Rise time and Fall time of both SDA and SCL signals are given as 20 + 0.1Cb ns in I2C bus fast mode electrical specifications version 2.1. Cb is bus capacitance and units in picofarad. How do the units match when Cb is in pF and time in ns?

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From page no-32 in the specification.

  • 1
    \$\begingroup\$ A link to the I2C spec and section/page reference would help us answer your question. \$\endgroup\$
    – The Photon
    Mar 12, 2013 at 5:23

2 Answers 2


The datasheet states 20 + 0.1Cb, but it doesn't say in what units 0.1 coefficient is. This is not uncommon in IC datasheets. You can still make practical use of the formula.

We can try to back out the units.

\$ \mathrm{ [ pF ] ={\cfrac{[mA] \cdot [ns]}{[V]} }}\$

For the units to match, 0.1 would have to be in the units of \$ \mathrm{{\cfrac{[V] }{[mA]} }}\$.

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    \$\begingroup\$ So the 0.1 is really 100Ω. \$\endgroup\$
    – starblue
    Mar 12, 2013 at 7:06
  • \$\begingroup\$ @starblue Yes, and 0.1Cb is an RC time constant. \$\endgroup\$ Mar 12, 2013 at 7:14

The units are taken care of by the footnote, which states that Cb is in picofarad, and the ns at the end of the row in the table.

So in the 0.1 * Cb the Cb is just a number (the number of pF in Cb), and to the resulting product (again just a number) the unit ns is added.

This is not the normal way to use units, but it is common in electronics, maybe because it is easier to do calculations that way.

But you lose the check that the units match in the calculations.


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