are you sure, Eric ?

Timer runs only for 50% of the cycle...
So, in this application, f=1/(2T) ?

What about the hysteresis in the shmitt-trigger ?

Better solution (i think) is:
detect falling edge on an INT pin
check if timer already is running :
No : reset and start timer
YES : stop timer, read it and calculate f=1/T

Using a 12MHz X-tal on the 8051 gives a count rate on the timer(s) of 1MHz : 1us...