Electronic – What does “Default Output State” mean for a digital isolator

Pull-up and Pull-downs are normally used to ensure a line has a defined state while not actively driven. They are used on inputs to prevent floating lines, rapidly switching between high and low and a middle "undefined" region. Outputs normally do not need them.

But most mcu pins are GPIO, and sometimes on startup are defined as inputs instead of outputs. As you said, sometimes you don't want an IC pin input floating on startup, especially like a reset pin that you would normally drive with your microcontroller's GPIO.

This is when you use a Weak Pull-up or Pull-down on the line. Because they are weak, and you choose the default state, they provide no interference with your circuit (If the input is supposed to be low at all times, then pulled high, you choose a weak pull-down, and vis versa), but they do consume a bit of current. This is why you choose a resistor weak (Higher the value, the weaker) enough for the job.

Another normal output setup that uses pull-ups (or pull-downs, rarer) is Open Drain or Open Collector connections. These only drive a connection low, or release the line, leaving it floating. The pull-ups are used to bring the line to a high logic state.

If the 17xx parts are like the 13xx and 11xx devices, it means that there is an internal pullup on the general purpose I/O pins and that the default configuration at reset is to enable these pullups. What surprises many of us is that the pullup devices do not pull the pins up to the supply voltage of about 3.3V, instead they only pull the pins up to around 2.5V. The data sheet is warning you that if you really need the pins pulled all the way up to 3.3V then you will have to add an external resistor pullup.

Also, you should be able to disable the pullup or chose a pulldown instead by modifying the pin's IO configuration register.