FAQ
TL;DR: A 1‑port mismatch (IDE on COM4 vs device on COM3) blocked uploads; “the UART driver is on the COM3 port.” [Elektroda, efi222, post #21746695]
Why it matters: If your ESP32 DevKit‑C v4 won’t connect, matching the actual COM port and confirming bootloader state solves most “no serial data” errors fast. This FAQ is for Arduino users fixing ESP32 USB/COM connection issues.
Quick Facts
- Reported IDE errors included: missing serial port, no serial data, and “port is busy.” [Elektroda, Roli, post #21746180]
- Device Manager showed the ESP32 UART on COM3 while Arduino targeted COM4. [Elektroda, efi222, post #21746695]
- Serial log showed reset + “waiting for download,” meaning bootloader mode was active. [Elektroda, Roli, post #21755603]
- Power‑only USB cables cause detection failures; use a data cable. [Elektroda, roman106, post #21746250]
- Reinstalling the CP210x driver restored connectivity for the poster. [Elektroda, roman106, post #21746950]
How do I fix “Failed to connect to ESP32: No serial data received” in Arduino IDE?
Confirm the COM port in Tools > Port matches Device Manager. In this case, the board enumerated on COM3 while the IDE targeted COM4. Select the matching COM port, then try uploading again. If the error persists, reset the board and retry. [Elektroda, efi222, post #21746695]
Why does Arduino say “Could not open COM4” but Device Manager shows COM3?
Windows assigned the USB‑UART to COM3. Arduino attempted COM4, so the port was nonexistent or busy. Select COM3 in Arduino, or replug the board and refresh ports. Changing numbers manually does not move the hardware endpoint. [Elektroda, efi222, post #21746695]
What does “waiting for download” on the ESP32 serial log mean?
Your ESP32 entered the ROM bootloader (Download Boot). It waits for a flashing tool to send firmware. This appears alongside rst and boot codes, then the prompt “waiting for download.” Start an upload from Arduino or esptool to proceed. [Elektroda, Roli, post #21755603]
Why do I see many “invalid header: 0xffffffff” lines before “waiting for download”?
Those lines indicate the bootloader read empty or corrupt flash headers. In the provided log, at least eight such lines appear before entering download mode. Flashing valid firmware clears them on next reset. [Elektroda, Roli, post #21755603]
Can a USB power‑only cable cause my ESP32 not to be detected?
Yes. Power‑only cables lack D+/D− data lines, so Windows never enumerates a COM port. Use a known data cable, or test with another data‑capable cable. “Use a cable with data transmission.” [Elektroda, roman106, post #21746250]
Which board should I select in Arduino IDE for an ESP32 DevKit‑C v4?
Select “ESP32 Dev Module” under Tools > Board. That board profile was used in the thread and confirmed as appropriate. Keep default flash mode and frequency unless your module specifies otherwise. [Elektroda, Roli, post #21746724]
How can I test the serial connection without Arduino IDE?
Open a serial terminal (e.g., Termite), pick the observed COM port, and press reset on the board. You should see the boot log. This verifies UART communication independent of Arduino. “I recommend a simple Termite.” [Elektroda, efi222, post #21746744]
The terminal shows output—does that prove my USB‑UART works?
Yes. Seeing the boot log confirms USB‑UART communication. As one expert noted, “Communication via uart is.” If uploads still fail, target the correct COM and ensure the board is in flashing mode. [Elektroda, efi222, post #21755632]
Do I need to reinstall the CP210x driver if detection is flaky?
It can help. One user restored connectivity by reinstalling the CP210x driver when enumeration became unreliable. Uninstall the device, disconnect, reinstall the driver, then reconnect and verify the COM port. [Elektroda, roman106, post #21746950]
How do I manually force upload if auto‑reset fails?
Try this 3‑step method:
- Hold BOOT on the ESP32.
- Click Upload in Arduino; release BOOT when “Connecting...” appears.
- Wait for “Done uploading,” then press EN/RESET.
This matches the scenario where pressing BOOT was attempted. [Elektroda, Roli, post #21746180]
Arduino shows “DISABLED” next to the COM port. What does that imply?
It implies the IDE did not open a valid serial port for the current session. The port may be wrong, missing, or in use. Select the correct COM, close other serial apps, and retry the upload. [Elektroda, Roli, post #21746180]
What should I do when I get “the port is busy or doesn’t exist”?
Close any terminals or background tools using that COM port. Ensure Arduino and only one terminal are open at a time. Then select the correct COM and retry the upload. [Elektroda, Roli, post #21746180]
Is changing the COM number in Windows a real fix?
No. Renumbering the port in Device Manager changes the label, not the device path Arduino targets. You must still select the actual active COM in Arduino after renumbering. [Elektroda, Roli, post #21746699]
Could attached peripherals (displays, DACs) block flashing?
They can, if they pull boot pins or draw excessive power. The poster tested both with and without PCM5102 and a display; behavior did not change. Disconnect add‑ons during troubleshooting. [Elektroda, Roli, post #21746740]
What does rst:0x10 (RTCWDT_RTC_RESET) indicate during boot?
It indicates a reset triggered by the RTC watchdog. Seeing it before entering download mode is normal during repeated flashing attempts and resets. Proceed with upload and then reset. [Elektroda, Roli, post #21755603]
Bottom line: what sequence confirms everything is healthy?
Match the COM port, open a terminal, press reset, and see the boot log. Then upload from Arduino on that same COM. If you see “waiting for download,” start the upload immediately. [Elektroda, Roli, post #21755603]
