TIMER1 intterupt of PIC 16f88 in C

Hi;
I'm trying to work ultrasonic sensor (HCSR-04) with PIC16f88. Since other interrutps (RB0 and INT0) were used I'm trying to operate HCSR04 with Timer1.When I connected trigger and echo to oscillator everyting seems OK. I can see trigger and corresponding echo related with distance. I think interrupt never occurs. When I read data sheet of microcontroller I understand as RB6 is related with TMR1 interrupt. Here is my code.


#include <xc.h>

#define _XTAL_FREQ 8000000 // crystal freq defined as 8 Mhz
int b;

void main()

{

OSCCON = 0b01111000;// 8 Mhz selected Internal Oscillator Freq

INTCON = 0b11000000; // GIE and PEIE is enabled

TRISB = 0b01000000;// RB6 selected as input(echo connected here)

T1CON = 0b01001000; // // System clock is derived from Timer1 oscillator with
// 1:1 prescale value and oscillator is enabled

PIE1bits.TMR1IE=1; // tmr1 overflow is enabled


ei(); // interrupt is enabled


while(1)

{

TMR1H = 0; //Setting Initial Value of Timer
TMR1L = 0; //Setting Initial Value of Timer
b = 0; // TM1 value is taken in b (look at interrupt at below)

RB5=1;
__delay_us(10); //trigger is sent
RB5=0;
__delay_ms(100); //Waiting for ECHO








if(b>2 && b<=20) // If measured distance is between 2-20 cm led
// at RB1 blinks
{
RB1=1;
__delay_ms(500);
RB1=0;
}

if(b>20 ) // If measured distance is higher than 20 cm led // at RB2 blinks
{
RB2=1;
__delay_ms(500);
RB2=0;
}

} // while closed

}// main closed

void interrupt echo()
{

if(PIR1bits.TMR1IF == 1) // Code enters if when flag condition occured

{
//PIE1bits.TMR1IE=0; // I also tried with that but it didn’t work

T1CONbits.TMR1ON = 1; //Start Timer

if(RB6 == 0) //If ECHO is LOW
{
T1CONbits.TMR1ON = 0; //Stop Timer
b = (TMR1L | (TMR1H<<8))/58; //Calculate Distance
}



PIR1bits.TMR1IF=0; //Clear PORTB On*Change Interrupt flag
//PIE1bits.TMR1IE=1;// Enable is opened if it is closed at top

}
}