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In NXP's I2C specification and user manual Rev. 6, it states we can put series resistors on SDA and SCL lines for protection against high-voltage spikes:

enter image description here

And it gives a graph, to show the relation between Rs and Rp:

enter image description here

I wonder how NXP get this graph, if there is a closed form equation to show the relation between Rs and Rp?

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  • \$\begingroup\$ SDA and SCL pin does have it's own capacitance (which limits switching speed), and Rs additionally limits charging current. I think they've calculated that figure from condition that allows to switch with needed speed (just as Rp calculates for long lines). \$\endgroup\$ – Looongcat Sep 24 '15 at 5:38
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Rp and Rs form a voltage divider. Rs must be low enough to keep the voltage on the line below VIL-VnL. They must also be small enough to sink at least IOL.

We therefore have the following:

Is - Ip >= 3mA

Vs / VDD <= 0.2

Vs + Vp = VDD

Solving for these simultaneously results in the output shown in the chart.

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