ac voltage and frequency measurement using clamping and 18f452

dear friends,
i want to measure ac voltage and frequency using clamping principle.i used 2 1k resistor to clamp at vcc/2.i am using this program. as result i am getting correct ac value,some times. but sometime i am getting wrong result. can ant body spot any error
i am using ccs compiler and pic18f452
void main()
{
int16 value,adc_val,adc_volt,count=0,freq;
int32 cum_adc=0;
float temp;
int8 a,b,c,d,e,data;
setup_adc_ports( ALL_ANALOG );
setup_adc(ADC_CLOCK_DIV_32);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_2); //26.2 ms overflow
//lcd_init();
//delay_ms(1000);
clear_interrupt(int_timer1);
set_timer1(0);
START:set_adc_channel(0);
delay_us(10);
adc_val=read_adc();
if((adc_val>512) & (adc_val<552))
{
adc_volt=read_adc();
if(adc_volt<adc_val)
goto START;
clear_interrupt(int_timer1);
set_timer1(0);
cum_adc=cum_adc+adc_val;
count++;
cum_adc=cum_adc+adc_volt;
count++;
while(adc_val>=512)
{
adc_val=read_adc();
cum_adc=cum_adc+adc_val;
count++;
if(interrupt_active(INT_TIMER1))
{
cum_adc=0;
d-0;
e=0;
goto skip;
}
}
freq=get_timer1();
temp=freq*.0000008;
freq=1/temp;
d=freq/10;
e=freq%10;
skip:lcd_display_char(2,4,'0'+d);
lcd_display_char(2,5,'0'+e);
value=cum_adc/count;
cum_adc=0;
count=0;
value=(value-506)*1.2550;
c=value/100;
lcd_display_char(3,4,'0'+c);
data=value%100;
b=data/10;
lcd_display_char(3,5,'0'+b);
a=data%10;
lcd_display_char(3,6,'0'+a);
}
goto START;
}