The setup:

I plugged an STEVAL-MKI224V1 board (basic eval board for baro sensor) on a nucleo F411RE. Communication happens through i2c bus.

I added 4.7k pull-up resistors on SCL and SDA.

The wiring:

  • SDO to VDD (set i2c address)
  • CS to VDD to activate i2s communication
  • VDDIO to VDD to nucleo 3.3V
  • GND to nucleo GND.
  • SDA and SCL to nucleo.

No internal pull ups are used.

The issue:

As communication was not successfull on first frames, I plugged a scope on SCL and here's what I get:

enter image description here

On the first bytes, the low level is not +/- 0V as it should be.

Other tests:

  • I tried a 10k pull-up, the result is the same.

  • I tried with only pull-ups (without eval board), result is ok.

  • I tried with another eval board for another sensor (Rohm), the result is far better but there's still a "rounded" 0. On this view, I zoomed a bit, code and i2c configuration is the same:

enter image description here

What did I miss?

Pictures of the setup:

enter image description here enter image description here enter image description here

  • 1
    \$\begingroup\$ The text doesn't mention it, but is GND connected between the boards? Consider editing the question to include a schematic of the connections between the boards. \$\endgroup\$ Commented Nov 13, 2023 at 15:32
  • \$\begingroup\$ Yes, it is. I did my best to improve the question but sorry, I don't have schematic. \$\endgroup\$
    – Julien
    Commented Nov 13, 2023 at 15:59
  • \$\begingroup\$ Looking at the picture of the connections I can't see anything obviously wrong. \$\endgroup\$ Commented Nov 13, 2023 at 16:21
  • \$\begingroup\$ You were not that far: I changed GND naughty dupont cable and it works. Thanks! – \$\endgroup\$
    – Julien
    Commented Nov 13, 2023 at 17:01

2 Answers 2


Looks like

  • either you've got some precharged capacitive load on your SCL,
  • or your SCL carries the involuntary job of exchanging current until ground potentials between two parts of your system have equalized.

Barring the possibility that you added a literal capacitor from SCL to ground or Vcc, I find the latter more plausible. Make sure you're not just connecting SCL and SDA between parts of your system – but also GND.

  • 1
    \$\begingroup\$ I changed GND naughty dupont cable and it works. Thanks! \$\endgroup\$
    – Julien
    Commented Nov 13, 2023 at 16:59

The black ground wire is loose and makes no contact.

That will explain the waveforms and the pin looks loose on the socket.


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