I2C Pull-Up Resistor Calculator

On an I2C bus, pull-up resistors are required to bring the SDA and SCL lines high between transmissions — choose the wrong value and signals either rise too slowly or draw excessive current. Enter your Supply Voltage (Vcc), Output Low Voltage (VOL), Output Low Current (IOL), Bus Capacitance (Cb), Rise Time (tr), and I2C Frequency into the I2C Pull-Up Resistor Calculator to get your Recommended Pull-Up Resistor value. Secondary outputs include the Minimum and Maximum Resistance bounds and the Calculated Rise Time for your bus configuration.

V

I2C bus supply voltage

V

Maximum low-level output voltage

mA

Sink current capability of I2C devices

pF

Total capacitance of I2C bus including wires and devices

ns

Maximum allowed rise time for I2C signals

I2C bus operating frequency

Results

Recommended Pull-Up Resistor

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Minimum Resistance

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Maximum Resistance

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Calculated Rise Time

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Frequently Asked Questions

What is the I2C protocol and why does it need pull-up resistors?

I2C (Inter-Integrated Circuit) is a two-wire communication protocol using SDA (data) and SCL (clock) lines. Pull-up resistors are essential because I2C uses open-drain outputs that can only pull signals low, requiring external resistors to pull signals high for proper logic levels.

How do I calculate the minimum pull-up resistance value?

The minimum resistance is calculated using Rmin = (Vcc - VOL) / IOL, where VOL is the maximum low-level output voltage and IOL is the sink current capability. This ensures devices can properly pull the bus low.

How do I calculate the maximum pull-up resistance value?

The maximum resistance is determined by the required rise time: Rmax = tr / (0.8473 × Cb), where tr is the maximum rise time and Cb is the total bus capacitance. This ensures signals rise fast enough for the desired frequency.

Where should I place the pull-up resistors on the I2C bus?

Pull-up resistors should typically be placed near the I2C master device, not distributed across the bus. Having a single pair of pull-up resistors (one for SDA, one for SCL) at the master ensures proper signal integrity and simplifies the design.

What factors affect the total bus capacitance?

Bus capacitance includes the input capacitance of all connected devices, the capacitance of PCB traces, and cable capacitance. Typical values are 10-50pF per device plus 1-2pF per cm of trace length.

Can I use different pull-up resistor values for different I2C frequencies?

Yes, higher frequencies require smaller pull-up resistors to achieve faster rise times. Standard mode (100kHz) typically uses 4.7-10kΩ, Fast mode (400kHz) uses 2.2-4.7kΩ, and Fast mode Plus (1MHz) may require 1kΩ or less.

What happens if my pull-up resistor value is too high or too low?

Too high resistance causes slow rise times, signal integrity issues, and communication errors. Too low resistance increases power consumption and may prevent devices from pulling the bus low properly, also causing communication failures.

Do I need separate pull-up resistors for each I2C device?

No, you only need one pair of pull-up resistors for the entire I2C bus (one for SDA and one for SCL). Multiple pull-up resistors in parallel would effectively reduce the total resistance, which is usually not desired.