Unexpected flashing of pin 13 LED with USB power and Win 8

I am getting strange behavior on my Arduinos when they are USB powered using Windows 8. This does NOT happen on Win 7 or with external power. Every 60 seconds, the LED on pin 13 flashes a random series of pulses lasting about 3-4 seconds. Pwr LED remains constant on. Tx & Rx LEDs remain off.

This happens on the Duemilanova, Ardweeny, and Mega 1280 with or without a sketch loaded.

If the blink example sketch is loaded, the random series of pulses overides the regular programmed blinking.

I get flashing if I run this sketch:
void setup() {
Serial.begin(57600);
}
void loop() {
//Serial.println (millis()); // note not printing, but flashing
delay(500);
}

I do NOT get flashing if I run this sketch:
void setup() {
Serial.begin(57600);
Serial.println (millis()); // note printing, but no flashing
}
void loop() {
Serial.println (millis());
delay(500);
}

Any help or hints would be greatly appreciated.

See this thread:

On the Uno R3 at least, it appears that a floating pin 13 might cause the LED to flash. It may be picking up some sort of noise. There is an op-amp amplifying/buffering that pin, so it can turn on for no obvious reason.

Try making it output.

Sounds like maybe something in your PC is scanning for open comm ports or opening and closing comm ports, as that would generate a auto-reset to an attached arduino board. A scope on the reset pin might be able to confirm that for some reason you are getting the DTR pulse toggled thus causing a board reset condition.
Got any bluetooth drivers on your PC, they seem to be infamous for doing stuff like that.

Edit: Of maybe what Nick said.

Lefty

Nick: When pin 13 is written LOW, I get the same flashing with the LED being normally off.
When pin 13 is written HIGH, I get the same flashing with the LED being normally on.
It seems that whatever is causing the flashing over rides whatever is programmed.

Retrolefty: No Bluetooth. I agree that it looks like some form of polling since it is so periodic and not random.

DTR is indeed being toggled hi to lo when using FTDI board ( I tried three different ones). 5V, CTS, Tx and Rx are constant during blinking episodes.

I have no way of monitoring USB connections, but will make up one today and report details. I have movie of blinking, but cannot get it to upload to Youtube as I have be doing in the past (yet another issue..sigh)

Thank you both for the responses. If anyone has Win 8, it would be interesting to confirm or deny all of the above...

update: I have monitored both USB data lines and they are active during blinking. I have not been able to capture the waveforms on my primative scope, but I have posted a video of the blinking here:

butch:
Nick: When pin 13 is written LOW, I get the same flashing with the LED being normally off.
When pin 13 is written HIGH, I get the same flashing with the LED being normally on.

Output and low.

Nick: Yes I understand. The code I used was:

void setup() {
// put your setup code here, to run once:
pinMode(13,OUTPUT);
digitalWrite(13,LOW);
}
void loop() {
// put your main code here, to run repeatedly:
}

As stated above, it does not matter if the pin is written HIGH or LOW or not at all, I still get the flashing.

Blinking stopped after I did the following:

Control panel > Ports(Com...) > properties > port settings > Advanced > miscellaneous options (lower right corner) > UNCHECK Serial Enumerator.

This was checked by default on my Toshiba Satellite.

If anyone can duplicate this, please post your results here so others may benefit.

butch:
update: I have monitored both USB data lines and they are active during blinking.

USB is a complex serial protocol. There is periodic traffic no matter what is going on. Activity on the bus tells you nothing except that the is pinging the device.