Function of a series capacitor in Arduino Schematics

Thread Starter

Tesla86

Joined Oct 25, 2016
34
Hello all,

Anyone knows which is the function of the marked capacitor in the pic attached? It has been taken from Arduino Mega Schematics, as you can see it is in series included in the RESET line, I guess it acts as a filter or something similar.

BW
 

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#12

Joined Nov 30, 2010
18,223
Another way to say it: When PD7 goes high, the capacitor allows a pulse to hit the reset pin on whatever it's going to, but you don't have to change PD7 back low to get the reset to stop. You have to set PD7 low some time before you need another reset pulse. When you need another reset pulse, change PD7 to high again.
 

ErnieM

Joined Apr 24, 2011
8,308
I would say without knowing what that line connects to you are just guessing what it does. Even knowing that, what is being done with the driving pin? PD7, if it is even the driving source. It could be the receiver of some reset function.
 

#12

Joined Nov 30, 2010
18,223
I would say without knowing what that line connects to you are just guessing what it does. Even knowing that, what is being done with the driving pin? PD7, if it is even the driving source. It could be the receiver of some reset function.
In that case, swap the words, "reset" and "PD7" in post #3.
The function is the same: A DC level changes, a pulse occurs.
 

wayneh

Joined Sep 9, 2010
17,175
The function is the same: A DC level changes, a pulse occurs.
And, it's a short pulse. You can play around with the cap and resistor to get the shortest possible pulse that accomplishes a reset, if that's what you're after. That can be handy if you want to do something with the next clock cycle that comes along, instead of potentially missing it if the reset is still held on.
 

wayneh

Joined Sep 9, 2010
17,175
I only knew the trick because I used it in my inductor ring tester (in Completed Projects, someone else's design). A pulse comes along that starts an LC tank ringing, and resets a 4017 timer. The shorter the reset pulse, the more likely you'll count the first oscillation. But if you get too short, it may fail to reset.
 

Thread Starter

Tesla86

Joined Oct 25, 2016
34
Yes, the circuit is connected to a reset pin.

Thank you very much, your explanations made this issue crystal clear!!!
 
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