It seems that more often than not, documents and datasheets for microcontrollers call for pull up resistors (ex. the I2C specification sheet).

Can anyone explain what these resistors from Vdd to the data/clock bus are for?

 

A simple Google search found lots of info.
http://www.seattlerobotics.org/encoder/mar97/basics.html

For I2C a pair of 4.7K resistors is common.

 

It seems that more often than not, documents and datasheets for microcontrollers call for pull up resistors (ex. the I2C specification sheet).

Can anyone explain what these resistors from Vdd to the data/clock bus are for?

There are many reasons behind using Pullup registers
Some of them are:

• Current Outsourcing capacity of IC is less than required for a output device, in that case we need to source extra current using pullup register.
• Output of IC is floating, i.e. neither 0 nor 1, in that case the voltage needs to be pulled high to VCC, i.e. logic 1

Let me know if you need any more help


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Thanks,
Rushikesh
www.projectsof8051.com

 

If you have something that just switches on & off, whether a physical switch (like a button or keyboard switch) or an open-collector transistor, with one side to ground or 0V, the state of the other side (call it the output) is not defined when the switch is off; the output is floating.

A pull-up resistor between that output and V+ means you have two definite states: switched to ground and pulled to V+.

Likewise, you can have a pull-down resistor to 0V when the switch is connected to V+.

The other common reason for a pull-ups is when more than one device needs to control the same signal. That is the case with the I2C bus, each device has an open-collector output switching to ground, so any device can pull the bus line low without 'fighting' other devices.

The actual value of the pull-up depends on the circuit. Something that needs a fast responce or has long wiring, or in an electrically noisy environment will use a fairly low value, eg. 220 Ohms.

A short connection between two components within a circuit board may use 4K7 or 10K, it's all down to the application.

 

You guys and your answers are amazing!

Thanks!

 

You guys and your answers are amazing!

Thanks!

You are always welcome....

Can you let us know your purpose behind using pullup register.

I mean what is your project/circuit details ?

 

There was no particular project for this one, being self taught (pretty much exclusively through datasheets) I understand the concepts without some of the basic terminology.