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Re: Can we come up with a Standard Plug/Socket for I2C

Posted: Wed Sep 11, 2013 9:12 am
by meltwater
Yep guess we should be thankful they have one add-on board I suppose.

Also, yes, there is possibly the case of "we can sell them an adaptor for that too" type approach. I guess it makes it more lego like, which is a good thing I suppose.

It is just the fact that I2C was so carefully designed to only need a few wires/connections, goes against the grain a little to make use 20+ extra.

Mind you, I guess if there was a standard-ish I2C connection, then they could keep the full header and easily add a few pins to allow daisy chaining of I2C devices (and an alternative way to connect) - the obvious advantage to them is selling multiple devices. If using the 2x3 header option, it wouldn't even matter if you had full 2x13 connection or not (as long as there is clearance).

Using the unit with breadboard/stripboard would be difficult, but I guess using M-F jumper wires would be possible. Also you could mount 2x3 connectors on a stripboard (with careful track cutting), as it is only a few pins to carefully solder in place (can keep the fall-back option of the 6x1 header anyway).

2x3 Connector which matches top pins of P1.
P1:
3V3 5V
SDA 5V
SCL GND

Colours (if available):
Orange Red
Purple Missing
Blue Back
Only confusing thing might be that the pin one wire will be orange.

We would still need a converter for P5, which while you were at it would need to include 1K8 pull-ups on anyway, so it would help to have a different connection here. Oh, and the added fly...have to consider top/bottom connections...which is helpful!

I think at a push a 1x6 variant can be used, for cases where it is easier design-wise.
3V3
5V
SDA
Missing for polarization
SCL
GND

I don't know if we would want to include a blocked hole/key in the cable for the missing pin (making it a problem if there is a pin there, like on the RPi itself or on a full header), or let it be inferred by missing pins on the devices by not having a wire going into it. My preference is the latter of the two. It could be optional to use a keyed connector as it should still work with open pins (as long as it doesn't get in the way of other pins on a full header).

I think in general the proposal has more advantages than disadvantages over picking some other arbitrary arrangement. The simple 1 to 1 wiring for beginners would be very helpful.

Re: Can we come up with a Standard Plug/Socket for I2C

Posted: Thu Sep 12, 2013 12:51 am
by AndrewS
meltwater wrote:It is just the fact that I2C was so carefully designed to only need a few wires/connections, goes against the grain a little to make use 20+ extra.
I2C stands for Inter-Integrated-Circuit (i.e. it's originally designed for board-level routing of hooking together multiple chips on a single PCB), not Inter-Pi-Addon-Board :twisted:
The advantage of it using so few wires is that it is easy to just use jumper wires to hook things up as necessary.
Mind you, I guess if there was a standard-ish I2C connection, then they could keep the full header and easily add a few pins to allow daisy chaining of I2C devices (and an alternative way to connect) - the obvious advantage to them is selling multiple devices. If using the 2x3 header option, it wouldn't even matter if you had full 2x13 connection or not (as long as there is clearance).
What, and upset all the people who don't want to use I2C but do want access to the SPI ports? ;) Can't please all the people all the time...

Feels to me like this discussion is just going round and round in circles...? :roll: (or maybe it's just because I'm tired and I've had a long day :oops: )

Re: Can we come up with a Standard Plug/Socket for I2C

Posted: Thu Sep 12, 2013 7:45 am
by meltwater
AndrewS wrote:Feels to me like this discussion is just going round and round in circles...? :roll: (or maybe it's just because I'm tired and I've had a long day :oops: )
I think we kind of have the result we could have hoped for, and that is a recommendation of a pin arrangement for I2C.

It doesn't mean that everyone will/should or is expected to use it (you can use what you like), but a recommendation is about all we can suggest, with fairly good reasons behind it. I think the result is easy for beginners to understand, which is always a bonus.
What, and upset all the people who don't want to use I2C but do want access to the SPI ports? ;) Can't please all the people all the time...
If you want to cover SPI, then we may as well "keep it simple" and go with the same concept, but such is life the 5V lines are nowhere nearby (so bang goes that plan!). Although there may be stronger standards in that case (and from what I recall just as much variation too). Having a I2C connector doesn't exclude using the SPI, particularly if the boards are designed nicely, if anything it may discourage full-header hogging boards which would.