One of the small innovations in the JeeNode is the use of “Ports”:
Most µC boards, including the Arduino, brought out all the I/O pins on one or more large headers, meaning that you had to pick and route individual pins for each case where some component or sensor was being connected. While flexible, it prevented adding stuff in a plug-an-play manner: hook up some sensors, use matching bits of C/C++ code, and done.
I also always kept running out of VCC and GND connection pins while experimenting. So instead of bundling all I/O pins, a decision was made in 2009 to bring out a couple of (nearly) identical “ports”, to any of which the same sensor could be attached. That meant you could connect up to at least four sensors independent of each other.
As it turns out, there are enough spare I/O pins on an ATmega to provide one digital and one analog pin to each port. Add GND, VCC (+3.3V) and PWR (the input voltage, could be up to about 13V), and you end up with a fairly general-purpose setup. One extra spare pin was provided as “IRQ” to allow connected sensors to draw attention to themselves via an interrupt, although this feature has admittedly so far not been used much on JeeNodes.
But the real improvement is the fact that the two I/O pins can be used as I2C bus. All of a sudden, each port is no longer limited to a single sensor. The I2C bus is so useful because many types of low-cost chips include an I2C interface in hardware, and it lets multiple chips operate on a single bus.
One of the first extensions added was the Expander Plug, an 8-bit general-purpose digital I/O interface. So now, with just two pins consumed on the ATmega, you could have up to 32 input/output pins (by daisy chaining 4 of these plugs and setting them each to a different I2C address). The Dimmer Plug takes this even further: up to 8 of them on one bus, each driving up to 16 LED’s – for a total of 128 dimmable LEDs on 2 I/O pins!
Officially, the I2C bus requires dedicated hardware with “slew control” to increase the accuracy of signals on the bus. And while the ATmega has such an interface, there is just one per ATmega. Fortunately, you can also “bit-bang” I2C in plain software, i.e. simulate the relatively slow pin changes with nothing but software on any 2 I/O pins. The speed is not quite as high, and the supported bus length is also limited to a few dozen centimeters, but for direct connection of a few plugs that still works out quite well.
And so the JeeNode has ended up having 4 “ports”, each of them individually capable of supporting either I2C, or one analog (AIO) and one digital (DIO) pin – or alternately 2 digital I/O’s, since each analog pin on the ATmega can also be used in digital mode.
Since then, lots of different types of “JeePlugs” have been created, some I2C, some not. Most of the plugs have a corresponding class and a demo sketch in the JeeLib library, making it very easy to hook them up and interface to them in software. With most plugs, you just have to define which port they are connected to – and in the case of I2C plugs, which bus address they have been set to.
The placement of these ports and the choice of very low-cost standard 6-pin headers took some experimentation, but I think it all turned out ok. Lots of expandability and flexibility.