problem with serial port connection with PC on Arduino Mega 2560

To replicate the issue:

- Use Arduino Mega 2560 with example Physical Pixel
- Try to send to board from Serial Terminal (works)
- Open an application, e.g. Java's Processing, that can connect to the port
- Run the Processing code at the bottom of the Physical Pixel example, it is supposed to go on when hovering on the square in the center of the application's window (it actually executes port.write("H") in Processing)
- The pin 13 LED will be always on, the TX LED is always on, which is an undesired behavior
- The expected behavior is that pin 13 should be off until "H" received from serial
- This has been replicated in two different boards using 1.8.5 and 1.8.3, both OSX and Linux
- This same example works fine on Arduino UNO in all of the above systems

Potential reason for the issue:

- I have the feeling this has to do with the serial-usb converter or with the bootloader on the Mega, but have no equipment to test with me

Start by reproducing the issue with an Arduino Mega 2560, the Physical Pixel example, and a Processing serial connection, then compare the behavior with an Arduino UNO on the listed OSX or Linux setups. Verify whether the pin 13 and TX LEDs remain on before receiving "H"; done means the Mega matches the expected Physical Pixel behavior.

Written by the indexing model from the issue text.