Where are the:
Revision 3 of the board has the following new features:
1.0 pinout: added SDA and SCL pins that are near to the AREF pin
I can't see anything near the AREF pin labled as SDA or SCL.
Would these differ in any way from pins 20 and 21 that are labed as SDA and SCL? Still trying to figure out why I can make I2C work on my Duemilanove but not on my Mega 2560 R3.
Also, what is the purpose of the second, unlabled ICSP six pin header?
I've not used the Mega, but since the same notes occurred with the Uno Rev 3,:
1.0 pinout: added SDA and SCL pins that are near to the AREF pin
I can't see anything near the AREF pin labled as SDA or SCL.
Check underneath the board, the 2 pins are labeled down there for the Uno. It should be the 2 pins closest to the USB socket, right next to the AREF.
Would these differ in any way from pins 20 and 21 that are labed as SDA and SCL?
They don't. Just another place to grab them from.
It goes to the USB->Serial chip, which is another Atemga processor (16u2): http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-sch.pdf
Yes, SCL/SDA connected in parallel to the other SCL/SDA pins.
Don't know about non-functioning vs Duemilanove - more info needed.
2nd ICSP header is for re-programming the 16U2 chip.
Thanks all. I've been struggling with an otherwise excellent Mega 2560 R3, that for some reason doesn't communicate with any I2C device. I now know that the devices, a compass and lcd work perfectly with my Duemilanove. That also verifies their address. When I run any of 3 I2C scanners on my Mega, nothing shows up for the I2C devices. I have another Mega on order, and perhaps it will be the same, or it might work. I've tried a variety of values of pull up resistors, and also no pull ups, and still get no I2C function on the Mega. I checked the output on the Mega's I2C pins with an oscilloscope, and there is certainly a 0 to 5v signal coming out of both SCL and SDA on the Mega. It all works so slick and easy like everything else with the Duemilanove, just not with the Mega 2560 R3.
For fun, I am going to try the other two SDA and SCL pins. Can any harm arise from accidentally swapping the SDA for the SCL? I'm thinking no, as its just a zero to 5 volt square wave on either. They don't appear to be labled on the added pins next to AREF, so picking the wrong one on this attempt is 50/50.
Northof49:
They don't appear to be labled on the added pins next to AREF, so picking the wrong one on this attempt is 50/50.
They are labeled, per my post above.
As for why I2C isn't working, well, sounds like you are debugging that one. Seems odd though, unless the Wire library needs changes to work with the Mega (which seems unlikely).
Northof49:
Where are the:Revision 3 of the board has the following new features:
1.0 pinout: added SDA and SCL pins that are near to the AREF pin
I can't see anything near the AREF pin labled as SDA or SCL.
Would these differ in any way from pins 20 and 21 that are labed as SDA and SCL? Still trying to figure out why I can make I2C work on my Duemilanove but not on my Mega 2560 R3.
Also, what is the purpose of the second, unlabled ICSP six pin header?
It's to allow ICSP programming of firmware to the USB serial converter chip.
The reason that arduino added two new I2C signals on the current version of Uno and Mega2560 board was to make life easier for shield manufactures that utilized I2C components on their shields, so that they would work on both kinds of boards, where prior it was more difficult and kludgy to make that so.