I2C Pull-up Resistor Calculator — InnovChip

I2C Pull-up Resistor Calculator

Compute the valid pull-up resistor range for I2C buses. Balance rise-time limits against sink-current constraints across 100 kHz, 400 kHz and 1 MHz modes.

Calculator

About This Calculator

I2C uses open-drain drivers, so every bus needs pull-up resistors. The value is bounded: low enough to meet the rise-time requirement (with bus capacitance) and high enough not to violate the controller's low-level sink current (Iol) at VOL. This calculator finds both bounds and a practical mid-point.

How to Use

  1. Enter the total line capacitance: trace estimate (~1-2 pF/cm on FR4) plus every device pin on the bus.
  2. Select the fastest speed mode you will use — this sets the allowed rise time (1000/300/120 ns).
  3. Set the bus supply voltage and the VOL/Iol from the weakest driver on the bus.
  4. A 'Valid = 1' means a workable range exists between the two bounds.

Formulas

Min pull-upR(min) = (Vcc − VOL) / Iol
Max pull-upR(max) = t(rise) / (0.8473 × C(bus))
RecommendationR(rec) = √(R(min) × R(max))

Design Notes

Rise time constant: V rises to 70.7% (0.8473 factor) of the rail in t = 0.8473 × R × C.

PMBus/SMBus may have additional sink-current limits — check each device's Iol spec instead of assuming 3 mA.