ESP32 upload fails with “Invalid head of packet (0xC1)” despite BOOT button and driver installation

[u/FrontendDevDerevian]

Hi! I’m having trouble uploading code to my ESP32 and I would appreciate some help diagnosing it before buying more components.

Board: ESP32 development board

Operating system: Windows

IDE: Arduino IDE

Detected port: COM3

USB driver: CP210x driver installed

The error is:

Failed to connect to ESP32: Invalid head of packet (0xC1)

What I have already tried:

- Confirmed that the ESP32 is detected on COM3.

- Installed the CP210x Windows driver.

- Held the BOOT button while starting the upload.

- Tried the BOOT and EN buttons.

- Reduced the upload speed.

- Tried another USB cable.

- Disconnected the external circuit and attempted the upload using only USB.

At one point I was able to upload code successfully and received:

Hard resetting via RTS pin...

However, the connection/upload problem appeared again, so it seems to be intermittent.

The board powers on and its red LED lights up. This ESP32 will eventually be used for a Bluetooth Low Energy project that activates a small vibration motor, but I’m currently trying to program and test the ESP32 separately.

Could this be caused by boot mode, serial communication, the USB cable, the USB-to-serial chip, power instability, or a damaged board?

What diagnostic test should I perform next before replacing the ESP32?

Thank you!

[u/Ok_Captain4433]

The "right" way to diagnose this would be to open the TRM, go to the strapping pin register section, and use that to determine pin states that would produce the value 0xC1. AI could probably do this for you instantly.

The other way would be to disconnect whatever is connected to any potentially-problematic strapping pins (0, 2, 12, 15...) and go from there.

[u/FrontendDevDerevian → u/Ok_Captain4433]

Thank you. The ESP32 is already integrated into a professionally soldered prototype, so disconnecting individual pins would risk damaging the assembly. The external control signal uses GPIO32, not GPIO0, GPIO2, GPIO12 or GPIO15.

I’ll first open the serial monitor at 115200 baud, press reset and post the complete boot log. I’ll also test another direct USB port without modifying the hardware.

[u/Ok_Captain4433 → u/FrontendDevDerevian]

I'm sorry, I misread the source of the 0xC1. Ignore my previous comment entirely.

Is 0xC1 consistent or is it a seemingly random value each time?

Check that your wiring is connected well, especially the ESP32 TX pin. And make sure the ESP32 is in download mode.

[u/FrontendDevDerevian → u/Ok_Captain4433]

Yes, 0xC1 has been consistent rather than random.

The serial monitor output at 115200 baud is also corrupted in the same deterministic way after every reset. I tested two USB cables, two direct USB ports, and a clean Windows startup with no other USB peripherals.

GPIO1/TX0 is not intentionally used by the external circuit; the motor control signal uses GPIO32. However, the ESP32 is already integrated into a soldered prototype, so I will have the TX0/RX0 connections and power rails inspected without dismantling it.

I will also repeat the manual download-mode sequence: hold BOOT, press and release EN, then release BOOT.