I2C GND Differs

[danielboe]

Hi,

I did some measurements today on our I2C bus and see that the GND level is different some times. The most time it is not GND level (about 800mV). I checked cross talk but dont see any reason and also it is perfectly timed. Has anyone an idea what could be the reason?

best regards

[ketiljo]

What's the value of the pull up resistors? It looks like one of the devices on the bus can't pull the lines down enough. Try using larger pull up resistors. 

[danielboe]

Hm, I tried different Values .4.7K to 800 Ohms. If I lower the value the problem moves to the top. I will try a bit more with different values. At the moment the I2C bus is connected to many BSS138 Mosfet and is routet to different devices all with the same address. So this works fine but still I wonder why the bus looks so strange

[danielboe]

I dont think it has to do with the pullups because also with 5K the problem exists and also there is a base line visible so the port can on some point reach GND

[ketiljo]

Use larger values, 10k maybe. Smaller values will of course make it worse. Some of the devices on the bus have less drive strength than others and will cause this. 

[danielboe]

okay this makes sense. maybe the sensor is not able to get the level back to gnd but the nrf chip does. We will check this out, thanks!