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

 

It couples to a change in the reset signal but removes any DC component.

 

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.

 

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.

 

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.

 

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.

 

And, it's a short pulse.

Yep. I did that in two places in my 1985 night detector circuit.
The, "fast pulse generator" calculates at less than a microsecond in the negative going edge.

 

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.

 

Yes, the circuit is connected to a reset pin.

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