2014/09/14: Revision 5, adding some more details of location and form. 
2014/09/08: Revision 4, allowing 4k7 for RX/TX serial for common values.
2014/09/08: Revision 3, note that the nIRQ and nSC lines can be left unconnected on some board variants.
2014/09/04: Revision 2, 14-pin connector, added nINT and nSC2 after dev mailing list suggestions
2014/09/03: Revision 1, 12-pin connector


Aim:

Trying to develop a simple I/O extension/shield system for OpenTRV to add more sensors (etc) easily.

Also this should allow an easy bridge to other systems such as RPi / BB / etc.

This must be simple and small and cheap enough to use in hobby / dev / prod environments and allow (in priority order):

1) I2C expansion.
2) In-Circuit Programming of AVR.
3) One-Wire expansion.
4) Serial off-take, eg for CLI interaction.

The connection should ideally be keyable or at least not cause instant destruction if reversed for example.


Suggestion:

14-way (2x6) pin stackable (pins or socket on board, suggestions please, RPi is pins).

0.1" pitch to be easy for DIY shields, eg with Veroboard as I just did for my RPi.

Following pin pattern:

MISO * .. * 3V3 (Vcc)
 SCK * .. * MOSI
nRST * .. * GND
nIRQ * .. * nSC
 SCL * .. * SDA
  RX * .. * TX
 N/C * .. * DQ

a) N/C means No Contact and can be used to key the connector.
b) RX and TX are protected by 10k series resistors from AVR.
c) nRST (input to main board) is active low and is pulled up to 3V3/Vcc with 4k7--10K series resistor at AVR.
d) SCL and SDA are pulled up at AVR with 4K7 resistors to 3V3/Vcc.
e) 3V3 can be anywhere from 1V8 to 3V6 absolute; 2V to 3V6 operating.
f) nIRQ (input to main board) is active low and is pulled up to 3V3/Vcc with a 10K--47K nominal resistor and a 1K series resistor: this should be driven with open collector/drain driver only.
g) nSC (output from main board) is the active low select for an SPI device on the bus, 
h) The nIRQ and nSC lines can be left unconnected unless the main board variant explicitly supports them.

Features:

1) Top 6 pins can be used directly with 6-pin AVR programmer.
2) Rest of pins carry I2C (SDA, SCL), OneWire (DQ) and serial (RX, TX, protected by 10k series resistors)
3) If entire connector rotated boards unlikely to be destroyed instantly.

Bit more on (3): RX and TX are safe to short to GND or 3V3 or anything else, nRST safe to short to either rail, DQ/SCA/SCL safe to short to GND.


Connector form/style and location, and constraints:

1) Should be near board edge but not in corner for REV2-style 5x5 box.
2) Should be placed prob near edge for easy dev access with 3D-printed board.
3) Should allow USB micro socket to go back in its REV2 position if possible.
4) Should allow I2CEXT board on 5x5 footprint to also be RPi HAT board.
5) Must not be single-source nor expensive nor anywhere near end-of-life.

Some agreed details:

a) Male 7x2 0.1" pitch connector like top part of Raspberry Pi A/B/B+ GPIO connector, with pins above the board (component side).
b) Stacked board to have female connector below the board with extra long pins up.
c) Location on standard 5cm x 5cm OpenTRV board:
   6.35mm (left) x 2.375mm (down) from top left corner of board to centre of top-left pin (3V3)
   with the connector running left to right
   with pins pointing vertically away from the plane of the board towards the viewer.










Note: see alternatives such as http://www.tag-connect.com/ for SPI/ICP part.