USB Serial vs UART

The ESP32-S3-DevKitC-1 features two Micro-USB ports with different capabilities. Understanding when to use each is essential for development.

Port Overview

Port Label	IC	GPIO	Capabilities
USB (J2)	Native ESP32-S3	GPIO19/20	USB-OTG, USB-Serial/JTAG, HID
UART (J4)	CP2102N	U0TXD/U0RXD	Traditional serial, reliable flashing

CP2102N UART Port

The UART port provides traditional serial communication:

Features

• Reliable programming with esptool.py
• Automatic reset/boot via DTR/RTS
• Works without any ESP32-S3 USB configuration
• Compatible with all standard serial monitors

Use When

• Programming new/unknown boards
• When USB stack crashes
• Debugging boot issues
• Using traditional serial libraries

Typical Usage

# Flash via UART port
esptool.py -p /dev/ttyUSB0 write_flash 0x0 firmware.bin

# Monitor via UART
idf.py -p /dev/ttyUSB0 monitor
# or
pio device monitor --port /dev/ttyUSB0

Native USB Port

The USB port connects directly to the ESP32-S3’s USB peripheral:

Features

• USB-CDC: Virtual serial port (faster than UART)
• USB-JTAG: Built-in debugging without external probe
• USB-OTG: Host mode for keyboards, storage, etc.
• USB Device: HID, MSC, custom USB devices

USB Modes

Mode	Description	GPIO19/20
USB-Serial/JTAG	Console + debug	Dedicated
USB-OTG Device	Custom USB device classes	Dedicated
USB-OTG Host	Connect USB peripherals	Dedicated
GPIO (disabled)	Regular GPIO pins	Available

Caution

When using native USB, GPIO19 and GPIO20 are not available for general purpose I/O.

Configuration

Arduino/PlatformIO

platformio.ini

# USB-CDC Serial (native USB as Serial)
build_flags =
    -DARDUINO_USB_MODE=1
    -DARDUINO_USB_CDC_ON_BOOT=1

# USB-JTAG for debugging
build_flags =
    -DARDUINO_USB_MODE=1
    -DARDUINO_USB_CDC_ON_BOOT=0

# Disable native USB (use GPIO19/20)
build_flags =
    -DARDUINO_USB_MODE=0

ESP-IDF menuconfig

Component config →
  ESP System Settings →
    Channel for console output →
      [*] USB Serial/JTAG Controller
      [ ] USB CDC

  Driver configurations →
    USB Serial/JTAG →
      [*] Enable USB Serial/JTAG

USB-CDC Serial Example

Using native USB as a fast serial port:

#include <Arduino.h>

void setup() {
    // USB-CDC initializes automatically with ARDUINO_USB_CDC_ON_BOOT=1
    Serial.begin(115200);

    // Wait for USB connection
    while (!Serial) {
        delay(10);
    }

    Serial.println("USB-CDC Ready!");
}

void loop() {
    if (Serial.available()) {
        String input = Serial.readStringUntil('\n');
        Serial.print("Received: ");
        Serial.println(input);
    }
}

USB-OTG Device Example

Create a USB HID keyboard:

#include <USB.h>
#include <USBHIDKeyboard.h>

USBHIDKeyboard Keyboard;

void setup() {
    Keyboard.begin();
    USB.begin();
}

void loop() {
    // Press a key every 5 seconds
    delay(5000);
    Keyboard.print("Hello from ESP32-S3!");
    Keyboard.write(KEY_RETURN);
}

USB Host Mode

The native USB port supports USB host mode for connecting peripherals (keyboards, flash drives, serial adapters). This requires a hardware modification — see the dedicated tutorial:

USB Host Mode Tutorial Hardware setup, OTG jumper, ESP-IDF class drivers (HID, MSC, CDC, UVC)

Simultaneous Usage

You can use both ports simultaneously:

#include <Arduino.h>

// Serial = USB-CDC (native USB)
// Serial0 = UART0 (CP2102N)

void setup() {
    Serial.begin(115200);   // USB-CDC
    Serial0.begin(115200);  // UART

    Serial.println("USB-CDC output");
    Serial0.println("UART output");
}

void loop() {
    // Echo between ports
    if (Serial.available()) {
        Serial0.write(Serial.read());
    }
    if (Serial0.available()) {
        Serial.write(Serial0.read());
    }
}

Debugging with USB-JTAG

The native USB port provides built-in JTAG debugging:

OpenOCD Configuration

# ESP-IDF includes OpenOCD
openocd -f board/esp32s3-builtin.cfg

VS Code Launch Configuration

{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "espidf",
            "name": "ESP32-S3 Debug",
            "request": "launch",
            "debugPort": 3333,
            "logLevel": 2,
            "openOCDLaunchRecipe": "board/esp32s3-builtin"
        }
    ]
}

Comparison Table

Feature	UART Port (CP2102N)	USB Port (Native)
Speed	Up to 3 Mbps	12 Mbps (Full Speed)
Programming	esptool, reliable	Possible, needs USB stack
Debugging	Serial only	Built-in JTAG
Reset Control	DTR/RTS auto-reset	Software reset only
USB Device	No	Yes (HID, MSC, etc.)
USB Host	No	Yes (with OTG adapter)
Driver Required	CP210x driver	None (built-in)
Boot Recovery	Always works	Needs working firmware

Troubleshooting

Native USB Not Working

1. Verify USB cable is data-capable
2. Check build flags include USB configuration
3. Try the UART port to flash correct firmware

Cannot Enter Download Mode via USB

Use the UART port and manually trigger download mode:

1. Hold BOOT button
2. Press and release RST
3. Release BOOT
4. Flash via UART

USB-CDC Serial Drops Connection

• Check power supply stability
• Reduce USB traffic rate
• Add connection recovery in code:

void loop() {
    if (!Serial) {
        // USB disconnected, wait for reconnection
        while (!Serial) delay(10);
        Serial.println("Reconnected!");
    }
    // Normal operation
}

Next Steps

• USB Host Mode — connect keyboards, flash drives, cameras
• ESP-IDF Setup
• PlatformIO Configuration
• Schematic Reference