Can anyone explain to me why the following occurs (See image below)?

When I connect the pins according to situation 1, everything works fine and I am able to upload my project. However when I connect the pins according to situation 2 it gives me the following error:

wrong direction/command: 0x00 0x08, expected 0x01 0x08 esp8266

enter image description here

Both supply the same constant 3.3V and after the project is uploaded it runs with the pins connected as in both scenarios.

(All the other factors are constant in both scenarios. Think of pulling down IO0 etc.)

3 Answers 3


This will not work for the same reason you can not measure the voltage of a battery by only connecting the measurement device to the positive battery terminal. You always need a ground reference.

Perhaps you were thinking all grounds, no matter where they come from, are the same. This is only true if all your power supplies are grounded. But this is usually not the case. In fact it can be dangerous and can lead to other problems such a ground loops.

So, connect your grounds together and try your circuit again.


You must always connect the grounds. Without the grounds connected, even when using a separate power supply, there can be no circuit. With no circuit, no electrons can flow.

I cover it in detail in this blog post.

  • Very nice blog post, made me understand it quite good!
    – Revils
    Dec 10, 2017 at 19:15

There's no problem with supplying a separate power supply to the ESP8266 but you always need to connect the grounds between the "programmer" and the ESP8266.

  • 1
    Can you also explain why? (Why is the ground of the separate supply not good enough for the esp8266)
    – Revils
    Dec 10, 2017 at 17:37

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