SEN0466 CAPACITANCE

Pullup Resistor Calculation A strong pullup (small resistor) prevents the I2C pin on an IC from being able to drive low. The VOL level that can be read as a valid logical low by the input buffers of an IC determines the minimum pullup resistance [RP (min)]. RP (min) is a function of VCC, VOL (max), and IOL: (1) The maximum pullup resistance is limited by the bus capacitance (Cb ) due to I2C standard rise time specifications. If the pullup resistor value is too high, the I2C line may not rise to a logical high before it is pulled low. The response of an RC circuit to a voltage step of amplitude VCC, starting at time t = 0 is characterized by time constant RC. The voltage waveform can be written as: (2) For VIH = 0.7 × VCC: (3) For VIL = 0.3 × VCC: (4) The rise time for the I2C bus can be written as: (5) The maximum pullup resistance is a function of the maximum rise time (tr )