SCL at 400k is not a good square wave

[StevenW807]

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? 

[awneil]

1. Your pullup resistance value is too high. See: electronics.stackexchange.com/.../473799

2. Where do you get these figures from? It's probably allowing for the fact that actual electronic circuits are not perfect.

[haakonsh]

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.