no signal detection, need help

[Greyjo]

Hello,

I'm trying to build an ultrasonic distance meter by using 16f877a. I can transmit 40kHz signal from an external oscillator and get the

reflected signal with no problem. However, when i try to transmit a signal by setting the output pin (C1 at the program) 'high' for about

200us to activate oscillator and wait for the reflected signal, program stucks at 'while(input(echo)) {}' loop which indicates no signal

detected (constant 'high'=no signal). I'm using LM311 comparator, and its output is directly connected to the microcontroller's input pin

mentioned as 'echo'. Btw, signal detection can be observed from the comparator's output with a square wave (low=signal detected).

#include <16F877A.h>
#device adc=8
#FUSES NOWDT, HS, NOPUT, PROTECT, NOBROWNOUT, NOLVP, NOCPD, NOWRT, NODEBUG
#use delay(clock=20000000) 


#use fast_io(b)        
#use fast_io(c)         
#define trig PIN_C1     
#define echo PIN_C0     
#define use_portb_lcd TRUE 
#include <lcd.c>   

int16 distance, time;          



void main()
{
   
   setup_adc_ports(NO_ANALOGS);
   setup_adc(ADC_OFF);
   setup_psp(PSP_DISABLED);
   setup_spi(SPI_SS_DISABLED);
   setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
   setup_timer_1(T1_INTERNAL|T1_DIV_BY_1);     
   setup_timer_2(T2_DISABLED,0,1);
  
   set_tris_c(0x01);  
   set_tris_b(0x00);  
   lcd_init();      
      
   
   printf(LCD_PUTC, "\f TEST");

   
   while(true)     
{

   output_high(PIN_C1);                       //starting the transmission  
   delay_us(200);                            
   output_low(PIN_C1);                      
   
  
   set_timer1(0);                            // no interrupts used, timer1's current value should be the time passed to calculate distance.
   while(input(echo))                       // echo (C0) , the input which is waiting for square wave to detect
   {}

   time=(get_timer1()/5);                       

   distance=(time*0.0341)/2 ;            
                           
   printf(LCD_PUTC, "\fTime:%Lu us\nDistance =%Lu cm",time,distance); 
                                     
  
}
}

The other parts of the circuit and program is working well. I haven't been able to figure out the problem since it's also working quite good

on proteus simulation. This is my first work with microcontollers and i cannot find the error here. Can anyone give me some advice?

Sorry for long text