Using external clock for 16 bit counter in Atmega 8

[alfie.max15]

I am using Atmega 8 16 bit counter.
It works fine with the internal clock.
But when tryin to use a signal as external clock on T1 pin on the uC the pin stays high all the time.

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Code C - [expand]
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DDRD &= ~(1 << DDD5);     // Clear the PD5 pin      
   // PD5 is now an input 
    
   PORTD |= (1 << PORTD5);   // turn On the Pull-up      
   // PD5 is now an input with pull-up enabled 
    
   TCCR1B |= (1 << CS12) | (1 << CS11) | (1 << CS10);     // Turn on the counter, Clock on Rise

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I used the above code so as to use the external clock on pin T1