Pull Down Resistors

Hi,

I cant to seem to find more detailed on Pull Down Resistors. I dont understand why do you need a resister there and not just a regular wire? Also how to you calculate the resister value that should go there? I see in most examples 10k resister is used but why?

thanks
Vadim

10K resistor connected to GND is a value that guarantees a LOW on a CMSO input.
It is reasonable value to keep power draw low.
Having a 100 or so on hand will last you years.

Edit:

.

They are to keep the pin a 0 V potential.

If you just use a wire if it has to go to a logic one or Plus volts then massive current would be needed.
This will destroy things.

Have a google at OHMS law.

Pullup and pulldown resistors are there to 'make sure' a signal is in a predictable state when it is otherwise disconnected or not getting data.

A resistor is needed so that you're not shorting out the input to power or ground while it is being pulled.
A wire and a resistor would both pull the signal in the same way, you are correct there, but without a resistor, it will pull as much power as it takes to break something since it's unrestricted. You choose a resistor value that will draw the least bit of power while still being recognized.

You want a trickle to set the pin's signal state, you don't need much. You pick a current that you figure is sufficient to trigger a low/high reading, and pick a resistor based on tha desired current and voltage.

great info. Thanks All!

BTW
Did you Google pullup pulldown?

This explains it all:-
http://www.thebox.myzen.co.uk/Tutorial/Inputs.html

Including why you should not be using pull down resistors anyway.

AVR I/O pins have INPUT_PULLUP mode that uses an on-chip 20K to 50K pullup for digital read.

It lets me use jumpers as switches. The pin is HIGH until I ground the free end of the jumper. I touch it to the grounded metal USB port box to "press the button" (holding the board to keep it from wobbling) and let the pin up from the box to "release". The pullup fills the pin and jumper to 5V quickly. PS - and there's a whole series of tiny spark jumps for a milli or two as the gap gets closed or opened (worse) called "bouncing".

Stick a jumper in a hole with pin mode INPUT. It starts out low local EMF can get the pin to HIGH unless it's terminated with a pulldown. AVR chip don't do that trick, you would have to wire it.

logicunit:
Hi,

I cant to seem to find more detailed on Pull Down Resistors. I dont understand why do you need a resister there and not just a regular wire? Also how to you calculate the resister value that should go there? I see in most examples 10k resister is used but why?

thanks
Vadim

Think of it as a return spring - it lets the voltage change if forced but otherwise it takes over and returns
the voltage to a default value. Like springs the strongest spring (or lever) wins.