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I have a pin multiplexer with a premade board: https://core-electronics.com.au/mcp23017-io-expansion-board-expands-16-i-o-pins.html My Pi 4 is powered by USB C. The multiplexer is powered with its own power source. It's little LED light is on. I connected the SCL and SDA pins from the pi to the multiplexer (Pi pins 2 and 3). If I run lsmod, I see that i2c_bcm2708 is there. I have /dev/i2c-1 as a valid file.

However, when I run the command i2cdetect -y 1 I get nothing:

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- --

Am I missing something? What else can/should I do to debug this?

Thanks in advance.

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  • If i2cdetect doesn't detect the device you have not connected it or you have something wrong, but you haven't explained WHAT you ACTUALLY connected. Why is the multiplexer powered with its own power source?
    – Milliways
    Jun 9 at 4:15
  • I connected the MCP23017 (the multiplexer) to the PI. It has its own power source because it's easier for me that way -- should I try and power it with the PI?
    – Merc
    Jun 9 at 4:17
  • "I connected the MCP23017 (the multiplexer) to the PI" is meaningless. WHAT you ACTUALLY connect to what. Using external power just seems an unnecessary complication; I have never used one with MCP23017 which can't draw enough to need one.
    – Milliways
    Jun 9 at 4:20
  • I am lost. The MCP23017 is a serial device that works on I2C. I connected it to the PI's I2C pins. Why is my statement meaningless?
    – Merc
    Jun 9 at 4:39
  • I fail to understand why people ask questions then seem to be reluctant to clarify. If you connected correctly it must work or is faulty. There are only 3 wires. Maybe if you post a photo someone may be able to help.
    – Milliways
    Jun 9 at 5:25

1 Answer 1

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In your description you write that "red by USB C. The multiplexer is powered with its own power source. It's little LED light is on. I connected the SCL and SDA pins from the pi to the multiplexer (Pi pins 2 and 3)". This is not enough. You also need to connect GND to GND for the communication to work.

Regarding the power, you could just power it from the pi, unless you have a specific reason not to. If the board draws a lot of power, I would use an external power source, but I am not sure that is the case here.

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  • You are a genius. I figured it out on my own not very long ago. Grounds need to be connected! Thank you!
    – Merc
    Jun 9 at 14:43

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