9.2.1.2.4 I²C Pullup Resistor Selection

Refer to the NXP Semiconductors, UM10204 – I²C-Bus Specification and User Manual for the specifications relevant to your application.

Use Equation 3 to calculate the maximum permitted pullup resistor value for the bus speed used in the application.

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Equation 3.

where

• t_r is the maximum permitted rise time (300 ns for Fast-mode)
• C_b is the capacitive load on each bus line

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If you do not know what the bus capacitance is in your application, start with a 1-kΩ pullup resistor and measure the rise time with an oscilloscope. Use Equation 3 to calculate the bus capacitance and thus the maximum permitted pullup resistor.

Use Equation 4 to calculate the minimum permitted pullup resistor value for different bus speeds.

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Equation 4.

where

• V_BUS is the I²C bus pullup voltage
• V_OL is the low-level output voltage (0.4 V)
• I_OL is the low-level output current (3 mA for Fast-mode)

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A pullup resistor value of 3.3 kΩ meets both of these requirements.