Arduino UNO - Blinking TX after upload

I use CLion with PlatformIO Core CLI.

I upload simple Blink sketch by using target generated for CLion and got Constantly blinking TX LED. It doesn’t happen if I upload via Arduino IDE

Could somebody help me understand:

  1. Why TX is blinking
  2. What does it mean

My ini file
platform = atmelavr
board = uno
framework = arduino

sketch code
#include <Arduino.h>

void setup() {

void loop() {
digitalWrite(LED_BUILTIN, HIGH); // turn the LED on (HIGH is the voltage level)
delay(1000); // wait for a second
digitalWrite(LED_BUILTIN, LOW); // turn the LED off by making the voltage LOW
delay(1000); // wait for a second

Could you reproduce this issue with PlatformIO IDE, for example, VScode A professional collaborative platform for embedded development · PlatformIO ?

Ok, I think I figured out my “issue”. My blink sketch also sends messages to Serial. and I think this lights TX LED.