I am controlling a MCP23017 to expand the number of GPIO through i2c.

Basically I use pins 3 and 5 for i2c.

When I power up the circuit using the +5v and ground from rpi pins 4 and 6 I can run my program written in golang and it works fine.

But when I use a computer power supply to power the MCP23017 with 5V (red) and ground the rpi cannot detect anymore the chip on the bus.

i2cdetect -y 1 does not detect anything despite my control LED is up.

I am then wondering if the i2c bus of the rpi is only enabled / active, when there is a minimum load on the other pins.

1 Answer 1


The ground of the power supply must be connected to a Pi ground.

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