A closer look at the “R3” in Arduino Uno

You may probably wonder what the “R3” means in board titles such as “Arduino Uno R3”. It may be simpler than what you think.

To start with, the R3 is a revision number of the PCB. This means there were earlier revisions such as R2 and the original board. So what is the difference between an R3 board and earlier PCB’s? Here are the two most significant ones we feel may affect your project:

  • Extra I2C Headers – the R3 has an SDA and SCL pins after the AREF pins. these are actually a duplicate of A4 (SDA) and A5(SCL) and was probably added to facilitate connection of two devices in the I2C bus.
  • The on board “L” led  controlled by pin 13 is buffered using an op-amp in the R3 while earlier boards directly connects the LED and resistor to pin 13. This eliminates degradation of pin 13’s performance especially when used as an input.

There are other differences like the upgrade of the USB chip from an Atmega8U2 to Atmega16U2 but this is essentially of the same function and is not necessarily the only or the best solution for other Uno compatible boards. The rest of the differences are changes you may never notice or does not really affect performance such as the positioning of some components on the board.

And while we are discussing these changes, it is worthwhile to note that almost all boards being sold today are using the R3 variant. Even Uno-compatible boards have now inherited the changes in R3 and with some  adding even better improvements.

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