Hello.
I think this may be usefull to others also.
I found a few old topics about the subject. Also my ethernet shield works only if I connect the Arduino board to the PC by way of USB, it doesn't work if it's powered with an external power supply.
What I found is:
- it's not a problem related to power supply or clean power or ground
- the program correctly runs
- only the ethernet shield doesn't connect to the ethernet network
- a manual reset by pressing reset button fix it
- I tried with Arduino Mega and Arduino Duo.
It seems that the USB connection to PC sends a late reset to the board: may be it's the PC USB driver that when connected resets the board. This late reset makes the ethernet shield work.
So apparently my ethernet shield is not able to correctly reset itself at power up.
I solved the problem with a simple external RC network connected to the reset pin, to externally reset the board after power is established. R is 10K and C is 47 microF.
Connections:
+5V----R-----C----GND
Get the signal between R and C and connect it to the RESET pin.
Thank you.
Roberto