how to select Pullup(down) Resistors?

[Yoking]

in digital circuit,we can find many pullup or pulldown Resistors in output our input port!but how to select that kind of resistors in our own circuit?anyone can help!i really have no idea about that!thanks!
:|:|:|:|

 

[IanP]

Try this link:
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html
Regards,
IanP

 

[cesare]

You have to take into account two things:

1) if you reduce the pullup value you increase the current when it is pulled down by the driver but when the line rises rapidly when the driver goes Hi-z (take into account the line capacitance that must be charged by the pullup resistor).

2) a low pullup value makes the line more robust against interference.

 

[nzahirov]

It boils down to a trade-off between speed/performance and power. A lower resistance pull-up will have higher current, consume more power but switch faster than a larger resistor.

I2C is a great example of a bus whose performance (in terms of speed) suffers due to the use of passive pull-ups on its lines.

However you must also be careful not to dammage your active components by using pull-ups or pull-downs that are too small. Check the datasheet for the chips that you are using and pay careful attention to the maximum currents that your output stages can sink/source. These impose certain hard limits on how small your resistors can be without causing dammage.