what will happen if pulse get high and do not be low for long time ?
[u]pulseIn()[/u] says you can go up to 3 minutes, or you can set a timeout. (But, you'll need a type long to hold more than 65,535 microseconds. 
).
i want to measure 50khz to 200khz ! and this part shouldnt take time more than 5ms !
If my math is correct, the period of a 50kHz square wave is 0.02mS so that's the longest it can take to "measure" (excluding any processing/calculating or I/O, etc.). In any case, it will NEVER be 5ms if the minimum frequency is 50kHz.