Arduino Mega R3 reset issue (serial monitor & different for switch vs auto)

Hi all,

I've encountered a curious issue with a new Arduino Mega R3 that I've not yet been able to solve.

When I open the 'serial monitor' with the new Arduino connected, it takes about 9 seconds after the window has opened for the Arduino to reset. However, it resets immediately if I press the reset button.

For instance, with the below sketch it takes ~9 seconds until "hello" comes through, followed by the stream of data after 1s:

void setup() {

void loop() {

  int sensorValue = analogRead(A0);
  Serial.println(sensorValue, DEC);

If I upload the blink sketch, the LED starts blinking immediately after the upload but again suffers this delay if I open the serial monitor.

The delay itself isn't a huge issue but none of my C++ programs can connect through serial and I suspect these problems are linked. It works with my other Arduinos.

Any help/suggestions/comments would be much appreciated

I think this is because the re-enumeration of the com ports. The old arduinos with FTDI chips had some options in the driver to avoid this, right now I always see how my arduino mega 2560 resets when I do some stuff over the serial port (open/close, etc).


Thank you for the reply.

So this should affect all R3s? If so, that seems a bit rubbish. It's a shame I don't have another to try.

I'll have a prod around the code to see if I can work around it. If not then I guess I'll have to bin it and buy another of the old ones.

For sure, but this is certainly something like a parameter in the driver descriptor. I remember with my old arduinos disabling the re-enumeration.

I found the thread detailing your old issues and subsequent modifications: I applied your fix and Serial Monitor now connects immediately but seems to be lacking any form of reset.

Following the above, I realised that my Unos are version 3. I tried them and they don't have any delay even without the above mod. As a result I'm thinking there might be a hardware issue on my Mega, maybe with the reset capacitor or something.

However, the reason I was looking in to this was because my serial connections in C++ were failing with the new board. I tried this with the Uno R3s and these didn't work either. As a result, I figured the two issues aren't connected and after a few hours of investigations, I've found that the serial library I'm using doesn't like COM ports numbered above '4'. Changing the port number of the devices to 3 or 4 solves the connection issue on all boards.

Thanks again