I bought this Arduino clone recently and it has lot of extra holes for pins, but some of them I don't know what they do. There was no instruction manual that came with it. Here's a picture of it, I added a red rectangle around the ones I don't know. (4 pins with X1, SCL/SDA, AREF and blank pin in the bottom) Arduino uno clone unknown pins

I thought I could use the pins SCL SDA 5V GND with my I2C LCD Display because the names of the pins are the same on it. Could it work? Usually I use analog pins to connect the display, does that mean that by using it some analog pins will be used indirectly? I didn't try connecting it because I don't want it to break.

enter image description here

What do these pins do?

  • I bought this one
    – Nicolas
    Commented May 14, 2016 at 14:59
  • I'v had the same question and searched for a manual... And where is the manual?
    – user46323
    Commented May 2, 2018 at 14:37
  • For the Arduino clone? Should be the same as standard Arduinos.
    – Nicolas
    Commented May 2, 2018 at 22:33

1 Answer 1


It's what they say they are.

SDA/SCL are I2C - what used to be A4/A5 on the old footprint. On the '328 based boards those are directly linked to the A4/A5 pins.

The blank pin isn't used. The 5V next to it is actually mislabelled and should be labelled "IOREF".

AREF has been there since the very first Arduino boards.

It's the standard "R3" Uno footprint, plus some extra breakouts for the I2C in different forms (probably to go with specific I2C devices they also sell).

All of that is on the Uno's product page if you'd care to read it some time...

The ones by the cheap Chinese USB interface chip will be something to do with that. Maybe RTS/CTS broken out on a header. You can ignore those - they are of no interest at all. In fact, just looking in their direction has a good chance of breaking the USB interface chip. Stay well away.

  • Does IOREF sets the voltage on I/O pins?
    – Nicolas
    Commented May 14, 2016 at 15:28
  • IOREF tells a shield what voltage the main board runs at.
    – Majenko
    Commented May 14, 2016 at 15:28
  • It's all in the manual. I seriously suggest you read it. At the moment you're just making a fool of yourself asking such basic questions.
    – Majenko
    Commented May 14, 2016 at 15:29
  • Would that make you happy?
    – Majenko
    Commented May 14, 2016 at 15:30
  • If you were happy?
    – Majenko
    Commented May 14, 2016 at 15:31

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