Electronic – Resistor parallel to an LED

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.

A link to the documentation of the board would possibly permit a more definitive answer, but general rules of thumb can be applied.

The reason a pull-down resistor is suggested for the serial in pin, is to prevent the input from floating when not driven by anything. If the pin were allowed to float, it would be sensed as a series of random voltage changes due to electromagnetic interference, and the device would attempt to interpret it as some (nonsensical) serial signal.

To prevent such floating, in general anything from 100 KOhms down to 10 KOhms will be used as a pull-down value. The smaller values will make the input less sensitive to EMI when not being driven, but will also make the output of the driving circuit work harder (more current) to generate a high on that pin. Wasted power, more heat at the driver, faster battery drain (if applicable), not nice.

Too high a resistance means that relatively small electromagnetic interference, such as from a nearby motor or electronic device, would permit the serial input pin to perceive state changes due to the noise, so the purpose is defeated.

In general, one would start with 100 K, and if interference persists, work downward in steps to the 10 K shown in the schematic. The exact value of the pull-down resistor is non-critical.