port of microcontroller

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Open drain simply means that the port or pin in question is not connected to VCC or VDD. Which means that for you to use the port, a pull up resistor is often required to make the port be functional or else there is an open circuit better the power supply and the internal drain of the port.
 




can you please tell why the word open DRAIN is there?
 

Open means that there is an open circuit between the port and vdd.
Drain means that the microcontroller in question is fabricated based on the MOS technology. A MOS has drain, source and gate. It is assumed that the source is internally grounded and the gate connected directly to the internal output of the microcontroller and the drain left open/ not connected to the Vdd.
So if you connect a resistor between the port and Vdd , you have successfully connected the drain to the power source thereby creating a complete connection.
 
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It's similar to open-collector outputs. The output is not "driven" but rather it floats when high and goes toward ground when low.
 
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