> Just question out of curiousity.
> 
> Why that impossible to one person to use current source
> in place of pull up resistor with the same amount of current
> as pull up resistor provides when it's grounded.
> Charging up the bus capacitance will become linear
> and thus takes less time.

Philips cover high-speed I2C and high-C busses. For normal
low speed operation resistors are fine, IM (very limited) E to
meet spec

Section 17

http://www.semiconductors.philips.com/acrobat/literature/9398/39340011.pdf

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