What is the proper length of a I2C flat cable of a temperature sensor probe?

Dear Digital Technology Experts,

I am planning to build a temperature monitor based on a I2C sensor probing hard-to-reach area such as refrigerator interior, ocean terminal container interior, etc.

Then, I have found this cheap China sensor.

Now my question is:

What is the possible maximum length of the probe's flat cable so that I2C data transmission between this temperature probe and the MCU is still kept intact?

Those 4 cables are:

I2C Clock
I2C Data
Voltage input
Ground

Thanks for joining this discussion.

Top Replies

0 Jan Cumps 30 days ago

Flat cables aren't ideal, but if you keep the 2 data lines at the outsides, and keep the wire away from other wires, you may get a reliable meter or 2 at speeds up to 100 kHz.

Tricks for longer distances: lower speed, lower pull-up resistors, line drivers (with twisted pair balanced data lines), shielding ...
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0 beacon_dave 29 days ago in reply to Jan Cumps

I recall seeing some MAX3816A IC's being used in AV display extenders for the DDC signalling.

https://www.analog.com/en/products/max3816a.html
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0 dougw 29 days ago

As Jan indicates, 2 meters should be fine at low speed. If you need to go further check back here for more advice. If 2 meters isn't quite working try with stronger pull-up resistors.

Note HDMI cables have I2C in the cable, so those lengths can work.
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0 wolfgangfriedrich 29 days ago

You could do the math and see if your system is compliant to the I2C spec and operational in general.

For the rise time of the leading edge of the signals, you need the pull-up strength and capacitance of the cable (dominant factor) + sensor chip + traces.

For the level of a driven low signal you need the driver current sink strength and again the pull-up value and rail voltage. With some added margin to account for the noise of the universe, you will know that your system will work.
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0 colporteur 29 days ago

Check out Clem's post How Far Can I2C Go? -- Episode 666 for maxing out a length.
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0 HKPhysicist 28 days ago in reply to dougw

I will start to try 1 metre. This length is good enough to probe most refrigerators from outside their cases.
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