user45486: 
Can anyone tell me why 1k resistor is used on pin 13 of ATmega16U2 in arduino uno schematics. I have also seen a 100k resistor used in this position? 
Link for schematics - https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

user-1: Its a pulldown resistor!
Comment there says "USB Boot Enable" so safe assumption that it's an digital pin.
They're handy because they mean if there is no input to whether that pin is headed, the voltage at the pin will be 0 instead of floating.
The value doesn't matter too much, "When in doubt 10k it out" is a pretty well known saying around electronics people, meaning just use a 10k Ohm resistor and you'll be fine.

45486: <p><a href="https://i.stack.imgur.com/494pj.png*" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/494pj.png*" alt="Arduino Uno Schematics"></a></p>

<p>Can anyone tell me why 1k resistor is used on pin 13 of ATmega16U2 in arduino uno schematics. I have also seen a 100k resistor used in this position? </p>

<p>Link for schematics - <a href="https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf" rel="nofollow noreferrer">https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf</a></p>
 -1: <p>Its a pulldown resistor!</p>

<p>Comment there says "USB Boot Enable" so safe assumption that it's an digital pin.</p>

<p>They're handy because they mean if there is no input to whether that pin is headed, the voltage at the pin will be 0 instead of floating.</p>

<p>The value doesn't matter too much, "When in doubt 10k it out" is a pretty well known saying around electronics people, meaning just use a 10k Ohm resistor and you'll be fine.</p>
