Cant flash ESP32U: No serial data received (ESPTOOL-1312)

[StrPowKno]

Operating System

Windows 11

Esptool Version

4.12.dev1

Python Version

3.12.0

Chip Description

ESP32U

Device Description

Connections

• i've connected the chip like this:

Prolific wire	ESP32U Pins
Red	5V
Black	GND (ground)
white	RX
Green	TX

Note: I used Prolific PL2303 UART to connect them.

Hardware Configuration

• Nothing is connected to the board

How is Esptool Run

Using VScode and esp extension with windows powershell

Full Esptool Command Line that Was Run

C:\Espressif\tools\python\v5.5.3\venv\Scripts\python.exe C:\esp\v5.5.3\esp-idf\components\esptool_py\esptool\esptool.py -p COM1 -b 460800 --before default_reset --after hard_reset --chip esp32 write_flash --flash_mode dio --flash_freq 40m --flash_size 2MB 0x1000 bootloader/bootloader.bin 0x10000 hello_world.bin 0x8000 partition_table/partition-table.bin

Esptool Output

esptool.py v4.12.dev1
Serial port COM1
Connecting......................................

A fatal error occurred: Failed to connect to ESP32: No serial data received.
For troubleshooting steps visit: https://docs.espressif.com/projects/esptool/en/latest/troubleshooting.html

 *  The terminal process "C:\Espressif\tools\python\v5.5.3\venv\Scripts\python.exe 'C:\esp\v5.5.3\esp-idf\components\esptool_py\esptool\esptool.py', '-p', 'COM1', '-b', '460800', '--before', 'default_reset', '--after', 'hard_reset', '--chip', 'esp32', 'write_flash', '--flash_mode', 'dio', '--flash_freq', '40m', '--flash_size', '2MB', '0x1000', 'bootloader/bootloader.bin', '0x10000', 'hello_world.bin', '0x8000', 'partition_table/partition-table.bin'" terminated with exit code: 2.

More Information

• since iran internet is kinda weird because of the war i cannot access sites or severs that are 3rd-party or arent related to programming.
• like i can open visual studio extensions marketplace but i cant open microsoft.com
• if theres a troubleshooting tutorial of the guide please tell me

Other Steps to Reproduce

Steps

• Hold the BOOT button
• Connect the chip while holding the BOOT button and release after two seconds
• Click Flash
• When the "Connecting ..." pops up press BOOT again

I Have Read the Troubleshooting Guide

• I confirm I have read the troubleshooting guide.

[github-actions]

Hi @StrPowKno! Please be aware that (1) the following suggestions are generated by a bot and haven't been fact-checked by Espressif Systems, (2) burning eFuses and enabling security features are irreversible operations and can damage your ESP32 device. We hope that this message will help you until an Espressif Engineer looks at your issue.

---

There are a few clear issues to address here. Let me go through them:

1. Critical Wiring Problem: 5V Power Supply

The most likely cause of the issue is that the chip is being powered with 5V (red wire), but ESP32 requires 3.3V. Connecting 5V can damage the chip or prevent it from working correctly. [Serial Connection]

⚠️ Do not connect the chip to 5V TTL serial adapters! The ROM serial bootloader uses a 3.3V UART serial connection only.

Please use the 3.3V pin from your Prolific PL2303 adapter instead of the 5V pin.

2. TX/RX Lines May Be Swapped

Double-check that the TX and RX lines are crossed correctly:

ESP32 Pin	Prolific PL2303 Pin
TX	RX (receive)
RX	TX (transmit)
GND	GND

It looks like the current wiring has white → RX and green → TX, but make sure these correspond to the ESP32's RX and TX pins respectively, and that they are crossed (ESP TX → adapter RX, ESP RX → adapter TX). [Serial Connection]

3. Boot Mode / Download Mode

The "No serial data received" error can also mean the chip is not entering download mode correctly. [Common Errors]

The correct manual procedure is:

1. Hold the BOOT button (pulls GPIO0 low)
2. Press and release the EN/RESET button while still holding BOOT
3. Release the BOOT button
4. Then run the flash command

The method described in the issue (connecting while holding BOOT, then pressing BOOT again during "Connecting...") is non-standard. The standard approach is to hold BOOT and press RESET/EN. [Flashing Troubleshooting]

4. Try a Lower Baud Rate

The command uses -b 460800. Try reducing it significantly to rule out baud rate issues:

esptool.py -p COM1 -b 9600 --chip esp32 write_flash ...

[Troubleshooting]

5. Verify the Serial Connection First

Before flashing, verify the serial connection is working by opening a serial terminal (e.g., using pySerial which comes with esptool):

python -m serial.tools.miniterm COM1 115200

Then reset the chip. If you see any output, the serial connection is working. If you see nothing, the hardware connection needs to be fixed first. [esptool DevCon Talk]

---

Summary of most likely fixes:

1. ✅ Switch from 5V to 3.3V power
2. ✅ Verify TX/RX are correctly crossed
3. ✅ Use the correct boot mode sequence (hold BOOT + press EN/RESET)
4. ✅ Try a lower baud rate like 9600

[StrPowKno]

this didnt work, and when i do this it get so hot

[radimkarnis]

Hi @StrPowKno,
a troubleshooting guide is here: https://docs.espressif.com/projects/esptool/en/latest/esp32/troubleshooting.html

Your wiring seems to be correct, but the prolific USB-UART bridge doesn't support automatic switching to download mode (no DTR/RTS lines available). That means you have to switch the ESP32 to download mode manually, which means the BOOT button has to be pressed while the ESP gets connected to power (or while the RESET button is pressed once).

Your steps mention

Hold the BOOT button
Connect the chip while holding the BOOT button and release after two seconds
Click Flash
When the "Connecting ..." pops up press BOOT again

This has some unnecessary steps, but should, in theory, work (BOOT button is held when the chip is being connected to power).

Unfortunately, if there are no signs of life (no serial data coming out of the ESP32) and you say the chip gets really hot, I believe the HW is dead. The devkit shouldn't be getting "so hot" to touch.

[radimkarnis]

Will close this, as we cannot help with HW issues. If you manage to get data out of the chip, but esptool doesn't work, feel free to open another ticket. Thank you.