TMR1 no está incrementando PIC16f877a

Estoy desarrollando un buscador de rango ultrasónico. Estoy usando PIC16f877a y el compilador MikroC. Estoy usando el pin CCP1 en modo PWM para generar un pulso ultrasónico. Este pulso se reflejará desde un obstáculo y se recibirá en el pin CCP2. Por lo tanto, estoy usando CCP2 en modo de captura. El problema es TMR1H y los valores de TMR1L permanecen en cero en todo momento. es decir, mi temporizador no se incrementa y no entiendo por qué es eso. También estoy confundido sobre el temporizador y los registros. Por lo que yo sé, TMR1 se va a utilizar. Aquí está el código:

unsigned int startTimerValueH;            //Global variable to store values of timer for calculation
unsigned int startTimerValueL;
unsigned int endTimerValueH;
unsigned int endTimerValueL;
unsigned int time_interval;
unsigned int display_value;
unsigned int distance;
unsigned int arr[4];


void Gen_UltraSound ()
{

     TRISB = 0x00;
     PORTB = 0;
     CMCON = 0x07;
     ADCON1 = 0x06;
     PWM1_init(40000);                  //initialize PWM module
     PWM1_set_duty(128);                //set the duty cycle to fifty percent

     T1CON.TMR1CS = 1;                  //external clock is used as clock source for timer1
     T1CON.T1SYNC = 0;                  //synchronizes the timer1 with external clock
     T1CON.T1CKPS1 = 0;
     T1CON.T1CKPS0 = 0;
     PIR1.TMR1IF = 0;                   //disables overflow
     T1CON.T1OSCEN = 0;                 //disables oscillator
     T1CON.TMR1ON = 1;                  //turns on the timer1

     startTimerValueH = TMR1H;          //stores the initial value of the timer as a binary number
     startTimerValueL = TMR1L;

     PWM1_start();                      //start generating the signal
     delay_ms(20);                      //delay during which the pulse is sent out to the speaker
     PWM1_stop();                       //stops the signal generator
}

 void displayDistance()
{
      TRISD=0x00;

      TRISB=0x00;
      PORTB=0x00;

      PORTD = 0x01;

}


void distanceMeasurer ()
{
       unsigned int remainder= 0;
       unsigned int quotient=0;
       unsigned int divisor=100;
       int i;

       distance = time_interval * 340;              //measures distance from the obstacle
       distance = distance / 2000;
       remainder = distance;

       for (i=0; i<3; ++i)
       {
            quotient = remainder/divisor;
            remainder = remainder%divisor;
            arr[i] = quotient;
            divisor = divisor/10;

       }

       displayDistance();                           //after measuring distance, display it

}


void main() {
     INTCON = 0b11000000;       //enables global interrrupts
     PIE2.CCP2IE = 1;           //enables peripheral interrupts so capture interrupt can occur

     CCP2CON.CCP2M3 = 0;        // sets CCP2 module to capture mode
     CCP2CON.CCP2M2 = 1;
     CCP2CON.CCP2M1 = 0;
     CCP2CON.CCP2M0 = 1;

     Gen_UltraSound();          //Ultra sound generator through PWM module
                 PIR1.CCP2IF = 1 ;   //for debugging purposes only
}

interrupt()
{
      TRISD = 0x00;
      if  (PIR1.CCP2IF == 1)
      {
           PIE2.CCP2IE = 0;                 //disables undesirable peripheral interrupts
           PIR1.CCP2IF = 0;                 //disables capture interrupts
           endTimerValueH = TMR1H;          //get timer value when echo received, in binary
           endTimerValueL = TMR1L;
           time_interval = endTimerValueL - startTimerValueL;  //time interval between transmission and reciept, in binary
           distanceMeasurer();                                 //measures the distance only when echo received
      }
}

EDITAR: He usado un reloj externo de 20MHz de cristal. Deseo sincronizar TMR1 con eso. El cristal está en los pines OSC1 y OSC2