Due: SerialUSB port busy after disconnecting and trying to reconnect

[odbol]

This is a regression in 1.6.6. 1.5.5 works correctly.

On Arduino Due, upload any sketch:

void setup() {
  // put your setup code here, to run once:
SerialUSB.begin(115200);
}

void loop() {
  // put your main code here, to run repeatedly:
  SerialUSB.println("hello");
  delay(100);
}

1. Upload it to the board using Native USB port.
2. Run Serial monitor.
3. It should connect and display the output correctly.
4. Close Serial Monitor.
5. Open Serial Monitor again.

Expected: it connects correctly.

Actual: get a Error opening serial port '/dev/cu.usbmodem1d111'. (Port busy) error. This happens in other apps too, like "Serial Tools" for OS X. It seems that when you close the connection on the PC side, the Arduino is not properly cleaning up and freeing the connection.

Tested on Arduino Due, OS X 10.10.5, with Arduino 1.6.6 (Due core 1.6.5).

[odbol]

Oops, looks like this also works in Arduino 1.6.2, so it must have been a change after that. (Specifically, git revision 72bf977)

[sandeepmistry]

Confirmed, I've narrowed it down to the following commits: 2acda41 or a261e6c

[sandeepmistry]

If I go back to b238150 and disable HID_ENABLED I see the same issue as well.

[odbol]

May be related: https://forum.pjrc.com/threads/31043-Teensyduino-1-26-Beta-1-Available

[sandeepmistry]

I've tracked it down to OS X sending a CDC_SEND_BREAK on close, and the SAM core not handling it correctly.

The following patch corrects the CDC_SEND_BREAK handling: #4171