Unable to flash esp32 devkitc via direct connection to the TX/RX pins (ESPTOOL-1197)

[fabianmejia]

Operating System

Windows 11

Esptool Version

esptool.py v4.8.1

Python Version

Python 3.11.2

Chip Description

ESP32-WROOM 32E

Device Description

ESP32-DevKitC V4 using esp32-wroom 32E,
Waveshare usb-to-ttl

Hardware Configuration

• TX/RX pins
• ground pin
• 3v3 pin

How is Esptool Run

vscode extension via terminal

Full Esptool Command Line that Was Run

esptool.py -p COM3 -b 9600 chip_id

Esptool Output

esptool.py v4.8.1
Serial port COM3
Connecting......................................

A fatal error occurred: Failed to connect to Espressif device: No serial data received

More Information

I can successfully flash the esp32 devkitc board using the usb connection.
However, I want to move forward, and flash it directly via TX/RX on the PCB I will integrate the wroom-32 with other components. That pcb will not include a usb port and will not include the cp2102, but it will be powered by 3v3.
I made the connections as described above. I can see that the board enters flashing mode by:

rst:0x1 (POWERON_RESET),boot:0x3 (DOWNLOAD_BOOT(UART0/UART1/SDIO_REI_REO_V2))
waiting for download

When I tried the command described above, I get the error "No serial data received".
I can confirm that the usb-to-ttl device is working after doing a loopback test.
Also, I can see the board output from the same usb-to-ttl.
So, I assume that:

• The usb-to-ttl is working
• The board can enter flash mode

Other Steps to Reproduce

No response

I Have Read the Troubleshooting Guide

• I confirm I have read the troubleshooting guide.

[github-actions]

Hi @fabianmejia! 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.

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Based on the information provided and Espressif’s official troubleshooting guides, the error “No serial data received” typically indicates a hardware issue with the serial connection. Here are the key points and recommended steps to resolve this issue:

1. Required Connections for Flashing

When flashing the ESP32 via a USB-to-TTL adapter (not using the onboard USB bridge), you must connect:

• TX (adapter) → RX (ESP32)
• RX (adapter) → TX (ESP32)
• GND (adapter) → GND (ESP32)
• 3.3V (adapter) → 3.3V (ESP32) (if powering from the adapter, ensure it can supply enough current)

Additionally, to enter download (flashing) mode, you must control:

• GPIO0: Must be held LOW during reset to enter download mode.
• EN (CHIP_PU): Used to reset the chip.

On the DevKitC, these are typically managed by the onboard USB-to-UART bridge and auto-reset circuit. When using an external USB-to-TTL adapter, you must manually ensure the correct boot mode:

• Hold down the BOOT button (GPIO0 LOW), press and release the EN button (reset), then release BOOT.
  If you do not have access to buttons, you must pull GPIO0 LOW and pulse EN (CHIP_PU) LOW to reset the chip into download mode.
  See: Manual Bootloader instructions

2. Common Causes for “No Serial Data Received”

• Missing GPIO0/EN control: If you only connect TX/RX/GND/3V3, the chip may not reliably enter download mode. You must ensure GPIO0 is LOW during reset.
• Wiring issues: Double-check that TX/RX are not swapped, and all connections are secure.
• Insufficient power: Ensure your USB-to-TTL adapter can supply enough current for the ESP32. Some adapters cannot provide enough power, especially during flashing.
• Serial port in use: Make sure no other program (including serial monitors) is using the COM port.
• Baud rate: You are already using 9600, which is good for troubleshooting.

3. Espressif Documentation References

• The error “No serial data received” is described as usually implying a hardware issue, such as RX/TX not connected or a problem with resetting into download mode.
  See: Common Errors – No serial data received
• Manual bootloader entry is required if you do not have DTR/RTS lines connected to GPIO0/EN.
  See: Automatic Bootloader & Manual Bootloader
• Required serial connections: TX↔RX, RX↔TX, GND↔GND, 3.3V↔3.3V (do not use 5V).
  See: Serial Connection

4. Recommendations

• Ensure you are manually entering download mode by holding GPIO0 LOW during reset (EN/CHIP_PU).
• Double-check all wiring (TX/RX, GND, 3.3V).
• Verify power supply capability of your USB-to-TTL adapter.
• Try a different USB cable or adapter if possible.
• Disconnect all other devices from the ESP32’s GPIOs during flashing.

If you follow these steps and still encounter issues, please provide details about how you are controlling GPIO0 and EN during the flashing process.

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Summary:
The most likely cause is that GPIO0 is not being held LOW during reset, so the ESP32 is not properly entering download mode when using a direct TX/RX connection. Please ensure you are manually controlling GPIO0 and EN as described above.
Reference: Espressif esptool troubleshooting – Common Errors

[fabianmejia]

There are no other pins connected to the bard.
I tried also pulling down gpio0. Same issue
I tried also pulling down gpio0 and gpio2. Same issue. Board enters download mode, but no response received
I tried holding the EN button longer. Same issue
Tried a different esp32 board. Same issue

[radimkarnis]

Hello @fabianmejia,
Have you tried swapping the TX/RX lines?

Hardware Configuration
TX/RX pins
ground pin
3v3 pin

This doesn't mention the EN (RTS), GPIO0 (DTR) pins connection for the auto-reset circuitry. How are getting into the download mode?

[fabianmejia]

Yes, they are swapped. I can confirm it works because I can see the output from any program I flash into Esp32.
I tried two different configurations for the download mode:

1. Using EN and Boot buttons on the DevkitC (In the correct order) to enter download mode. It correctly shows the "Download mode" in the terminal
2. Setting both Pin0 and Pin2 low and powering the device. It also shows the "Download Mode" in the terminal.

However, none of these methods worl

[radimkarnis]

In that case, your TX line might be faulty. Your PC (host) is receiving data correctly, but the chip might not be receiving commands from the host. Esptool waits for a response from the chip to SYNC commands, but if they never get to the chip, there is nothing to respond to.

Is it possible that the onboard CP2102 is still running? Since it is connected to the same TX/RX lines, it might be mangling the communication if it still runs.

[fabianmejia]

I have the feeling that's what is doing. (which was my initial assumption)
The question then is whether manual serial flashing is possible with a board that has the cp2101 in it?
If that's the case, then it may be the case that the documentation is not clear because it mentions the use of both Boot and EN buttons on the board, but that is not possible if manually flashing via TX/RX due to the cp2102.
To give more info, I am designing a product, and I can't include the cp2102 in it.

[radimkarnis]

The question then is whether manual serial flashing is possible with a board that has the cp2101 in it?

I believe it depends on the devkit design. I just tried it with ESP32-DevKitC V4 with ESP32-WROOM32 module, and I see the same behavior as you. But if I switch to a ESP32-C61-DevKitC-1 (also has a CP2102 onboard), everything works.

the documentation is not clear because it mentions the use of both Boot and EN buttons on the board

The use of buttons is unrelated to the not working TX/RX paths. If you press the buttons, the chip will still correctly switch to the download mode - you even confirmed this.

To give more info, I am designing a product, and I can't include the cp2102 in it.

In that case, the best possible scenario here is to look at some of Espressif's newer chip with the USB-Serial/JTAG or USB-OTG peripherals (e.g., ESP32-C3, or ESP32-S3). This allows designs without USB-UART bridges.

[fabianmejia]

Thank you very much for looking into it, and confirming the suspicions.
I have already considered esp32-c3, but the BT stack is different, meaning that we have to rewrite the code. Same with the pcnt driver.
My next step is to build a custom board and program the esp32-wroom32 without any cpo2102 attached.