I2C Pull-Up Resistor Calculator

Choosing the correct I2C pull-up resistor value is critical for signal integrity. This calculator follows the NXP I2C specification to determine the valid Rp window based on bus capacitance and required rise time.

Formulas

Rp_max = Tr / (0.8473 × Cb)

Rp_min = Vcc × 0.1 / Iol (≈ 3 mA)

I2C Speed Reference

Mode	Speed	Max Cb	Max Tr
Standard	100 kHz	400 pF	1000 ns
Fast	400 kHz	400 pF	300 ns
Fast-Plus	1 MHz	550 pF	120 ns

Tips

Smaller Rp gives stronger drive but higher power consumption.
Multiple devices share bus capacitance — add all I/O capacitances together.
Consider active pull-up drivers to achieve Fast-Plus with higher Cb.

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