I2C Pull-up Resistor Calculator
Calculates a compliant I2C pull-up resistor range and recommended value based on bus speed mode (Standard/Fast/Fast-plus), supply voltage, and bus capacitance.
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How to Use
- Select the I2C speed mode: Standard Mode (100kHz), Fast Mode (400kHz), or Fast-plus Mode (1MHz).
- Enter the supply voltage Vcc (typically 3.3V or 5V).
- Enter the bus capacitance Cb (in pF, range 1–400pF, depending on device count and trace length; default 50pF).
- The minimum resistance Rp(min), maximum resistance Rp(max), and recommended resistance Rp(rec) (geometric mean) are calculated and displayed in real time.
- A status indicator below shows whether the recommended value falls within the common 1kΩ–10kΩ range: green for in-range, yellow warning for out-of-range. If Rp(min) > Rp(max), no valid resistor exists for the current bus capacitance — reduce Cb or switch to a slower mode.
- Click "Load Sample Data" to restore the default example: Fast Mode, Vcc=3.3V, Cb=50pF.
Features
- Supports all three standard I2C speed modes: Standard (100kHz, tr=1000ns), Fast (400kHz, tr=300ns), and Fast-plus (1MHz, tr=120ns), each with fixed maximum rise time tr, max output-low voltage Vol(max)=0.4V, and sink current Iol=3mA.
- Computes the minimum pull-up resistor needed for a valid bus-low level using Rp(min) = (Vcc − Vol(max)) / Iol.
- Computes the maximum pull-up resistor allowed to meet the bus rise-time requirement using Rp(max) = tr / (0.8473 × Cb).
- The recommended value Rp(rec) is the geometric mean √(Rp(min) × Rp(max)) of the min and max values.
- Automatically checks whether the recommended value falls within the commonly used 1kΩ–10kΩ engineering range, with three status states (in range / out of range / no valid solution).
- Displays the intermediate parameters used (tr, Vol(max), Iol) alongside the results for verification.
Use Cases
Selecting pull-up resistors for MCU-to-sensor I2C buses
When designing a circuit connecting a microcontroller to multiple I2C sensors (accelerometers, temperature/humidity sensors), determine the right pull-up value based on bus speed and estimated trace capacitance.
Diagnosing I2C bus communication issues
When I2C signals show slow rising edges or waveform distortion, check whether the current pull-up value exceeds Rp(max), indicating the resistor is too large to meet timing requirements.
Evaluating bus capacitance for multi-device chains
More devices and longer traces increase bus capacitance Cb; adjust Cb to see how Rp(max) changes and assess whether timing requirements can still be met for a given topology.
Re-selecting resistors when upgrading from Standard to Fast-plus mode
When a product needs to move I2C speed from 100kHz to 1MHz (Fast-plus), recompute the pull-up range since the shorter tr at higher speed lowers Rp(max), possibly requiring a smaller resistor than before.
FAQ
Why does Rp(min) sometimes exceed Rp(max)?
This typically happens when bus capacitance Cb is too large or too fast an I2C mode (e.g., Fast-plus) is selected — the computed maximum allowable resistance ends up smaller than the minimum required, meaning no single resistor value satisfies both the low-level and rise-time requirements. Reduce bus capacitance (shorter traces, fewer devices) or switch to a slower I2C mode.
Can I still use a recommended value outside the 1kΩ–10kΩ range?
1kΩ–10kΩ is the most common engineering rule of thumb (the tool shows a yellow warning outside this range), but it's not strictly unusable. Too small a value increases bus power consumption and driver current burden; too large slows edge rise times and reduces noise immunity. Evaluate against actual bus length and EMI environment.
How should I estimate the bus capacitance Cb?
Bus capacitance is the sum of trace parasitic capacitance and each device's pin capacitance — typically each I2C slave contributes about 5–10pF, plus a few pF per 10cm of PCB trace. Typical total bus capacitance ranges from 10–400pF; the tool defaults to 50pF as a common estimate. For precision, estimate from your actual topology or measure the rise time on an oscilloscope.
Are Vol(max)=0.4V and Iol=3mA fixed values?
Yes, the tool uses the typical values shared across all three I2C speed modes per the I2C-bus specification (NXP UM10204): max output-low voltage 0.4V and sink current 3mA, which most I2C devices follow. If your device datasheet specifies different Vol/Iol values, you'll need to recompute manually with the formulas.