I just finished a project (ArduXWeatherStation) of a weather station that sends data on the portal Xively remotely and locally on a second Arduino Mega 2560 with TFT touch screen 3.2 ".
The project runs smoothly, I had still the problem of the ethernet shield that inserting the plug of the power supply 9V 1A would not start, I just had the first time, press the reset button.
Now I solved the problem by inserting a capacitor on the RESET pin of the ethernet shield and GND.
On a post in this forum is listed capacitor to use and that is a 47nF, trying this ceramic capacitor I saw that the project starts regularly but led the shield flash intermittently for at least 5 to 6 seconds.
Trying instead an electrolytic capacitor 100uF or 330uF times were almost instantaneous and with that from 470uF in practice the project starts immediately.
Some of you know explain why? The electrolytic capacitor on the shield to act as a bridge between RESET and GND is more appropriate than ceramic or not?
I would be grateful if someone help me understand.