Arduino Uno SCK(Pin13) Blink when open COM port

Projects General Guidance
1

Hello,

I have create the arduino project for control some relays, and use the digital port number 13(SPI SCK) conected in relay, but when open the COM port by USB this relay conected in PIN 13 blink.

I check the board and this pin have the LED L also conected.

I know this pin is the Clock for SPI, but not have ideia why pulse when open the COM.

My question is, is possible to disable it? and use the pin 13 only for digital port... like other pins?

Thanks

2

The bootloader code blinks pin 13. The bootloader code runs when the microcontroller on your Arduino board resets. The microcontroller resets whenever a serial connection is made to it. The reason for this is so that the bootloader will automatically started when you upload a sketch to your board.

If you don't want the board to reset when a serial connection is opened to it, you can temporarily disable the auto reset circuit by connecting a 10 uF capacitor between the reset and ground pins on your Arduino board. If you want to more permanently disable the auto reset circuitry, you can cut the solder jumper marked "RESET-EN" that you will find on most Arduino boards. After disabling the auto-reset circuit, you will need to manually reset the Arduino board during the upload at just the right time. The right time is when you see something like this printed in the black console window at the bottom of the Arduino IDE window:

[Sketch uses 3462 bytes (12%) of program storage space. Maximum is 28672 bytes. Global variables use 149 bytes (5%) of dynamic memory, leaving 2411 bytes for local variables. Maximum is 2560 bytes.](http://Sketch uses 3462 bytes (12%) of program storage space. Maximum is 28672 bytes. Global variables use 149 bytes (5%) of dynamic memory, leaving 2411 bytes for local variables. Maximum is 2560 bytes.)

Another option would be to do without a bootloader. If you connect an ISP programmer to your board and do a Sketch > Upload Using Programmer, then the bootloader will be erased. Without a bootloader you will no longer be able to upload to your Arduino board via the USB cable. You will always need to use your ISP programmer. You can replace the bootloader on your board via Tools > Burn Bootloader.

Another option is to install a modified version of the bootloader that does not blink pin 13. You will likely need to compile the bootloader from source and then flash it to your microcontroller. That is an advanced process.