Software PWM issues at 250Hz

/*
Software PWM

20070403 : Target AT89C52

*/

#include <AT89X52.H>
#include <stdio.h>

unsigned char requiredDutyCycle[] = {10, 20, 30, 40, 50 , 60, 70, 80}, timerISRCount = 0;

/* Serial port setup macros */

#define XTAL_HZ         11059200
#define BAUD_RATE       9600
#define TH1_BAUD_VALUE  (0x100 - ((XTAL_HZ / (12 * 32)) / BAUD_RATE))
/* TODO : Calculate the error percentage for XTAL_HZ and warn the user accordingly */

/* Software PWM Macros */

#define PWM_FREQ_HZ     250
#define PWM_STEPS       100
#define PWM_TIMER_VALUE (0x100 - ((XTAL_HZ / (12 * PWM_STEPS)) / PWM_FREQ_HZ))

/* This port must be 8 bits wide */
#define PWM_PIN P1

#define CHANNEL1 0x0
#define CHANNEL2 0x1
#define CHANNEL3 0x2
#define CHANNEL4 0x3
#define CHANNEL5 0x4
#define CHANNEL6 0x5
#define CHANNEL7 0x6
#define CHANNEL8 0x7

/*
Interrupt Vector at 000BH, Reg Bank 1; generates the PWM.
Any function that this ISR calls must also be in bank 1
*/
void softPWMisr(void) interrupt 1 using 1
{
        if(timerISRCount < PWM_STEPS)
        {
                if(timerISRCount == requiredDutyCycle[CHANNEL1]) PWM_PIN &= ~(0x1 << CHANNEL1);
                if(timerISRCount == requiredDutyCycle[CHANNEL2]) PWM_PIN &= ~(0x1 << CHANNEL2);
                if(timerISRCount == requiredDutyCycle[CHANNEL3]) PWM_PIN &= ~(0x1 << CHANNEL3);
                if(timerISRCount == requiredDutyCycle[CHANNEL4]) PWM_PIN &= ~(0x1 << CHANNEL4);
                if(timerISRCount == requiredDutyCycle[CHANNEL5]) PWM_PIN &= ~(0x1 << CHANNEL5);
                if(timerISRCount == requiredDutyCycle[CHANNEL6]) PWM_PIN &= ~(0x1 << CHANNEL6);
                if(timerISRCount == requiredDutyCycle[CHANNEL7]) PWM_PIN &= ~(0x1 << CHANNEL7);
                if(timerISRCount == requiredDutyCycle[CHANNEL8]) PWM_PIN &= ~(0x1 << CHANNEL8);

                ++timerISRCount;
        }
        else
        {
                /* timerISRCount reached PWM_STEPS */
                timerISRCount = 0;
                PWM_PIN = 0xFF;
        }
}

void main(void)
{
        unsigned char delay = 0, i = 0;
        unsigned char count = 0;

        TCON = 0x0;
        PWM_PIN = 0xFF; /* Port0 init to all high */

        /*
        Setup TIMER1 for the serial port.

        Our XTAL = 11.0592 Mhz. This is divided by 12 to get 921.6 kHz
        The UART divides this by 32 to yeild 28800 Hz; to get 9600bps, we need to divide furthur by 3
        Setting up TIMER1 in 8 bit autoreload mode, we need to init TH1 = (0xFF - 3 + 1) = 0xFD
        */
        EA = 0x00;
        SCON = 0x50; /* Mode 1 : 8 bit data with start and stop bit, enable reciever */
        TMOD &= 0x0F; /* Reset TIMER1 settings */
        TMOD |= 0x20; /* Timer 1, Mode 2 : 8 bit autoreload */
        TH1 = TH1_BAUD_VALUE;
        TR1 = 0x1;
        EA = 0x01;
        /* Setup serial port done */

        printf("SoftPWM Program starting....\r\n");

        /*
        Initialize TIMER0
        We need 40us period : yeilds 100 unadjusted steps for a 250 Hz PWM Freq
        Our XTAL = 11.0592 Mhz. This is divided by 12 giving time period of 1.085us
        Hence, we need to load TH0 with (0x100 - (40/1.085)) = 0xDB
        NOTE : (40/1.085) gives 36.87 which is rounded to 37 (0x25)
        */

        EA = 0x00;
        TH0 = PWM_TIMER_VALUE;
        TMOD &= 0xF0; /* Reset TIMER0 settings */
        TMOD |= 0x02; /* Mode 2 : 8 bit Autoreload */
        ET0 = 0x01;
        TR0 = 0x1; /* Start TIMER0 */
        EA = 0x01;

        printf("SoftPWM Program running....\r\n");

        while(1)
        {
                for(i = 0; i < 8; ++i)
                {
                        /* Increment in steps of 10% of fullscale */
                        requiredDutyCycle[i] += (PWM_STEPS / 10);
                        requiredDutyCycle[i] %= (1 + PWM_STEPS);

                        printf("\r\nrequiredDutyCycle[%Bu] = %Bu", i, requiredDutyCycle[i]);
                }

                for(count = 0; count < 0x1F; ++count) for(delay = 0; delay < 0xFE; ++delay); /* Delay a little bit between changing the duty cycles */

                printf("\r\n-----------------------\r\n");
        }
}