which pins should i take for i2c on arduino uno

Question

I am trying to create an i2c communication bus with an arduino UNO.

i2c requires SDA and SDL pins. I see on arduino UNO there is twice SDA and SDL:

The 2 first pins side to usb connector are SDA/SCL according to documentation.

And the documentation says A4 and A5 pins can also be SDA/SCL

So i do not understand which pins i can use.

Thanks

Answer 1

Look at the complete diagram: http://pighixxx.com/unov3pdf.pdf

SDA and SCL pins are PC4 and PC5 pins of the microcontroller (pin number 4 and 5 of port C). Pins A4 and A5 are also PC4 and PC5. This means they are connected together.

So you can use any of these, it does not matter.

Answer 2

The earlier versions of the Uno did not have the separate SDA/SCL pins. Then as they started making other boards (such as the Leonardo) there was a problem that the Leonardo SDA/SCL pins were not A4/A5 (they are D2/D3).

To make it easier to make shields which use I2C the manufacturer added the SDA/SCL pins in a known location. For both boards they are connected to the correct pins for that board.

A similar remark applies to the later versions of the Mega2560 board.

You can use either pins: the ones marked SDA/SCL or the appropriate pins for the board to hand. They are connected together so it doesn't matter which ones you choose.

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Partial schematic from the Uno showing the two sets of pins are on the same "net" which means the pins are internally connected.