Why is there a 100nF capacitor in the Arduino Severino reset line?

Question

While looking at the Arduino Severino (Single-Sided Serial Board V3) schematics I notice it has a ceramic 100nF capacitor in the reset line (marked in red in the picture below).

My questions are:

1. What is the capacitor for?

2. Would the auto-reset line still work ok if one removed the capacitor?

The context for this question is that I'm trying to find what's wrong with an RS232-to-TTL converter board of mine (see Problems with homemade RS232-to-TTL converter board based on TI's MAX232), and was studying the reset line (which is similar to the reset line in my board) when I stumbled across this capacitor.

Answer 1

The reset capacitor is there to block DC signals. The capacitor acts as a pulse transmitter so to speak, where the auto-reset line will be pulled low briefly by an external source, and the capacitor will act as an AC pull-down, for the effect of a single reset trigger on the microcontroller. If there was a reset button, and it was pressed down by a human (or other object) then it will exert low for as long as you hold it down. If the external trigger device through the capacitor were to hold the reset line low, the capacitor would only show the 'edge' and would return to the normal reset voltage.

Thus, it is important to have that capacitor there. I did the same on my own "arduino pro mini" board I made, so that my FTDI serial programmer could use the DTR signal and reset my ATMEGA328P ready for Arduino bootloading a new program.

I hope my explanation was clear enough. please tell me if it's not, I will clarify for you.