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Physical / electrical concepts

Open-drain

An output type that can only pull low. Needs an external pull-up to go high. I²C lines are open-drain.

How it looks

Drawing
AN OPEN-DRAIN OUTPUT3.3 Vground 0 Vpull-up (outside the chip)inside the chipN switchoutput
The chip only has the bottom switch. The top half of the job is done by a resistor outside it.

How it works

AN OPEN-DRAIN OUTPUT3.3 Vground 0 Vpull-upswitch onoutputLOWswitch offoutputHIGH (via the resistor)switch offoutputHIGH, slowlyonly a resistor lifts it

Where it fits

I2C lines are open-drain. Compare push-pull; the two side by side: open-drain vs push-pull.

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