I recently bought a new Arduino Mega R3.
I noticed that when I upload a Blink sketch to it, pin 13 blinks on and off. perfectly.
then when I upload a blank sketch:
void setup(){}
void loop(){}
LED on pin 13 STILL lights up! I have 3 of these arduinos and the problem is the same with all of them.
So, I decided to connect an external LED to pin 13, and the built in LED and the external LED were both off.
Then I connected just a resistor from pin 13 to GND. Pin 13 remained off, like it's supposed to.
Does anyone know if there is an issue with the Arduino R3? It looks like its missing a pulldown resistor...
or is this just part of the design?
I know this is not normal because I have an older arduino mega and it does not do this...
The LED on the board is driven by an op-amp which is "floating" if you don't drive the pin high or low. Thus the LED may well light. This op-amp takes the load off the pin, however it does have this side-effect.
If you configure the pin for output and set it high or low it (the LED) will obey you. Earlier revisions of the Uno did not do that.
It's a good thing its not a design error. I soldered an SMD resistor (random value) between 13 and GND just to settle my nerves.
Thanks
Well I would consider it a design flaw in not an outright design error. Changing to using the unused op-amp section to drive the led was a good idea/good intentions, but they really should have wired a high value resistor from the op-amp input pin and ground, as the pin 13 led is often mentioned in tutorials about how the bootloader works, etc and I can understand the be-wilderness of beginners if their rev 3 boards are exhibiting this always on led 13 symptom.
I soldered an SMD resistor (random value) between 13 and GND just to settle my nerves.
What, exactly, is a random value resistor? Does the resistance change randomly?
No, I suspect he just reached into his SMD resistor drawer and picked a random one from the pile, decidedly not a good practice as too low a resistance value could damage pin 13.
