Strange power issue between Arduino UNO and ESP8266

Other Hardware General Electronics
1

I have a strange issue between my Arduino UNO and an ESP8266 (ESP 12-F).

Connection:

UNO ESP8266
D5(SoftwareSerial RX) TX
D6(SoftwareSerial TX) RX
GND GND

Baudrate is set to 4800.

My Arduino UNO gets its power via USB (5V).
The ESP8266 gets its power via an external power supply (3,3V).

The issue is if my Arduino UNO is not connected to the USB port (no power), but is connected to the ESP like above mentioned, then the "ON" led of the UNO is slightly on.

Do you have an idea why this happens?
Or is this behaviour normal?

Thanks in advance
mtdshare

2

Phantom powering? Yes, that happens, and it's not a good thing. Try to avoid it.

3

You should also do some voltage level conversion between the arduino D6 (5v) and the ESP RX (3.3v)

Jimmy

4

Voltage spec on any input pin is Vcc + a fraction of a volt, see datasheet, absolute maximum rating section. When the Arduino is off, what is Vcc? Zero. What are you doing? Applying 3.3v to it.

The pins are internally connected to Vcc and Ground via diodes (to prevent damage), and so if you apply a voltage in excess of Vcc to a pin while the part is not powered, it will be "backpowered" through the protection diode - with a relatively low impedance thing connected (like the output pin of some other micro), this grossly exceeds the maximum current for for the protection diode, and is the most common way to blow pins.

Additionally, the same restriction applies to the ESP8266, a 3.3v device - And you're connecting it to 5v serial. With no level shifting or protection. This is how you blow pins on your ESP8266.

Use a level shifter. Those el-cheapo FET levelshifters will also protect against backpowering (or at least limit the current such that the protection diodes can handle it safely).