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tlfong01
tlfong01
1:49 AM
@RobH. I just saw that you are using Rpi3B+, and there is a big problem. It was some two years ago I found a rpi pro complaining that he could not set his Rpi3B+ I2C speed to higher or lower than that default flat 100kHz. So I joined in the swearing discussion group that I also discovered the bug, verifying by three Rpi3B+ and my oscilloscope.
Then the Rpi "experts", including one who claimed that he "participated" in the "foundation". All experts then said according to official doc, Rpi (3B+), you could change the frequency, by this doc, this and dtoverlay statements. Me and other angry swearers of half the galaxy verified and verified to concluded that Rpi3B+, also confirmed by some insiders that ．．．
the official doc was misleadingly wrong. Anyway, it was only when Rpi4B and buster came along that the "system bug" was fixed. In other words, if you are using Rpi3B+ and you thought you were changing the I2C speed to 50k, 400k, whatever, and if you had a scope you can immediately verified that you were a fool, cheated by the official doc and other elites and experts (just reading the doc without any practical I2C experience).
If you skimmed by long list of posts there, you might also found me complaining that not just for Rpi, but also a couple of famous brands do "sometimes" created bugs in there hardware devices, well they called "special features", and won't disclose them until the developers press hard enough then they would admit.
Thank you for reading my long paragraphs. What I want to say/repeat is the following: "RPI3B+ HAS A SYSTEM BUG, AND CANNOT CHANGE I2C SPEED". Cheers.
 
 
16 hours later…
Rob H.
Rob H.
6:02 PM
@tlfon01 I indeed noted in the links you posted that people complained that on the Rpi3B+ the speed could not be set. That would explain why it did not have any noticeable effect on performance.

Either way, I by now have one sensor on the end of a ~5m Cat5 cable that has been returning values non-stop every 10s for over 24hrs now. So I'm quite confident that on the electrical / hardware side this configuration with the bypass caps fixed the problem.

Remaining issue is that it does not seem to be possible to add multiple stable I2C busses extra to the Rpi3B+. Looking at the specifications …
 

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