Hi Ellappan,


In general, "pull up" and "pull down" resistors are used to prevent circuit nodes from being at ambiguous voltage levels while inactive.

Consider the scalar version of Ohm's Law that most electronics books cite. I = E/R, R = E/I, and E = IR. Now lets do a bit of a thought experiment.

Imagine that a resistor has one end tied to a particular voltage level (+5V = Vcc, or maybe 0V = GND. It doesn't matter for our thought experiment. Pick a voltage, any voltage.). Now the voltage on the other side of that resistor is going to be the voltage from the first end plus the voltage dropped across the resistor.

So how much voltage is dropped across the resistor? It depends on how much current is flowing. If the other end of the resistor is tied to something floating, like an open collector transistor, then no current is flowing. Thus, by Ohm's Law, the voltage dropped across the resistor is E = IR = 0•R = 0V. Thus, whatever voltage is on the one end of the resistor, the same voltage is on the other end as well.

Thus, if you attach a resistor to an open collector transistor circuit and tie the other end to 5V, as long as no current flows through the resistor you are applying 5V to the open collector of the transistor. On the other hand, if you turn on that transistor, it will try to hold the collector at approximately the same voltage as the emitter. Now, assuming that you've tied the resistor to Vcc and the open collector is trying to go to GND, and assuming you've chosen a resistor value wisely, the resistor will develope a voltage drop of Vcc long before the transistor reaches saturation, and the once open collector will be at GND.

This is a "pull up" configuration, meaning that the resistor tends to hold the circuit node to which it is applied at a higher voltage level than it will go to when active. Of course, if you have a circuit node which floats (goes HiZ) when inactive and is held at a higher voltage when active, you could use a resistor to hold that circuit node at a lower voltage level when inactive. This would be called a "pull down" configuration.

I hope this helps.