I2C Pull-Up Resistor Calculator
Calculated Resistance Range
What is an I2C Pull-Up Resistor?
The I2C (Inter-Integrated Circuit) bus is an open-drain communication protocol. This means that devices on the bus can only pull the signal lines (SDA and SCL) low; they cannot drive them high. To return the lines to a high state, external pull-up resistors are required. Choosing the correct resistance value is critical for ensuring signal integrity, especially at higher clock speeds.
How to Use the I2C Resistor Calculator
To use this tool, input your system's operating voltage (typically 3.3V or 5V) and the estimated bus capacitance. The bus capacitance is the sum of all trace capacitances and the input capacitance of every peripheral connected to the bus. Select your target speed mode to automatically populate the maximum allowed rise time according to the I2C specification.
Calculating Minimum Resistance (Rmin)
The minimum resistance value is limited by the current-sinking capability of the weakest driver on the bus. If the resistor is too small, the current flowing through it (Iol) will be too high for the transistor to pull the line low enough to be recognized as a logic "0" (Vol). The formula used is:
Rmin = (Vcc - Vol) / Iol
Calculating Maximum Resistance (Rmax)
The maximum resistance is limited by the total bus capacitance and the maximum allowable rise time for the selected speed mode. If the resistor is too large, the RC time constant will be too high, and the signal will not reach a logic high fast enough before the next clock pulse. The formula is:
Rmax = tr / (0.8473 × Cb)
Frequently Asked Questions
What happens if the resistance is too high?
If the pull-up resistance is too high, the signal rise time will be too slow. This results in rounded waveforms that may fail to reach the logic high threshold, causing communication errors or data corruption.
What is a common default value?
For most 3.3V or 5V systems operating at 100kHz with short traces, 4.7kΩ is a very common starting point. For 400kHz (Fast Mode), 2.2kΩ is often preferred to counteract bus capacitance.