Unable to flash ESP32

[kolban]

The last question comes down to electronics ... not an area that I am particularly good at ... so the facts here are probably way off or reversed .. but it loosely works like this ...

When you connect your UART wires to the ESP32, there must be a "path" where the small electrical currents can flow. Electricity flows from one location to another ... for example ... from the 3.3v line down to the GND line. If there isn't a path from the source of the current to somewhere it can sink (end up at) then the current won't flow.

Now imagine a signal from the TX line of the the UART ... that needs to have a current flow down it to be sensed by the ESP32 ... and there must be a path from that line BACK to the UART. So what we do is connect the UART and ESP32 GND lines together. So a current coming from the TX line of the UART will flow down that wire, arrive at the ESP32 ... be sensed and then complete the circuit back to the UART because both the UART and the ESP32 share the same ground line. Now consider the TX line from the ESP32 ... same story ... for the current to flow through the UART from the ESP32, there must be a path back to the ESP32 ... again, the common ground line.

And that's how it all works ... circuits must share a common ground for the circuit to flow. There simply isn't a place in the story for attaching another SOURCE of current such as the 3.3V line. It just isn't meaningful to the story. There is nothing that needs to be continuously powered.

[martinayotte]

Do you mind explaining why I should not connect the USB UART's 3.3V to ESP32 though?

The USB-Serial is self powered from the USB line. Therefore it's 3.3V is an output, and your ESP32 has already it own power supply.
Shorting 2 power supplies together could cause damage.

[electromania]

I had same issue with one of new esp32 dev kits i bought. but later i realized that the USB cable was too long and of not good quality. Once i changed the cable to short length and better quality cable, problem was solved.

This is for all those who are facing this issue, if esp fails to upload codes, first thing would be is to check the USB cable and port.

good luck