Ethernet Shield + Uno stack power problem

Hi everyone,

I recently bought an Ethernet Shield and initially everything worked fine. When stacked on my Uno, all leds were blinking happily, i tested successfully the Webserver and SD cards examples. I wanted to power the stack with a 9V battery, so i plugged (+) in the Vin and (-) in Gnd of the Ethernet Shield…and then problems started. First, when on battery, nothing worked. When i switched to usb power, the leds on the Ethernet Shield did not come on at all, and the leds on the Uno faded out after a few seconds. I tried several times with the same results. I removed the Ethernet Shield, the Uno works fine and the voltage at +5v pin is healthy. When stacked with the Ethernet Shield, the voltage at +5v pin shows about 1.7 v.

I know i probably messed up (thanks Google for showing me that setup with the battery), but what ? And is it recoverable ? Would like to know before buying another one…any hint welcome.


I found this on the net:

“[…] Any 5V Arduino or Arduino-compatible board needs a minimum of a solid 4.50 Volts DC at the board at all times to run. If the voltage drops even a fraction below this for a very short time like when loaded up by an IC power-up, LED turning on, LCD backlight or whatever, the board will go into reset and won’t operate. The ATmega microcontroller in all Arduino boards has a built-in “brown-out detector” which is activated and set to a nominal 4.30V, below which the board will be held in reset and not run. The nominal 4.30V BODLEVEL fuse setting has a true range of 4.10V min, 4.30V typical, 4.50V. […] Also if you’re using an EtherTen or any other board with the current Arduino Ethernet Shield that has a microSD card, the STM811 Reset IC that it has controlling the W5100 as explained above has a 4.38V typical, 4.25V min, 4.50V max reset voltage. […] If your computer outputs just a little lower voltage than this and some do, or if your board is drawing a high current close to 500mA or have a high startup current requirement, the voltage at your Arduino board may drop lower than the 4.50V needed. This can also happen if the USB cable being used for connection and power is quite long or has weak conductors as you could get say a 0.5V drop along the length of the cable.”

Can it be that the shield is working just fine but that when stacked it draws enough current to have the Uno go and stay in reset?


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