Signal Problems with PCA9615 - can't see MCP23017

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1

Hi, I have been trying to debug an issue with not being able to use MCP23017 when hooked up through a pair of SparkFun PCA9615 boards. I soldered headers to connect to Raspberry Pi Zero, I am using about 15ft cat5 cable. Both boards have bounded VDDa and VDDb. I checked everything with a volt meter. I had some trouble seeing voltage on the far end at first, but I sorted it out.

Still I can’t see the device on the MCP23017 bus. I also hooked up an oscilloscope on the far end (PCA9615) and on the near end (Pi 0). What I noticed is a difference in the data line, specifically the low on the far end is not getting as low as on the near end. Please take a look at the attached screenshots. The wave is from the “detect devices” command

This is my closest lead as to why I can’t see the MCP23017.

I would really appreciate some help getting this to work.

detect devices at pi.png
detect devices at pi.png1492×709 77.6 KB

detect devices endpoint.png
detect devices endpoint.png1492×709 78.7 KB

2

I have done some more testing and digging.

I hooked up another sensor - ADS1115 and it showed up on the bus w/o a problem. Then I dug into the data sheet for MCP23017 and I realized that the max voltage for a low on SDA/SCL, etc is 0.2V, and from what I observed I get only as low as 0.7V on the far end. So I think I found the culprit, but these questions remain:

  • Why - do I get 0.7V and not 0.2V or below for low value (it is 0.2V on the Pi0)

  • What can I do about it?

  • Is one (or both) of the boards defective?

Thanks for any advice

3

If you would be willing to humor me, can you share a clear picture of your setup or provide a clean image representation of your setup?