What is a pull-up resistor? (newbie)

[Richardcavell]

Hi, everyone. I have an Atmel AVR microchip that has a RESET pin. One is told to supply current of approx +5 volts to it at all times. One is also advised to supply that current through a "pull up" resistor of at least 2.2 kohms to prevent a too-large current from going through the microchip.

1. "Pull-up" means simply that the resistor is connected to the positive rail, right?

2. If this is true, then it's not really the resistor that pulls up the voltage, but the positive rail itself, correct?

3. If there were nothing attached to the pin, it would be "floating". I recognize that this means that there is no connection on one side to establish its voltage, but wouldn't any electric potential in the remaining part of the "circuit" simply quickly drain?

4. The 2.2k resistor and the AVR chip will then form a voltage divider, correct?

5. Can I assume that the resistance of the chip is sufficiently large that the 2.2k resistor is relatively trivial?

6. If not, then wouldn't the 2.2k resistor drop sufficient voltage that I can no longer rely on the input to the reset pin being 5 volts?

 

[Ian Rogers]

The reset pin on the pic requires a pullup as there is an onboard cap... The choice of resister determines the boot time ( newer chips don't need this as there is a power up timer )

I don't know if its the same for AVR.... It should be.

 

[dr pepper]

1 correct

2 correct (its a term given to the function of the resistor)

3 correct, the pin would be suscepatble to noise change of state may occur at random

4 the resistor is much lower than the input impedance of the micro so the voltage on the pin is well above logic 1 level

5 as above, I'm not an avr man however the logic 1 level will be less than 5v so a little drop is tolerable

You say at least 2.2k, there will also be a maximum value (not sure for the avr) if you go above that the resistor will not provide enough current to lift the pin to logic 1 level.

 

[JimB]

Similarly, if the resistor was connected to 0v it would be a "pull down" resistor.

One thing to note when you have an input or reset pin pulled up to the "1" state through a resistor, all you have to do to set the input to the "0" state is to make a connection to 0v. To do this you could use a wire link, a switch or a transistor.

JimB