8051_PWM

#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
// PWM generation code is written inside the Timer0 ISR
}
// 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
}