I'm reading a few manuals about I2C , SPI where it says I2C ports are open drain,which I don't know what it is. I know about sinking mode and sourcing mode of ports in electronic circuits.
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\$\begingroup\$ Open drain means that the line is pulled high to VCC by default and is actively pulled low to ground (so it's an active-low configuration). \$\endgroup\$– JashaszunCommented Jun 22, 2017 at 6:24
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1\$\begingroup\$ Sorry, this is a lazy question - you haven't looked for yourself. An internet search gives you plenty, including en.wikipedia.org/wiki/Open_collector#MOSFET \$\endgroup\$– TonyMCommented Jun 22, 2017 at 6:50
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Open drain means that the output port is directly connected to the "drain" (normally named "D") pin of a MOSFET (NMOS most probably). It is similar with the term of "open collector", where the transistor at the output is a BJT.
simulate this circuit – Schematic created using CircuitLab
In practice this means that the port is only possible to drive its state to LOW (connect to ground) and not to HIGH. That's the reason why it is necessary to put an external pull-up resistor that will pull the line HIGH when the pin is left "floating" from inside.
I guess the terms "sinking mode" and "sourcing mode" are somewhat more general. We are referring to "sinking mode" of a port when current is flowing from the load to the port and to "sourcing mode" when current is flowing from the port to the load.
In this sense, the open drain ports can only be in the "sinking mode", whereas other ports (not open drain) can be in either mode.
