Hi,
Does anyone have any idea why my R3 Uno would be trying to perform I2C communications on the previously used A4/A5 pins rather than the newer dedicated SCL/SDA. I can see appropriate signals using an oscilloscope when I run the I2C address scanner but only on A4/A5, not on SCL/SDA. I can't see anywhere in the IDE where I specify the Uno revision.
On the UNO the A4/A5 pins are internally connected to the SDA/SCL pins. Do you really have an original UNO from Arduino? Because some clones have the new pins but they aren't connected as in the original. Can you post a picture of your board (both sides)?
Thanks for the reply. I can well believe I may have a counterfeit. I bought my supposed R3 arduino as part of an Arduino starter kit on Ebay. After the package arrived I was sent a message by the seller saying they had been delisted by Ebay on account of being an unauthorised reseller....needless to say I haven't replied or had any further contact with them. My board has performed as expected in other circuits that do not use the I2C bus. Here are the photos as requested.
On a '328, A4/A5 have the alternate function of SDA/SCL (or SCL/SDA - I always have to look).
On bigger chips, like 1284P, they are seperate pins.
It looks like a not so bad made clone and on the pictures it seems that the SDA/SCL pins are really connected. Can you check (on the unpowered Arduino, using a multimeter) if the SDA pin is connected to the A4 and SCL to A5?
I checked...open circuit on both. Thanks for your help. Now I'll order a real one....this has been pretty much my first go with programmable chips and is proving to be a rewarding experience...trying to build a vario for hang gliding and got lots of the other bits working but was/am going to use the I2C bus to get data from a pressure sensor
Do chip pins 27 & 28 make connections to anything? Like headers A4 & A5? If so, take your I2C connections from there.
Looking at the Arduino UNO product page on this very site shows the SDA/SCL on the back like the original poster has. I doubt his was a counterfeit based off that information. Unless Arduino's own site is using pictures of counterfeit Arduino boards. I find this very odd considering their documentation clearly states A4/A5.
I am using I2C to communicate with a custom board and I can confirm those other pins actually do work as I2C. I was able to communicate with my custom board.
Looking at the Arduino UNO product page on this very site shows the SDA/SCL on the back like the original poster has.
But the OP is not using an original UNO but an (illegal) clone. We've often seen clones that have labeled pins that doesn't have that functionality. So just because your original Arduino UNO works with these pins doesn't mean the cheap Chinese copy does too. But CrossRoads is correct, the OP can just use A4/A5 and he's fine.
Reading over the posts again I can see your point now. I don't think I fully understood the situation at the time I first read this post. 
