Drive Notes #41: Why Does I2C Need Pull-Up Resistors?
I2C uses just two wires — SDA for data and SCL for the clock — with start conditions, addressing, ACK and stop conditions completing a transfer. So why are pull-up resistors mandatory?
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I2C uses just two wires — SDA for data and SCL for the clock — with start conditions, addressing, ACK and stop conditions completing a transfer. So why are pull-up resistors mandatory?
Because I2C devices have open-drain outputs: they can pull the bus low but cannot drive it high. Returning the bus to the high level is the job of the pull-up resistor.
The resistor value sets both edge rate and static current. Too large and the edges slow down and the bus speed drops; too small and power consumption rises while devices sink more current. Kewei Technology stocks the common 2.2k to 10k SMD values, and recommends reducing the value and checking waveforms for multi-device or long-trace buses.
Kewei's take: what this means for procurement
Passive components look unglamorous but they are the easiest way to stop a production line — one missing resistor and the board cannot ship. Hold about three months of the passives you use most.
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