Arduino UNO I2C TWI timing

Hello.

As far as I understood from the code, The TWI library for Arduino UNO communicates at 100KHz clock frequency by default (which can be changed).
Do you perhaps know, what is the rise and fall time of SCL line for default frequency?

Thanks!

Why do you ask ? For which device do you want to know that ? It is a strange question.
The rise time depends on the capacitance of the wires and the connected I2C devices and the pullup resistor.
With 10k pullup resistors it will be slower, with 2k2 it will be faster.
A level shifter might also influence the rise and fall time.
With short wires, the speeds can often be set to 400kHz with an Arduino Uno. But when 400kHz is used with a 10k pullup resistor, the rise time is often too slow.

As Koepel said, the rise and fall time depend in large part on the external hardware. You can measure it with an oscilloscope, and somewhere recently on here I saw a link to somewhere where someone had done that while trying different pullup resistor values. But the physical wiring will impact this, too.

http://gammon.com.au/forum/?id=10896&reply=5#reply5

Function Wire.setClock(uint32_t) exists but is not documented.
Have a look in :
Wire.h
Wire.cpp

I2C of Atmega328p works fine until 533 kHz.