I have built a custom circuit board using the ESP32-C3-MINI-1-H4 module (datasheet).

Here is a schematic:

enter image description here

I have the GND, Tx, and Rx lines hooked up to an Arduino Uno without the ATmega. The 3.3 V comes from an external power supply.

I have successfully put the chip into download mode (as shown from the serial output below).


Build:Feb 7 2021

rst:0x1 (POWERON),boot:0x5 (DOWNLOAD(USB/UART0/1))

waiting for download

I attempted to upload an example sketch using the following settings (Arduino IDE).

Arduino Settings

I held down the reset button (S4) until the Rx light on the Uno started flashing.

It works about 5% of the time, but 95% of the time it ends in this:

A fatal error occurred: Failed to connect to ESP32-C3: No serial data received.

I'm not sure how that is possible, as the serial data appears on the oscilloscope, and it is clearly in download mode as shown by the serial output.

I have tried removing the voltage divider from the RX line, but that didn't help. Is there any way to make uploading more consistent?

  • "I held down the reset button(S4) until the Rx light on the uno started flashing", the correct sequence for putting the device into flash mode is to press and hold the FLASH button (io9) and press RESET(EN) once, then release(or hold) the FLASH button. BTW, read the Figure 7 of the datasheet on the reference design and the note below the diagram, you need a 1uF in parallel to your RESET switch otherwise your switch will bouncing during the reset.
    – hcheung
    Sep 2, 2022 at 15:45
  • // ESP32C3 USE DIO flash mode or it will not work !!! Sep 15, 2023 at 11:48

1 Answer 1


You must be in flash mode DIO and watch the "USB CDC on boot" which must be on if you use native C3 usb and off if you use an external usb-serial converter.

You can upload OK in QIO mode but program wont start, that is EXACTLY what OP is asking.

  • Welcome to Arduino:SE Please take a moment to read the Stack Exchange Expected behavior Sep 15, 2023 at 13:42
  • This sounds like anecdotal evidence. Do you have any sources to back this up?
    – StarCat
    Oct 17, 2023 at 9:13

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