The open-collector adds inconveniences like requiring an external pull-up resistor and slows down the throughput. Why did the I2C designers choose to use open-collector/open-drain?
Electronic – Why does the I2C interface use the open-collector/open-drain
i2copen-collectoropen-drain
Related Solutions
You can do open drain on any digital pin on the PIC yourself, it's easy! I'll show through a quick chunk of example code using RB0.
_TRISB0 = 1; // Set the pin to high impedance
_LATB0 = 0; // Set the output low (this would be _PORTB0 on some pics)
// As long as you're using pin RB0 as a open-drain, don't touch _LATB0
// To output a "low" (drain) do this:
_TRISB0 = 0; // Set the pin to output, it's already low, so it will "drain"
// To output a "high" (open) do this:
_TRISB0 = 1; // Set the pin to high impedance, the pullup resistor will pull high
The I2C module will do something similar to this internally, without you doing anything. In other words: yes! You'll need a pullup resistor!
EDIT: To answer your second question which I did not directly address, the pin does not need a pullup resistor during normal digital output operation. My example was to show you how a normal, fully functioning digital I/O pin could operate as a open-drain without any additional behavior or behaving unexpectedly. I should also note that I have done this exactly what I showed here to do open-drain in real programs, so this isn't a toy example, it's really how you do it!
There really isn't an issue with that. Open drain and open-collector are essentially the same in this application.
Two things to watch out for:
Make sure you get your polarities right and that you need an OR gate and not a NOR gate because the open-collector/open-drain style often invert your signal. If the output of either opamp is high, then the output of that IC will be pulled low, and vice versa.
Just make sure each IC can sink (Vcc/R1)+(Vcc/R2). This will be the worst case current if one output is low, and the other is high.
Picture for reference.
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Best Answer
Open drain allows there to be multiple masters on the same bus. If two masters try to transmit at the same time with push-pull drivers, they can damage each other, and even if they don't it's hard to tell which master will win. Open drain acts as a "wired AND", which makes it easy to share the line and arbitrate collisions. CAN does something similar, although the physical layer is differential instead of single-ended.
In I2C in particular, open drain allows the receiver to stretch the clock by holding it low when the receiver needs more time to process the data. Whether this is a bug or a feature is a matter of opinion.