Qwiic Cables - 4 ft limit. 28 AWG maximum when we make our own? What AWG size are the official ones?

Qwiic System
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The rule of thumb of 4 foot maximum length for all of the Qwiic cables in a row, is that due to the fact that the maximum physical cable size is 28 AWG for the Qwiic connector? At 3.3V, you are going to start to see some considerable voltage drop as well, especially if the conductor size is so small. The issues we keep coming up against are wire length here and there doesn’t seem to be really any guidance from Sparkfun other than “try it yourself” with regards to number of devices supported on the Qwiic chain in practice. To be fair, different devices draw different amounts of current. Are there any resources out there that speak to this regarding the Qwiic system though? It looks like the official Qwiic cables are 28 AWG based on the datasheets though Sparkfun doesn’t go out of their way to mention this anywhere I could easily see in the product descriptions.

It looks like the braided CAT5/CAT6 style extender product such as your SparkFun Differential I2C Breakout - PCA9615 (Qwiic) BOB-14589 product is suitable to extend I2C further and transparently to the device at least but it also has a few quirks we have to keep in mind as well. Like trying to make sure we are only sending 3.3V out to the devices that can only tolerate that much. We are aware that we need to do so but somewhat unclear at present exactly how to ensure that is happening as the PCA9615 seems to happily tolerate up to 5.5V.

Here is a neat voltage level translator that is compatible with the Qwiic system that we recently came across that lets you use 5V with 3.3V devices (or other options as well) as well as clarifies that each output on it “resets” the 4 foot distance limit.

https://www.smart-prototyping.com/Zio/Z … or-PCA9306

Lastly, do you sell a 4 pin version of your through hole https://www.sparkfun.com/products/9750 product? If not, is that a common Mouser or Digikey part? Preferably at a 0.1" pitch.

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Would edit that post but it doesn’t let me anymore.

https://www.smart-prototyping.com/Zio/Z … oster.html

Sparkfun doesn’t seem to offer something like this exactly though it has been suggested before but it might be a nice go between instead of the RJ45 style one to extend Qwiic connections more than 4 feet but maybe less than what the RJ45 style one allows you to do?

Would shielding the Qwiic cable make it work any better over longer distances or is the reason it stops working reliably less to do with noise and more to do with voltage drop and the like? Or both?

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The length limit doesn’t have anything to do with wire size, it’s how much capacitance the wires have. It’s a fairly technical subject and more than I’m able to explain, but the link below might be helpful to you.

I2C Bus Electrical Specifications:

[I2C transmission distance, data rate, cable capacitance, and cable length limit

I’m not exactly sure what wire size we use on our Qwiic cables, but it’s probably 28 to 30AWG.](I2C Bus Range and Electrical Specifications, Freescale 9S12 HCS12 MC9S12 I2C Hardware)

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It’s 28 AWG. And does that hold true for both I2C and SPI though?

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SPI is a different bus with different rules. You should be able to run longer wires on SPI though. Unless you’re running power, wire size isn’t terribly important for either bus.