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I2C Pull-up Resistor Calculator
Calculate the valid pull-up resistor range for an I2C bus using supply voltage, bus capacitance, rise-time limit, sink current, and selected pull-up value.
Input Parameters
Results
Use the largest valid resistor that still meets rise time if low power is important; use smaller values when bus capacitance is high.
Equations Used
Maximum pull-up: Rmax = tr / (0.8473 × Cbus)
Minimum pull-up: Rmin = (VCC - VOLmax) / IOLmax
Estimated rise time: tr ≈ 0.8473 × Rp × Cbus
Use capacitance in farads, resistance in ohms, and current in amperes.
Frequently Asked Questions (FAQ)
Q1: What is an I2C pull-up resistor calculator used for?
It helps choose SDA and SCL pull-up resistors that satisfy rise-time and sink-current limits.
Q2: Why does I2C need pull-up resistors?
I2C lines are open-drain or open-collector, so pull-up resistors are required to return the bus to logic HIGH.
Q3: What happens if the pull-up resistor is too large?
The bus rises too slowly and may fail at Fast Mode or Fast Mode Plus speeds.
Q4: What happens if the pull-up resistor is too small?
Devices must sink more current to pull the line LOW, which can violate VOL or current limits.
Q5: Should I calculate pull-ups for both SDA and SCL?
Yes. Both lines should meet the bus capacitance and rise-time requirements.
Q6: Is 4.7 kΩ always correct for I2C?
No. 4.7 kΩ is common, but the correct value depends on VCC, bus capacitance, speed mode, and device sink current.
