Given a few posts have been looking at the i2c bus/protocol, I thought it might be worth a read.
On doing so, I found a few things that might be worth discussion.
To get started, after looking over bith the NXP Document
src: https://www.nxp.com/docs/en/user-guide/UM10204.pdf
and 2nd document
src: http://esd.cs.ucr.edu/webres/i2c20.pdf
On uses the term “must be” and the other "should be:
and state that if the length > 10cm then the connecting wire/trace must/should be…
So with that in mind, why is the qwicc cable not in that order ?
While i understand that the qwiic has become a common plug on dev boards, I would have assumed that either the never expect it to be > 10cm or it was never considered… or a 3rd option that I cant think of, hence the question.
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Hi Michael
I think there are several things the designers / manufacturers of some of these “hobby” type boards and add ons that are not considered or missed. I think some think that if a good percentage of devices work under iffy conditions (like outside individual component specs) then that is OK. Good practise I don’t think.
I have found over the years that specs are there for a reason. It would be interesting if someone did some MTBF numbers for some of these offerings wouldn’t it. Mandatory in some circumstances. Another cute one is MTTR (Mean Time To Repair) Which I came across in a couple of projects years ago. I once had to redesign a complete cabinet rack layout because with my existing layout a cooling fan in one unit could not be replaced in the allotted time (30 min).
Cheers Bob
Cheers Bob
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Yeah, Im trying to see if there is a reason for it or it it was just not thought of?
People tend to remember what the first learn, so if you buy something and its not correct, yet works, people will assume its correct and build into their designs. in fact with sometime like this, you are forced to use it if you want to use that port on modules.
I remember in my early days we where running some cat5 cable. I said to my helper, lets go 568A termination standard. Then when we come to testing, none passed. turns out the TAFE teacher told them something that was not the A or B standard and was just wrong; even showed me the hand out he was given.
So if the data comes from a source that is seen as an “expert” people will trust and run with it.
All that said, there still could be a valid response.
i.e. if the length is < 10cm I did not see any reference to the wire order, as such, the modules that use QWIIC might simply be for < 10cm runs.
I would assume the able run in a bus is end to end, so if you cascade 4 modules 5cm apart, you would have a 20cm + board tracks at each device cable run.
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Becasue it’s not ribbon cable. The ‘should’ that you quote seems to be applicable only to cases where there is a risk of significant cross-talk - ribbon cable and PCB traces. The implication is that where there is not that risk then the order doesn’t matter.
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Hi Michael
Well said and very true. Especially this bit.
And it is very difficult to UNLEARN bad habits and learn new ones.
Not any fault with anyone involved but (me included) tend to take notice of people you would expect to know and are usually on a higher pay scale. After all you probably don’t know and you have got to get the information somewhere so higher up would be the go to place.
Practical example. While working a new addition to our department arrived. An adult motor mechanic looking for a change of scenery. Knew nothing about electronic type projects but keen and eager to learn.
Sat him down for a couple of days with the company Standard Practise Manual and the invitation to ask questions which he did. We explained what his job entailed and importantly EXACTLY how to do it and the QA standards to achieve.
Well he was a very quick learner and with his mechanical background and this new set of skills he turned out to be an absolute gem when it came to project installation work. A great asset who had the ability to work unsupervised to a great extent.
I guess what I am emphasising here is regarding our work he had NO habits to UNLEARN and he did learn his new tasks very well indeed.
Cheers Bob
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