Arduino nano (clone) needs serial connection

Development Tools IDE 1.x
1

Hi, I have got a strange bug.
I bought Arduino nano clone for less than $2 with the CH340 USB serial converter to see if it is even possible - I thought the ATMega alone costs more (it definitely does in our local store). Everything looks fine apart from one thing.
I need to have Serial Monitor opened for the Arduino to work - even if Serial is not used at all. If I close the Serial Monitor the Arduino stops doing whatever it does and start blinking the "L" LED regularly with about 1 or 2 Hz frequency. When I open the Serial Monitor again the sketch runs as expected (I know it restarts the Arduino) and after closing it the blinking starts again. There is my minimal example:

void setup() {
  pinMode(13, OUTPUT);
  digitalWrite(13,LOW);
  pinMode(8, OUTPUT);
}
void loop() {
  digitalWrite(8,HIGH);
  delay(1000);
  digitalWrite(8,LOW);
  delay(100);
}

With Serial Monitor open the LED connected to pin 8 is blinking while onboard LED is off. As soon as I close Serial Monitor the external LED turns off while the onboard LED is blinking (with duty cycle approx 5-10%).
Thanks for any advice!

EDIT: I tried measuring voltages on pins. In general voltages are the same as on my (working) UNO: stable 5V is on 5V output and TX and RX pins (digital 0 and 1), on pin 13 is alterning 0 and 5 (as the LED is blinking), all other I/O pins are floating.

2

Apparently there are problems with Arduino IDE 1.6.8. Try IDE version 1.6.5.

3

Thank you, it's really the Arduino IDE. When plugged into USB the Arduino is working as long as Arduino isn't runnig. When I start Arduino strange behavior happens (on board LED blinking) and when I close Arduino the external LED start blinking.
Does anyone know how is it possible? Will try it on older Arduino when I get time.

4

The reason it keeps blinking is due to a bug in how the 1.6.8 IDE looks at serial ports. They totally messed it up on windows, and the IDE polls the serial ports every couple of seconds in a way that makes the serial adapter act like it's opening the port. When a serial port is opened, it resets the arduino board (unless autoreset has been disabled) - and the bootloader which runs immediately after reset blinks the led.

5

Any workaround other than downgrading the IDE?

6

Go up to 1.6.9?