FAQ
TL;DR: Build log shows 11% dynamic memory used [Elektroda, CC_PL, post #20512534] “This is not a USB port” [Elektroda, khoam, post #20512593] If Arduino IDE on Ubuntu only shows /dev/ttyS0 and your ESP32 vanishes as /dev/ttyUSB0, fix cable/power, select the USB port, and check Snap permissions. For Ubuntu/Arduino users whose ESP32 isn’t detected or won’t auto-reset.
Why it matters: It saves hours of trial-and-error when your ESP32 fails to appear or drops connection during upload.
Quick Facts
- /dev/ttyS0 is a built‑in UART; ESP32 boards enumerate as /dev/ttyUSBx via USB‑UART [Elektroda, khoam, post #20512593]
- Many ESP32 dev boards use CH340 (VID:PID 1a86:7523) USB‑UART bridges [Elektroda, CC_PL, post #20512674]
- USB 2.0 ports supply up to 500 mA; marginal power causes device disconnects [USB in a Nutshell — BeyondLogic].
- esptool toggles DTR/RTS for auto‑reset; boards without proper wiring won’t auto‑reboot [esptool README].
- Typical monitor baud for ESP32 logs is 115200 bps (IDF monitor default) [IDF Monitor — ESP‑IDF].
Why does Arduino IDE show /dev/ttyS0 but not my ESP32 on Ubuntu?
/dev/ttyS0 is a motherboard UART, not your USB device. ESP32 boards should appear as /dev/ttyUSB0 (or higher). If only /dev/ttyS0 shows, the USB connection failed or is blocked. Check cable, power, and Snap permissions if using the Snap IDE [Elektroda, khoam, post #20512593]
My /dev/ttyUSB0 appears and disappears. How do I stop the port from dropping?
Use a short, data‑capable USB cable and try a different USB port. Weak ports and long/poor cables cause brownouts and re‑enumeration. In the thread, a shorter cable and another socket fixed it [Elektroda, CC_PL, #20512615; khoam, #20512617; CC_PL, #20512690; CC_PL, #20518044]. USB 2.0 ports provide up to 500 mA; marginal power triggers dropouts [USB in a Nutshell — BeyondLogic].
How do I make the Arduino IDE Snap see my ESP32 serial port?
Connect Snap interfaces: sudo snap connect arduino:raw-usb and, if exposed, sudo snap connect arduino:serial-port. Then replug the board. Ensure your user is in the dialout group and re‑login: sudo usermod -a -G dialout $USER [Snapcraft Interfaces: raw-usb; Arduino IDE on Linux: Serial Permissions].
Upload fails with “Could not open /dev/ttyS0, the port doesn’t exist.” How do I fix it?
Select the correct USB device (/dev/ttyUSBx) in Tools → Port, not /dev/ttyS0. If /dev/ttyUSBx is missing, check cable/port and permissions. In the thread, picking the USB port and fixing the cable resolved uploads [Elektroda, CC_PL, #20512534; khoam, #20512593; CC_PL, #20518044].
What does “Hard resetting via RTS pin…” mean, and why doesn’t my board reboot?
esptool toggles DTR/RTS to drive EN/BOOT for auto‑reset. Some boards lack proper wiring, so auto‑reset fails. Press RESET manually after upload, then open Serial Monitor at the correct baud [esptool README; Elektoda, khoam, #20512661; CC_PL, #20516611]. “Maybe the board doesn’t have autoreset support?” [Elektroda, khoam, post #20512661]
How do I reliably see Serial Monitor output after flashing?
- Upload the sketch.
- When “Hard resetting via RTS pin…” appears, press RESET if the board doesn’t auto‑reset.
- Open Serial Monitor at the same baud as Serial.begin(), e.g., 115200 [Elektroda, khoam, #20516902; CC_PL, #20512690; esptool README].
Do baud rates need to match between my code and the Arduino Serial Monitor?
Yes. Set Serial.begin(115200) and select 115200 in Serial Monitor. Mismatched baud yields gibberish or nothing. The user matched 115200 in code and IDE during troubleshooting [Elektroda, khoam, #20512681; CC_PL, #20512690]. 115200 is a common default for ESP32 logs [IDF Monitor — ESP‑IDF].
How can I confirm the ESP32’s USB‑UART bridge and driver on Linux?
Run lsusb and look for CH340 (1a86:7523) or similar. Linux will create /dev/ttyUSBx on detection. The thread shows CH340 enumerating correctly [Elektroda, CC_PL, post #20512674]
Could a weak USB port really cause flashing or serial issues?
Yes. Low‑current ports and hubs can sag voltage, causing device resets and vanished /dev/ttyUSBx entries. Switching ports stabilized the connection in the thread [Elektroda, khoam, #20512681; CC_PL, #20512690]. USB 2.0 allows up to 500 mA for configured devices [USB in a Nutshell — BeyondLogic].
Why does Arduino IDE not react when I press the board’s RESET button?
The IDE doesn’t detect hardware resets; it only opens the serial port. Open Serial Monitor after resetting, and ensure the baud matches. The user’s reset worked on hardware but IDE showed nothing until monitoring correctly [Elektroda, CC_PL, #20512636; khoam, #20516902].
Is my cable the problem if uploads succeed sometimes but Serial is flaky?
Yes. Charge‑only or noisy cables can upload sporadically yet break Serial enumeration. In the thread, replacing with a shorter data cable fully resolved issues [Elektroda, khoam, #20512617; CC_PL, #20518044]. “Try a different USB cable… maybe even a shorter one” [Elektroda, khoam, post #20512617]
What if my ESP32 board uses CP2102 or FTDI instead of CH340?
Linux supports CP210x and FTDI bridges out of the box. They also appear as /dev/ttyUSBx. Install udev rules or permissions as needed, then select the port in Arduino IDE [Establish Serial Connection — ESP‑IDF].
How can I quickly check if the port really exists?
Run ls -l /dev/ttyUSB0 and dmesg | grep -i ttyUSB after plugging in. If only /dev/ttyS0 shows, your USB device didn’t enumerate or you lack access. The user initially saw only /dev/ttyS0 in logs [Elektroda, CC_PL, post #20512534]
Why does the USB port number sometimes change after reconnecting?
Kernel re‑enumeration can assign the next ttyUSB index, especially with hubs or multiple converters. Re‑select the new /dev/ttyUSBx in IDE when it changes [esptool README].
Bottom line: what fixed the thread’s issue?
A better, shorter data cable and a different USB port. After that, uploads and Serial worked as expected [Elektroda, CC_PL, #20518044; CC_PL, #20512690].
I'm giving shorter and shorter cables, but the port is not visible at the moment. 