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I2C Multiplexer - Simple?

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Siwastaja:
Maybe nctnico meant these 100k pull-ups as extras, only to maintain idle states, in addition to to the "main" pullups (maybe 2.2k) somewhere else (e.g. at the master); that would work. Although I fail to see the advantage in this as you need two more resistors in total.

fourfathom:
I used a strong pull-up on the controller side of the analog mux, and two weak pull-ups at the mux pins that feed the devices.  I guess two strong pull-ups on the device side would have been just fine, but I didn't want to risk having the controller side left floating during mux switching transitions.  No doubt the bus capacitance would have been adequate to hold the levels, but I decided on "an abundance of caution".  And it's just one practically free resistor.

nctnico:

--- Quote from: Siwastaja on July 01, 2023, 08:41:55 am ---Maybe nctnico meant these 100k pull-ups as extras, only to maintain idle states, in addition to to the "main" pullups (maybe 2.2k) somewhere else (e.g. at the master); that would work. Although I fail to see the advantage in this as you need two more resistors in total.

--- End quote ---
The 100k Ohm is to maintain idle state indeed. IMHO you'll need those as I2C pins on I2C peripherals chips typically don't have pull-ups and will float. An open pin can easely pick up enough from the pin next to it to make it see a different logic level.

Siwastaja:
Probably simplest and most robust solution would be 1:1 distributed between the two sides (at master, at the slave), so that if you desire 2.2kOhm, you would use 4.7kOhm on both sides for 1 BOM line and 2.35kOhm effective. In practice 2.2kOhms at master and 100kOhms at slave of course works equally well.

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