SCL at 400k is not a good square wave

As you could see from the below image, the SCL signal at 400kHz is NOT a good square wave.

1) Why & how to solve it, please?

2) Also, why SCL is between 0.4V to V_DD=3.25V? Shouldn't it be 0V to V_DD=3.25V? 

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  • Also:
    1. Your probe typically adds a 12pF capacitive load when attached, requiring stronger pull-ups. 

    2. Where on your circuit is your probe's ground reference attached? It seems like it has a lower voltage potential than the TWI circuit's ground reference. 

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