Basically, “General Purpose” means those consisting of little more than the microcontroller, USB interface, and voltage regulator.
In my last post I mentioned the Uno as the base-line board, with the historical lineage from the original Arduino board. But today you can get many different boards differing in size and microcontroller. In my next book I intend to look Far Inside them all, at least those using AVR microcontrollers.
The boards come in essentially four sizes. The Mega size, the Uno size, the Nano size, and the Mini size. And they come basically in four microcontroller families in roughly increasing modern design, ATmega328P, ATmega2560, ATmega32U4, and ATmega4809. The chart below shows the offerings with an added column for the ARM based Arduinos.
The asterisked boards contain additional features, but I put them in the table to fill empty spots. For the traditional Uno sized boards there is the Uno, Leonardo, Uno Wifi, and Zero. These try to mimic the functionality and pinout of the Uno.
If more I/O pins are necessary, there is the Mega and Due which are larger boards that still accept Uno-sized shields but have extra headers to present more microcontroller pins.
Moving smaller, the Nano gives the functionality of the Uno with minor omissions (not buffering the LED) but with two additional analog input pins thanks to the surface mount package of the 328P. This size also has the Micro, in a slightly longer board to fit four more pins, and the Nano Every. The Nano Every uses the most powerful AVR and is also the least expensive Arduino board. It’s only real problem is that it is not completely backwards compatible with the Nano (or Uno) and people have become mystified when their Uno programs fail to compile. The new ARM-based MKR series board are midway between these sizes. See Is the SAMD21 the future for Arduino Boards?
Finally there are some sub-Nano sized boards basically for when the space is at a premium. The Mini, a discontinued board for which clones can still be found, leaves off the co-processor and USB port, so is considered a board for professionals. The Pro-Micro is not an official Arduino board but was designed by SparkFun and also is available as clones. Since it uses a 32U4, the USB port is available, but is missing 7 pins that are on the Micro. This seems to be an easily usable board when one can get by the fewer pins.
My next post will discuss the differences between the microcontrollers.