When do you pull up a line?

[jbs87]

I'm not sure whether this is an analog or digital question. I know that for an I2C line, you have to pull up SDA and SCL. What other lines do you have to pull up? Someone told me that you have to pull up lines that come from open drains. In general, what are guidelines for pulling up lines? Thnx for any replies

 

[akilpatrick]

Yes. Generally for I2C you would use a 4.7K resistor to pull up each line.

In terms of other types of lines, anything with an open collector or open drain would need a pull-up if you want to convert the output to a voltage to use for controlling something. If the "lines" are microcontroller outputs or TTL logic outputs then you can use something like 10K ohms or 4.7K. Don't use a too small value or you'll waste power. But don't use a too high value or the thing that you have connected to the output might load down the pull-up too much and the voltage will drop.

A good rule of thumb is to make sure all inputs are teminated somehow. The easiest way to do this on microcontrollers is to make all unused pins set as output. Or if you want, you can set them as input and connect them to the power supply voltage or ground. (some people say +5V is better than ground for this, but I don't know)

 

[A.Anand Srinivasan]

i dont think open drain outputs are used generally...
the pull up is mainly for the open collector system....

 

[akilpatrick]

That's untrue. Open drain is the FET equivalent of open collector. Many modern ICs such as microcontrollers use FET type outputs instead of bipolar technology.