user32126: I know this that when you try to program a pro mini without a DTR pin it will give you a error as the atmega needs to be reset during programming. So I had a doubt why does the microcontroller needs to be reset and clearly at which time you know at which moment does it reset ?

user13432: A Reset is necessary to activate the Bootloader and allow programming. Without the DTR signal the Reset button has to be pressed just before starting the upload from the Arduino IDE. 

32126: <p>I know this that when you try to program a pro mini without a DTR pin it will give you a error as the atmega needs to be reset during programming. So I had a doubt why does the microcontroller needs to be reset and clearly at which time you know at which moment does it reset ?</p>
 13432: <p>A Reset is necessary to activate the Bootloader and allow programming. Without the DTR signal the Reset button has to be pressed just before starting the upload from the Arduino IDE. </p>
