When you need many sensors at once...

Question

I have a project where I may need 25+ accelerometers and magnetometers.

I see three options for sensors:

-Direct wire, which could be possible with shift registers

-I2c, however many sensors of the same brand only ship with 1 or 2 address options and I dont want to have dozens of different brands with with different protocols

-SPI, same problem as I2c.

I found the I2C sensors to be cheap, is there a way to use numerous sensors with the same address within one bus?

dougw · Accepted Answer

Linear Technology makes chips to handle this situation - up to 127 devices with the same address can be handled on one bus, but you may need one of these chips at each sensor. They cost about $2 each. LTC4316 LTC4317 LTC4318 Linear Technology - Product Page http://cds.linear.com/docs/en/datasheet/4316fa.pdf

shabaz · Answer

yet here you are again pushing the easily dis-proven idea that I2C is only good for short distances between boards Where did I state with the explicitness that you suggest, that it is only good for short distances? Engineers try not to speak in absolutes, but they certainly make approximations, to convey ideas.

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You yourself stated a similar guideline yourself,.. I'm only quoting you from your earlier thread.. "so keeping distances as short as possible and routing away from any noise sources is a good idea" The video interface (built to a low cost) is still an exception, people don't use I2C for any significant distance generally. ("Generally" does not mean "only" by the way..). And I suspect there are plenty of cables from cheap vendors that fail for this reason, leaving the user wondering why their video devices need manual configuration.. And, in some products ("some" does not mean "all", just to be clear, although I'm beginning to feel only you need this level of clarification since you tend to see things that are not really written) it would be a joke to even consider I2C outside the enclosure. Furthermore it would be a dumb thing to promote the use of I2C over a long distance for people trying to learn by asking advice, and setting them up for a fall if I suspect they may have an undefined cable and undefined level of test equipment. I'd rather make them aware, and learn more about the use-case, and give them some information that they can take and use with confidence. This isn't "pushing" a non-I2C agenda/conspiracy. That would be as daft as it sounds. Sure, NXP recently created a marketing paper promoting multi-kilometre use (with many repeaters!) - took them 25 years to get around to it, so clearly not a popular scenario - it was published in 2014. Anyway, reaching out to my latest copy of 'Art of Electronics' - famous for practical real world guidance- the authors, Paul Horowitz and Winfield Hill too suggest (on page 1032) that typically [amongst other serial busses, I2C is] an internal serial protocol, chip-to-chip and within an instrument . You might well wish to use I2C for long distance use but real engineers tend to ("tend to" != "only") use it conservatively for short distances because there are better long-distance protocols.. (by the way, I am qualified and was a Chartered Electrical Engineer before I moved on to other equally technical jobs, i.e. recognised by my peers and my professional body (IEE, now IET) to have satisfied practical experience in the field of electronic engineering as well as achieving educational qualifications in the same field (electronics). In the 4 or 5 years I've been a member here I have never had to state that and put my cards on the table, but it's tiring discussing this mundane I2C topic with you. Sensible readers can decide for themselves if they wish to use I2C over long distances and what precautions they would take. If they ask for such advice, then it can be stated as fact that I'm giving my advice based on experience designing with it in industry , not just from using the controlled design cable that is VGA.. I'm not 'trying to push' an idea as you suggest, for any other reason - if I did, I wouldn't be living up to the standards that I try to always maintain..

shabaz · Answer

Not that old story again. You discussed this quite a while back:

https://www.element14.com/community/message/72520/l/i2c-buss#72520

If we take the time to analyze the signal on the wire then sure you can push I2C - but I think it is fairly clear that if the person asking the question doesn't know how to connect multiple same-address sensors then they may not have access to an oscilloscope or even know what to look out for - hence the gentle probing about their use-case.

At a first approximation, I2C is typically used for on-board and board-to-board communication within an enclosure, with some exceptions being the cable you refer to - it's an exception that works because engineers examined the signal on the wires in detail. If you have (possibly) limited hardware and test tools and using whatever cable is at hand, then you wouldn't use it across 25 feet.

In fact you yourself came to some similar conclusion when you stated on that thread

so keeping distances as short as possible and routing away from any noise sources is a good idea

clem57 · Answer

I will start by asking why 25+ sensors? Maybe the way you use them is too narrow? Curious tho. Clem

jc2048 · Answer

Perhaps you could use a repeater like this. http://www.ti.com/product/pca9515b (This one's for 3.3V use, but there are plenty of alternatives if you look through that product section) It's got an enable pin, so if you had one in front of each of your identical slaves they could be enabled individually one at a time as you wanted to talk to them. Think it would work, though I've not done it myself so an idea rather than a recommendation.

When you need many sensors at once...

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