I just received my new Uno Rev 3 board and I noticed that the on-board LED associated with pin 13 stays on when any sketch is running. This is not the case with my original Uno. The LED flashes normally with the blink program, but is on steadily when any sketch not using the LED is run. Is this a normal condition for Rev 3?
It appears to be and several have reported this LED behaviour lately. The Uno and Mega R3 boards use a op-amp section to drive the pin 13 LED rather then driving it directly from the pin 13 output. So the op-amp is going to see a 'floating input pin condition' if pin 13 remains in input pin mode as it does when the bootloader finishes running and jumps to the loaded sketch. So if your sketch does nothing to pin 13 then there is a chance that the pin 13 may light up, blink, or remain off according to the phase of the moon and other unknown factors.
So if the behaviour is a problem for you you can do one of the following:
1. Set the pin 13 mode to output and set it LOW in your sketch code.
2. Wire a high value resistor say 10K ohms or higher between shield pin 13 and ground, which will act like a pull-down forcing the op-amp off.