Do circuits with pull up resistors waste more electricity than circuits with pull down resistors?

Question

A pull up resistor provides a constant voltage (and therefore some current) to an input pin on arduino. https://learn.sparkfun.com/tutorials/pull-up-resistors/all A pull down resistor connects an arduino pin to to ground so no current should be flowing through the pin.

When a button is used in a pull up circuit (see ref above) current flows through it only when the button is pressed resulting in a decreased voltage at the arduino pin. When the button is in its native state, the arduino pin sees 5v so more current is flowing through the pin of the arduino. I am simplifying an arduino and thinking of it as a resistor .

Since a button is normally in the open state and spends most of it time this way, if it is used in a pull up circuit, then current is always flowing through the arduino.

A button used in a pull down circuit allows voltage at the arduino pin only when it is pressed, so this seems less wasteful. But apparently, pull up circuits are much more common. why this desire to waste electricity?

Answer 1

Let's look at a simplified cmos input and an analogy. While the atmega in an arduino uses a more complicated non-inverting Schmitt trigger for the input circuit, the same would still apply.

It's a complementary pair of fets. One p-channel at the top one n-channel at the bottom. And they experience leakage through their gates just like any mosfet.

When you have a pull up, the input is at VDD, turns off the p-channel and turns on the n-channel. Current flows through the n-fet.

And of course the opposite happens when pulled to ground. Top fet is turned on and bottom is off.

Of course these are mosfets, so current barely flows unless you are switching the state, and where leakage occurs in different ways, drain-source leakage, gate and body leakage.

Generally, this means the pull up and the pull down will result in the same amount of leakage for a modern cmos input.

Answer 2

You are mistaken that no current flows through a pulldown resistor. There is leakage current at the processor pin, regardless of whether it is pulled up or pulled down. With a pulldown resistor current will flow from the power supply, through the processor, out of the I/O pin, and through the pulldown to ground.

Answer 3

The current consumption does not depend by itself about there being pull-ups or pull-downs, because virtually no current flows into or out of an Arduino pin. Leakage current can be ignored as it is likely to be symmeteric, and assumption is that no internal pull-ups are used because obviously internal pull-ups do cause current to flow out if input is grounded.

Which setup wastes more current depends on if the actuator such as slide switch spends most of the time open or closed in which case current either flows via the resistor or not.