Programming of built in timer of arduino

[Don (don_1414)]

Hello Everyone ,
I am new to programming in IDE. I am using built-in timer of arduino.I 
am using TDR method to capture reflection from fault location in cable 
and for that I am using timer.Timer should start as soon as output is 
send and stop at reflection. I have a reference code for it but I dont 
able to understand it, so if anyone know about it , it would be great.I 
have attached the code for it.
Thank you

[FOp (Gast)]

In your example there is code that directly uses the hardware without 
help of Arduino functions. This results in being bound to one kind of 
microcontroller. So this software won't run on all Arduinos. Looks like 
its for ATmega328P, which is used in NANO, UNO and MINI, but not in 
the DUE.

If you like to understand, what's going on there, you should download 
the complete datasheet of the ATmega328P from the microchip website and 
study the chapter about the timer used in your code in detail.