Suffice to say that the I2C world, as simple as intended, can get a bit hairy.
Multiple devices with the same address will lead to bus conflicts. There can be missing (or multiple!) sets of pull-up resistors to contend with on occasion, and don’t even get me started on the wonderfully loosely defined part of the specification that deals with clock stretching — even the Raspberry Pi has had issues with that — requiring either a cripplingly slow I2C hardware clock, or software I2C implementation to fix the issue.
When you are cranking out as many I2C prototyping modules as the gang over at Adafruit do, you don’t want to be spending what adds up to a significant amount of time, hooking up a logic probe every time you don’t get an ACK on the bus.
To this end, they have put together a really handy looking gadget, named the STEMMA Friend, aimed at easing the pain that can be experienced when bringing up a board with a I2C device loadout!
Read more, excellent overview by Tom Fleet at Hackster.io
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