I2C Pull-up Resistor Calculator

I2C bus issues that only show up at certain clock speeds or bus lengths are almost always a pull-up resistor problem. This calculator finds the valid pull-up range straight from the I2C-bus specification's sink-current and rise-time limits, for your actual bus capacitance and target clock speed, rather than defaulting to whatever value ended up in the last design.

I2C Pull-up Resistor

Bus pull-up range from the I2C-bus specification's sink-current and rise-time limits

Show your work

Bus speed mode

Vdd

Total bus capacitance

Sum of every device + trace on the busVOL max

Sink current (IOL)

Rp min (sink-current limit)

967 Ω

Rp max (rise-time limit)

3.54 kΩ

Suggested E24 value

1.80 kΩ

Rise time at suggested value

153 ns

Formula

R_{p,min} = \frac{V_{DD} - V_{OL,max}}{I_{OL}}

R_{p,max} = \frac{t_{r,max}}{0.8473 \cdot C_{b}}

R_{p} \approx \sqrt{R_{p,min} \cdot R_{p,max}}

t_{r} = 0.8473 \cdot R_{p} \cdot C_{b}

Standard mode (100 kHz): rise time up to 1000 ns

Fast mode (400 kHz): rise time up to 300 ns

Fast-mode Plus (1 MHz): rise time up to 120 ns

0.8473 = ln(0.7 / 0.3): the I2C spec measures rise time from 30% to 70% of V_DD. The suggested value is the standard resistor nearest the geometric middle of the range.

V_DD = supply the pull-up connects to (V)
V_OL,max = highest voltage that still counts as a logic low (V)
I_OL = current a device can sink while holding the line low (A)
C_b = total bus capacitance: every pin, trace and connector (F)
t_r = rise time, 30% to 70% of V_DD (s)
R_p = pull-up resistor, one on SDA and one on SCL (Ω)

Two pull-ups are needed — one on SDA, one on SCL — each sized from this same range. Most designs use matched values on both lines.