ESP32-C3 in USB CDC mode can hang if Serial.flush() called

[6v6gt-duino]

Board

ESP32-C3 Supermini (no USB/UART adaptor)

Device Description

ESP32-C3 Supermini is basically an ESP32-C3 chip with a USB-C socket and little else.
https://www.tindie.com/products/adz1122/esp32-c3-development-board-esp32-supermini/

Hardware Configuration

USB-C cable to either a PC (windows 10) or a simple phone charger

Version

v2.0.14

IDE Name

Arduino IDE 1.8.19

Operating System

Windows 10

Flash frequency

80MHz

PSRAM enabled

no

Upload speed

921600

Description

Using "USB CDC on boot : enabled":
If the supermini is powered via a phone charger with no PC connection, Serial.flush() appears to cause a hang.

Sketch

/*
    esp32-c3 supermini test
    similar to: https://www.tindie.com/products/adz1122/esp32-c3-development-board-esp32-supermini/

    set up IDE with
    (a) Board ESP32-C3 dev module
    (b) "USB CDC on boot : enabled"
    (c) JTAG adapter disabled

    Test generates lots of serial output and then forces a Serial.flush

    This test appears to function correctly whether or not Serial.setTxTimeoutMs(0) is set (but see comment below for case(c)) :
    (a) if the supermini is connected via USB to a PC and the serial monitor is active. 
    (c) if the supermini is powered via a phone charger with no PC connection.

    This test appear to work only if Serial.setTxTimeoutMs(0) is set :
    (b) if the supermini is connected via USB to a PC and the serial monitor is NOT active. 
    
    Serial.flush() appears to cause a hang in case (c)

*/


const uint8_t led = 8 ;  // led wired HIGH side





void setup() {

  pinMode( led, OUTPUT ) ;
  digitalWrite( led, HIGH ) ;

  Serial.begin( 115200 ) ;

  // only defined if option "USB CDC on boot" selected in IDE
  // Serial.setTxTimeoutMs(0) ; // https://github.com/espressif/arduino-esp32/pull/7583

  while ( !Serial && millis() < 10000 ) {
    delay(500) ;
  }

  // pulse led for 4 seconds if serial connection found.
  if ( millis() < 10000 ) {
    Serial.println( "found Serial") ;
    digitalWrite( led, LOW ) ;
    delay( 4000 ) ;
    digitalWrite( led, HIGH ) ;
    delay( 1000 ) ;
  }
  else {
    // no Serial found
  }

}



void loop() {

  for ( int j = 0; j < 10 ; j++ ) {
    for ( int i = 0; i < 500 ; i++ ) {
      // Serial.print( i ) ;
      // Serial.print( '\t' ) ;
      // Serial.println(  "the quick brown fox jumps over the lazy dog" ) ;
      Serial.printf( "%4d \t %s \r\n",  i, "the quick brown fox jumps over the lazy dog" ) ;
      delay(1) ;
    }
    digitalWrite( led, LOW ) ;
    delay( 1000 ) ;
    digitalWrite( led, HIGH ) ;
    delay( 1000 ) ;
    if ( j == 9 ) { 
     // this kills it if there is no serial monitor connected 
     Serial.flush() ;
    }
  }
}

Debug Message

No debug messages with this use case (no serial monitor)

Other Steps to Reproduce

I have checked existing issues, online documentation and the Troubleshooting Guide

• I confirm I have checked existing issues, online documentation and Troubleshooting guide.

[tshcherban]

those might be related? #6983, #7779

[6v6gt-duino]

@tshcherban Yes, thank you. Following the 2 cases I found a reference to another which quite closely matches mine. #7554
Unfortunately I did not find this during my searches.

Duplicate of #7554