I'm kind of curious as to why there is a 0.1uF capacitor on this Arduino Pro Mini (https://www.arduino.cc/en/uploads/Main/Arduino-Pro-Mini-schematic.pdf). It seems like it would work just fine without it.
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2"This question does not show any research effort..." ie, you really should do a web search before posting a question here, as if you had you would already know the answer. – Chris Stratton Aug 19 '16 at 16:51
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1One aspect of it wouldn't work fine without it. Do a little digging in the Arduino documentation, and you'll find out what. – Neil_UK Aug 19 '16 at 17:01
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1The capacitor provides one of the most basic functionalities of the Arduino. If you don't know what that is, then google it. – Majenko♦ Aug 19 '16 at 17:44
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Most USB to serial chips set the DTR to LOW on connecting, but it keeps it LOW.
This would reset the arduino, but since the DTR never goes HIGH the Arduino will never get out of this reset state, and not run your code.
The capacitor will AC couple the signal. The the DTR goes low, the other side of the capacitor will go LOW too, but the pull-up resistor will slowly charge the capacitor, till the reset pin is HIGH again.
So it will convert a LOW transition to a LOW pulse.
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2It is not the USB serial chip, it is the operating system serial driver's most common default mode settings. And regardless of that, the Arduino programming process typically toggles it explicitly. – Chris Stratton Aug 20 '16 at 17:41
