[OpenTRV-dev] I2CEXT connector first pass: comments please.

[gareth]

>From my experience i2c doesn't travel well - and grey beards have advised me that off board i2c wires are an excellent way to broadcast emc!

Damon Hart-Davis wrote:
> Hi,
>
>
> 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:
>
> 12-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
>  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 is active low and is pulled up to 3V3/Vcc with 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.
>
> 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.
>
> Rgds
>
> Damon