what are pull ups/internal pullups?

[syeda amna]

what are pull ups? what are the use of thier use in microcontrollers? Any advantage of using them?

 

[IanP]

Pull-up resistor - Wikipedia, the free encyclopedia

:wink:

 

[b4bb4ge]

they are used to pull a line high / or low. they are just resistors connected to power or ground. One application they are needed in is I2C communications or anything that needs to pull a line high but can only sink current not source it.

 

[syeda amna]

are pull ups use particularly with microcontrollers or have any other applications also?

 

[b4bb4ge]

They are usually used for things attached to microcontorollers or FPGA's, such as ADC, DAC, shift registers, accelerometers and things of that nature

 

[syeda amna]

Thanks for your replies.
so they are used at the output terminals??
or they can be used for the input also.

 

[b4bb4ge]

usually they are used for inputs to a microcontroller because a device can only drive a line high and not low.

 

[Tahmid]

Hi,
Usually they are used at the input stage. They can be needed in the output stage when the device can both sink and source current. eg In SG3524, it can either sink or source current, but can not do both, so you need to connect pull-ups or pull-downs at the output.

But usually, they are used at the input stage. Let's take an example. Say you have a microcontroller pin connected to a switch which is in turn connected to +5V. When the switch is closed, the pin receives +5v. However if the switch is open, the switch receives no input, it's not high or low, it's just a high-impedance (high-Z) input state, which can not be made use of in the code. So, you connect a pull-down resistor from the pin to ground, so that when the switch is open, the microcontroller pin is 0 - connected to ground.
Other applications maybe in communication, eg. I2C, etc, or converters, eg. ADC, DAC, etc.

Hope this helps.
Tahmid.

---------- Post added at 11:57 ---------- Previous post was at 11:44 ----------

The switch can source current when closed, but cannot sink current when opened. That's why the pull-down is required. Pull ups are required for the same reason as I explained with the SG3524 example.

 

[ship1024]

Pullups can be used in two distinct modes input and output, some microcontrollers have open collector outputs, they don't have the ability to source current only sink it, you must attach pullup resistors to these ports to be able to drive them to logic 1, they will then have the ability to sink the curent for a logic 0. Some microcontrollers have build in weak pullups for input ports, what that will do is internally connect the pin to a pullup resistor so that you can eliminate the external resistor and just short the pin to ground to indicate logic 0 otherwise the pin will read 1. If you don't use a pullup resistor on a microcontroller (internal or external) you might encounter porblems with pins floating between 0 and 1.