help for programming 16F877

Hi, I calculated the phase shift between two sine (50 Hz) and displayed on an LCD, I compile my prog on picc the problem is I can not calculate the phase shift for low frequency (0.05 Hz) had good results by cons for 50 Hz are obtained false values ​​of phase shift here is my source code and the diagram on isis
*

#include <16f877.h>
#device ADC=10
#fuses xt,nowdt,NOPROTECT,noput
#use delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6,rcv=PIN_C7)
#include <lcd.c>
#include <math.h>


int32 n=0;


#int_timer0
void timer0_isr()
{
n++;
}


void main ()
{
int1 test_1, test_2;
//***********programme de dephasage******************//
//float a,b,c,e,f,g,R,X;
//float tens_max,courant_max,dephasage,Veff,Ieff ;

float temps, dephasage;
float tens_****_10,tens_****_1,tens_num_1,tens_num_2,tens_****_2,tens_****_20;
int8 Iter;
float Freq=10;

tens_****_10 = 0.1;
tens_****_20 = 0.1;

setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
enable_interrupts(int_timer0);
enable_interrupts(GLOBAL);


disable_interrupts(int_timer0);
clear_interrupt(int_timer0);

setup_port_A(all_analog);
setup_adc(adc_clock_internal);


while(true)
{
while(!input(pin_C0))
{
test_1=1;
test_2=1;
set_adc_channel(0);
delay_us(10);

while(test_1==1)
{
tens_num_1 =read_adc();
tens_****_1 = (tens_num_1 *(0.0048876)*2)-5;


lcd_init();
printf(lcd_putc,"Tens1=%2.5f\r",tens_****_1);
//printf("T1=%2.3f\r",tens_****_1);

if ((tens_****_1>=0) && (tens_****_10<0))
{
set_timer0(0);
n=0;
clear_interrupt(int_timer0);
enable_interrupts(int_timer0);
test_1=0;
tens_****_10=tens_****_1;
//lcd_init();
//printf(lcd_putc,"Tens1=%2.5f\r",tens_****_1);
printf("Detected Tens1=%2.5f\r",tens_****_1);
}
else
{
tens_****_10 = tens_****_1;
}
}

set_adc_channel(1);
delay_us (10);

while (test_2==1)
{
tens_num_2=read_adc();
tens_****_2 = (tens_num_2 *(0.0048876))-2;

lcd_init();
printf(lcd_putc,"Tens2=%2.5f\r",tens_****_2);
//printf("T2=%2.3f\r",tens_****_2);

if ((tens_****_2>=0) && (tens_****_20<0))
{
Iter=get_timer0();
disable_interrupts(int_timer0);
clear_interrupt(int_timer0);
temps = 0.0000512*n+Iter*0.0000002;
dephasage =2*PI-((2*PI*Freq)*temps);
test_2=0;
tens_****_20=tens_****_2;

//lcd_init();
//printf(lcd_putc,"Tens2=%2.5f\r",tens_****_2);
printf("Detected Tens2=%2.5f\r",tens_****_2);
printf("temps=%3.5f\r",temps);
}
else
{
tens_****_20=tens_****_2;
}
}
lcd_init();
printf(lcd_putc,"Deph=%3.5f deg\r",dephasage*57.295779513);
printf("Deph=%3.5f deg\r",dephasage*57.295779513);
}
}
}


the isis diagram https://obrazki.elektroda.pl/8_1303829582.png

There'a lot of time consuming actions (LCD related, float arithmetic) interleaved with measurement and timer actions, I can't see, that these critical code parts are precisely timed.