I'm using a 38pin wroom dev board. It works fine when it is powered by USB, however, if I cut VCC line from usb, and directly supply the board at the 5V (as it is supposed to have an internal vreg to 3.3V that in turn powers the board), it stops working.

I kept the GND common (external supply, board, usb), and in this case, it starts to spill nonsense at the serial:

17:27:20.785 -> rst:0x10 (RTCWDT_RTC_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
17:27:20.785 -> flash read err, 1000
17:27:20.785 -> ets_main.c 371 

So, as far as I understand, linux on my pc could establish connection with the board fine, but the board fails to boot properly... right? If so, how to fix it?

Or, are there another way to supply the board with external power and read its serial?

(My main objective are to debug another program which fails to boot because it makes the board draws so much current that my pc usb can't give. So, Im trying a more reliable power source )

Edit: Grammar isn't on my side... sorry if it's confuse. I'll do my best to clarify it.

  • \$\begingroup\$ vcc needs to be 3.6v max, not 5v. there should be a vin pin that ties into the LDO's input that you can feed 5v. \$\endgroup\$
    – dandavis
    Jun 14, 2022 at 21:18
  • \$\begingroup\$ a board that overloads a PC's USB socket is defective. The ESP+"FTDI" should never draw anywhere close to enough power to do that. If you have other hardware powered from the board, that's the cause. But you shouldn't draw more than 500ma from the Vin pin, because that all goes through a SMT diode which isn't able to handle a lot of current. \$\endgroup\$
    – dandavis
    Jun 14, 2022 at 21:26

1 Answer 1


I'm not sure if you can do it like that. Depending on your hardware the USB controller senses VBUS and this might be disabled by a diode. Here is a schemtic of a ESP32_DevKitc_V4. If you need to supply with external 5V it is probably easier to cut the VBUS in the USB cable and supply it from there. Leaving the PC end of VBUS open of course.

enter image description here


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