Help with pcf8574

To add to all of these question, The pcf8574 datasheet sepcifies the max i2c input rise time to be 1μs but it does not specify a minimum rise time. i am also confused about the first figure on page 7 of this datasheet,

the first figure show the use of a 1kohm resistor?

I submit that you should only concern yourself with the measurable resistance from SDA & SCL to
5V . Use the scope on those two signals and compare to Nick Gammons screenshots but focus on
the resistance.
Also, you need to investigate the overvoltage . Where is that coming from ? (the arduino ?)

If you look at the floor levels in the two yellow trace images, you can see it comes off the 0v level.

470 ohm
image

Example, here you can see a HIGH is 5v but the low level is 520mv above the 0V that we need.

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That's a good reason to avoid pullup values less than 2k. (or 1k)

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could that cause the pcf8574 to randomly reset itself? will 500ohm resistors on the i2c lines cause the pcf8574 to sink or source any current?

A few hundred millivolts probably not, but let’s remember there can be switching noise (from other components) on top of the floor voltage.

These can add up to a level where unpredictable results can happen.

We always what to maintain solid HIGHs and LOWs and appropriate Rise times in our designs to ensure we stay away from indeterminate voltage levels.

A solid state switch should always saturate.

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Yesterday I got the resistance up to 2.5k.. im probably going to bring to to around 5k. I put a decoupling cap on the pcf8574. So far u have not had an unexpected pin changes.

However I'm in the processes currently building another exact device. I will have time to put my oscillocope on the new one while hopefully the other one works in the mean time.

"Hope for the best, plan for the worst"

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