Pull Up Resistor Basics - Value & Function

[zak8000]

hi there

i have been reading up on the function of pull up resistors from the text:http://ti.tuwien.ac.at/ecs/teaching/courses/mclu/theory-material/Microcontroller.pdf and on page 37 it talks about it. in their example of the pull up resistor with the switch. i was wondering if they circuit would still work if the resistor was removed and if not what would be a suitable value for the resistor?

 

[vk6kro]

hi there

i have been reading up on the function of pull up resistors from the text:http://ti.tuwien.ac.at/ecs/teaching/courses/mclu/theory-material/Microcontroller.pdf and on page 37 it talks about it. in their example of the pull up resistor with the switch. i was wondering if they circuit would still work if the resistor was removed and if not what would be a suitable value for the resistor?

No, it wouldn't work.

With the resistor, the supply voltage appears across the switch when it is open, but no voltage appears across it when it is closed. So the voltage changes and the processor can use this to tell if the switch is open or closed.

Without a resistor, there is no voltage across the switch whether it is open or closed, so the processor can't tell if the switch is open or closed.

A typical value of pullup resistor is 4700 ohms.