[OpenTRV-dev] I2C expansion connector round 3

[Richard Chilton]

Can I ask what the reasoning behind optionalising nIRC/nSC was, as I think the answer will help me understand the requirements better.

Thanks,
Rich

On 8 Sep 2014, at 16:18, Damon Hart-Davis <EMAIL ADDRESS HIDDEN> wrote:

> Well, that don’t last long!
> 
>  In response to comment from production engineers I am making nIRC/nSC support optional for now, depending on the main board.
> 
> 
> 
> 
> 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 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.
> 
> 
> 
> Note: see alternatives such as http://www.tag-connect.com/ for SPI/ICP part.
> 
>> 
> 
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