Ok, the 8 little board designs are done and have been sent to manufacturing (doesn’t that sound impressive?). Here’s the final layout check:

(This is a screenshot from ViewMate, with all the Gerber files generated by Eagle loaded in.)

Here is a summary of what each board does, from left to right, top row:

• The Blink Plug is as simple as it gets: two LEDs and two push-buttons.
• The Expander Plug has 8 general-purpose digital I/O lines – based on the MCP23008 chip, it uses an I2C bus and allows daisy-chaining. Two jumper pads allow up to 4 of these plugs on the same bus.
• The Thermo Plug ties the AIO pin to either an NTC (thermistor), or a K-type thermocouple (via an expensive AD597 chip), or a DS18B20 1-wire sensor. The DIO signal drives either an on-board buzzer or an external relay / light / DC motor via a transistor.
• The I2C Connector is not a plug but hooks up to the 4-pin PWR/I2C header on the JeeNode and provides a “port-like” 6-pin bus for I2C-based plugs.

In the bottom row:

• The Room Plug is a shield-like daughterboard which plugs into ports 1+4 or 2+3. It supports an SHT1x temperature/humidity sensor or a DS18B20 1-wire sensor, a PIR or ePIR module, and an LDR.
• The Memory Plug is an I2C-type daisy-chainable plug with 1 .. 4 EEPROM chips (up to 512 Kb).
• The Clock Plug is an I2C-type daisy-chainable plug with a DS1307 real-time clock and battery backup.
• The Breadboard Connector is the thing marked “plug” in this previous post. It’s not a plug, just a bit of wiring between a bunch of pin headers. It also works on the rightmost pins of the Expander Plug.

I2C plugs are at least 0.1″ wider than daughterboards, so that they can’t accidentally be plugged into two ports at once (which would short-circuit just about everything!).

I haven’t tried all of this yet, the I2C stuff in particular is untested. I have no idea how many of these plugs could be daisy chained reliably, or what the limits are in terms of power drain. But with the I2C Connector, things should be fairly robust since it has pull-up resistors to bring the I2C bus in spec, as well as a 3.3V regulator.

Note also that all these I2C buses (up to 4 bit-banged ones and 1 with MPU hardware support) use 3.3V logic levels. Just like the rest of the JeeNode, I’ve decided to use 3.3V as standard voltage for everything. The “PWR” pin carries whatever voltage is fed into the system, though it will most likely be about 5V in many cases. More and more sensors and memories come in 3.3V versions, these days.

It’s far too early to say how far this can be taken. Can we connect up to 160 individual I/O pins on a JeeNode? (5 buses x 4 expander plugs), or 2.5 Mbyte of EEPROM memory? (5 buses x 1 memory plug) – I doubt it. Besides, if connecting the maximum number of I/O lines were a goal, I’d probably combine several 16-line expander chips on a special-purpose plug instead, or even use 40-line expanders.

Done! The next challenge is to maintain all this flexibility in the software.

Update – I’ve added some docs, very preliminary for now, i.e. until the boards get a proper workout.