;
                                TH0  = 0xFF - (temp>>8)&0xFF;
                                TL0  = 0xFF - temp&0xFF;
                }
                TF0 = 0;     // Clear the interrupt flag
                TR0 = 1;     // Start Timer 0
}