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Open drain can only sink current. You have to provide a pullup resistor if you want to source current. Open drain is useful if you have a bus with several devices on it, any of which can pull down the output to logic "0". Logic "1" is provided by the pullup resistor.
With some limitations! - you need to be aware that it's open-drain and design accordingly. It's effectively just an on/off switch - when switched on the pin is taken to ground, when switched off the pin is O/C (just as if you'd disconnected the wire from the pin). Using a pull-up resistor is limited by the amount of current the resistor can provide - this current has to be absorbed by the open-drain pin when it's taken low, so it can't provide more current than that will sink.
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