arduino
I'm trying to reduce power as much as possible in an Arduino board I have created. How should the unused input pins be configured? There are a few answers (here, here) already for this, but I'm after something specific to the ATMega328P.
What you need is a pull-up resistor.
The Arduino whebsite has a nice tutorial (with software) on using a pushbutton.
We connect three wires to the Arduino board. The first goes from one leg of the pushbutton through a pull-up resistor (here 2.2 KOhms) to the 5 volt supply. The second goes from the corresponding leg of the pushbutton to ground. The third connects to a digital i/o pin (here pin 7) which reads the button's state.
The default is INPUT which sets up the pin as an INPUT. If the pin is totally disconnected, it will randomly read HIGH and LOW. If you tie it to +5V or 0V, it will obviously read HIGH or LOW.
Internal to the Arduino, the Atmega chip has internal pullup resistors with a value around 20k ohm. (See the DigitalPins documentation for more details) These resistors can be optionally connected internally using INPUT_PULLUP. This is functionally (and electrically) equivalent to connecting a ~20k ohm resistor between the pin and +5V, the only difference is that it requires no external components and you can turn it on and off in software during the execution of your program.
So why pull-ups and not pull-downs? There are likely several reasons for it, but when wiring buttons or switches or anything "normally open", you only have to tie them to ground, you don't need to run +5V out to them. Since most boards are going to be designed with large ground pours for shielding reasons anyway, tying to ground is practically reasons.
Some more featured ICs like ARM chips have both pull ups and pull downs, but the 8-bit AVR line only comes with pull-ups. You just have to remember that HIGH is "open" and LOW is "closed".
Best Answer
After digging through the datasheet, I found this:
update in relation to comment/question:
According to table 14-1, the pull-up resistor is only active when the the following conditions are satisfied:
The only way you'll get significant current flowing through the pull-up resistor is if the pin experiences a low level with the pull-up enabled. This means either Atmel messed up badly (unlikely) or you have the pin configured as input with the pull-up enabled and the pin is somehow connected to ground.
Section
14.2.5discusses digital input enable and sleep modes. To summarize, the digital input is clamped to ground at the input of the Schmitt Trigger to prevent a floating level while in sleep mode, unless the pin is configured as an external interrupt. I can't tell if digital output is disabled in sleep mode. It doesn't look like it is disabled according to figure14-2, though I wouldn't be too surprised if it was. The best bet is to use either an internal or external pull-up resistor.