Dumb question about AVR pullups

[Dan Moos]

So say I set a particular pin as an input, and, and activate that pin's internal pullup.

Is there anything bad that could happen if a HIGH signal is sent to that pin, such that it's getting 5 volts from two sources at once?

Related question. Let's say I don't enable the internal pullup. The MCU can still read the pin even if it's tri-stated, right?

App specific question. I'm gathering input with a 74HC165 shift register. I'm not confident how to properly set up the MCU pin that gets the serial output from the shift register. My gut says no internal pullup, but I'm not sure.


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[ataradov]

The voltage will still come from the same source - your power supply.

There is no harm having a pull-up, especially given that it is pretty weak. If connected device do drive the line, then there is no need for it, however.

[Dan Moos]

Ah, good way to put it. If I use the internal pullup, and my shift register sends it HIGH, its the equivalent of just using two parallel resisters from Vcc as a pullup, which is obviously harmless if you think about it that way.

Is there an accepted standard practice to this? In other words, are there pros and cons to using/not using the internal pullup?

[ataradov]

If your driving device always actively drives its output, then no pull-up is needed. 74HC165 has no way of making any of its outputs tri-state, so it can be used without a pull-up.

[Dan Moos]

Right on. Thanks as usual. I've just never thought about the consequences of using one or the other setting. All is clear now!

[bobaruni]

There is no harm in using a pull-up with an active device except that there will be slightly more current consumption when the active output is low, this should be avoided if you are trying to conserve battery power.
 

[MagicSmoker]

One other thing to keep in mind is that all I/O pins go Hi-Z (tri-state) during power on and reset; you can get into a lot of trouble with output pins, especially, if there isn't an external resistor to define their state.

[Ian.M]

If you are using SPI slaves that can tristate MISO when deselected, then a pullup on that line would be a good idea so it isn't floating between data blocks.