Hello everyone. Ive written this code for the PWM control of the full bridge dc-dc converter. My switching freq is 100kHz(10us period) and I'm using PIC18F4550(bought from http://www.kynix.com/Detail/506330/PIC18F4550-I%2FML.html) and "xC8" compiler. I have made various cases for the selection depending upon the A/D conversion from the feedback resistive network, as I want to maintain 330Vdc at the output. I am also using the PLL functionality to make use of the 48MHz from the 4MHz crystal. I hope you all will understand the code as it is clearly written in a very easy possible way.
The problem is that during the simulation and debugging, I found that the program is taking about 39 cycles i.e., about 17us for the A/D conversion and I am reading this after every cycle. By the methodology I used, this much delay is causing my switching frequency to change a lot. I want a constant switching frequency of 100kHz, i.e, the time period must remain 10us that I've ensured in each case statement.
Any effective ideas to solve this problem?
Thankyou!

Code:

#include<xc.h>
#define _XTAL_FREQ 4000000
unsigned int answer=0;
unsigned int x=0;

void main()
{
TRISC = 0;
PORTC = 0;
TRISB=0;
PORTB=0;
RB0=0;
RC1=0;
RC2=0;
ADCON0=0b00000001;
ADCON1=0b00000000;
ADCON2=0b10000100;

while (1)
{
unsigned int temp=0;
GODONE=1;
while(GODONE==1);
temp=ADRESH;
temp=ADRESH<<2;
temp=temp|ADRESL;
answer= 4.883*temp;
RC1=0;
if(answer>3771)
{
x=x+1;
}
else x=x-1;

switch (x)
{
case 1 :
{
RC1 = 1;
__delay_us(18);
RC1 = 0;
__delay_us(42);
RC2=1;
__delay_us(18);
RC2=0;
__delay_us(42);
break;
}
case 2 :
{
RC1 = 1;
__delay_us(19.5);
RC1 = 0;
__delay_us(41.5);
RC2=1;
__delay_us(19.5);
RC2=0;
__delay_us(41.5);
break;
}
case 3 :
{
RC1 = 1;
__delay_us(21);
RC1 = 0;
__delay_us(39);
RC2=1;
__delay_us(21);
RC2=0;
__delay_us(39);
break;
}
case 4 :
{
RC1 = 1;
__delay_us(22.5);
RC1 = 0;
__delay_us(38.5);
RC2=1;
__delay_us(22.5);
RC2=0;
__delay_us(38.5);
break;
}
case 5 :
{
RC1 = 1;
__delay_us(24);
RC1 = 0;
__delay_us(36);
RC2=1;
__delay_us(24);
RC2=0;
__delay_us(36);
break;
}
case 6 :
{
RC1 = 1;
__delay_us(25.5);
RC1 = 0;
__delay_us(35.5);
RC2=1;
__delay_us(25.5);
RC2=0;
__delay_us(35.5);
break;
}
case 7 :
{
RC1 = 1;
__delay_us(27);
RC1 = 0;
__delay_us(33);
RC2=1;
__delay_us(27);
RC2=0;
__delay_us(33);
break;
}
case 8 :
{
RC1 = 1;
__delay_us(28.5);
RC1 = 0;
__delay_us(32.5);
RC2=1;
__delay_us(28.5);
RC2=0;
__delay_us(32.5);
break;
}
case 9 :
{
RC1 = 1;
__delay_us(30);
RC1 = 0;
__delay_us(30);
RC2=1;
__delay_us(30);
RC2=0;
__delay_us(30);
break;
}
case 10 :
{
RC1 = 1;
__delay_us(31.5);
RC1 = 0;
__delay_us(29.5);
RC2=1;
__delay_us(31.5);
RC2=0;
__delay_us(29.5);
break;
}
case 11 :
{
RC1 = 1;
__delay_us(33);
RC1 = 0;
__delay_us(27);
RC2=1;
__delay_us(33);
RC2=0;
__delay_us(27);
break;
}
case 12 :
{
RC1 = 1;
__delay_us(34.5);
RC1 = 0;
__delay_us(26.5);
RC2=1;
__delay_us(34.5);
RC2=0;
__delay_us(26.5);
break;
}
case 13 :
{
RC1 = 1;
__delay_us(36);
RC1 = 0;
__delay_us(24);
RC2=1;
__delay_us(36);
RC2=0;
__delay_us(24);
break;
}
case 14 :
{
RC1 = 1;
__delay_us(37.5);
RC1 = 0;
__delay_us(23.5);
RC2=1;
__delay_us(37.5);
RC2=0;
__delay_us(23.5);
break;
}
case 15 :
{
RC1 = 1;
__delay_us(39);
RC1 = 0;
__delay_us(21);
RC2=1;
__delay_us(39);
RC2=0;
__delay_us(21);
break;
}
case 16 :
{
RC1 = 1;
__delay_us(40.5);
RC1 = 0;
__delay_us(20.5);
RC2=1;
__delay_us(40.5);
RC2=0;
__delay_us(20.5);
break;
}
case 17 :
{
RC1 = 1;
__delay_us(42);
RC1 = 0;
__delay_us(18);
RC2=1;
__delay_us(42);
RC2=0;
__delay_us(18);
break;
}
case 18 :
{
RC1 = 1;
__delay_us(43.5);
RC1 = 0;
__delay_us(17.5);
RC2=1;
__delay_us(43.5);
RC2=0;
__delay_us(17.5);
break;
}
case 19 :
{
RC1 = 1;
__delay_us(45);
RC1 = 0;
__delay_us(15);
RC2=1;
__delay_us(45);
RC2=0;
__delay_us(15);
break;
}
case 20 :
{
RC1 = 1;
__delay_us(46.5);
RC1 = 0;
__delay_us(14.5);
RC2=1;
__delay_us(46.5);
RC2=0;
__delay_us(14.5);
break;
}
case 21 :
{
RC1 = 1;
__delay_us(48);
RC1 = 0;
__delay_us(12);
RC2=1;
__delay_us(48);
RC2=0;
__delay_us(12);
break;
}
default :
{
RC1=0;
RC2=0;
}
}

}
}