what is pull-up resistor for outputs of ICs?

ics pull-up

some ICs require their output pins to be connected with a pull-up resistor. i wonder what this is for. another Q, what does open drains mean? 8O

szpeatech said:

some ICs require their output pins to be connected with a pull-up resistor. i wonder what this is for. another Q, what does open drains mean? 8O

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Pull up resistors are used for open collector or open drain outputs. Usually, a logic gate output have two switching elements (either bipolar or mos), one which produce "1" logic by connecting the output to the positive power supply, and one which produce "0" by connecting the output to negative (ground) power supply. There is another type of output, when only one switching element is connected to logic output, between the output and ground. The switching element can be bipolar, in this case the emitter is wired to ground and the collector to the output. This is the open collector connection. Similar, when the output element is MOS, you have open drain output. But if you want to use the output,, you have to bias it, and this is the purpose of pull up resistor. Open drain or open collector outputs exists because some applications require it (ex. when you want to switch an element powered with a voltage higher than power supply of the logic circuit, or when you want to use wired-or connections between outputs)
/pisoiu

The pull-up resistor "pulls up" the output pin to high voltage level when it is made high impedance by the chip.

When the transistor or FET inside the chip turns on, the pin is pulled low. The current supplied by the pull-up resistor is then diverted to ground via the conducting transistor.

An output of a logic device which only has the "pull down" active device is termed an "open collector" or "open drain" output.

If you look at the circuit diagram for chips that are open collector you will see that is exactly what they are Open Collector. This enables you to decide on the type of interface you require and of course load resistor.

Barrybear

An intial rule of thumb is to have 1K per Volt, so 5v logic would use use the nearest resistor which is 4K7. This is just a general guide, depends on what you are trying to do, as to whether this is a suitable value or not. Works for most instances.

Open drain logic is used to realize wired AND logic. U can refer I2C cells. they are used for this purpose. Like in case of I2C bus( where several devices are connected to a common bus). & there it is not fixed which device is operating @ which voltage . so in this scenario, the bus high logic "1" is decided by the common pull up resistor (which is connected to the bus) & the devices just pull down to 0 when they have to transfer the data.

4.7K to 10K is usually good.

You have to think too that if, for some reasons, you end-up with two pull-up resistors on the same net, that their total equivalent value will lower. So, if you use 10K, and you end-up with two 10K pull-up resistors on the same net (which is often the case for connectors, where each devices have it's pull-up resistor, and you cable the two devices together, for example, a PC and a printer, where each would have a pull-up in case where the cable isn't connected yet), the total resistance will give 5K, which is still between 4.7K and 10K.

open drain <=> open collector <=> high impedance (Z) <=> tristate

All are the same, meaning the drain is kind of floating in mos or collector is floating in bipolar as if the circuit was electrically disconnected at the pin which is in high impedance mode hence a pull up is required to take the voltage to a logic level.

Basically pull up/down resistors are used to avoid unknown state of a signal line, when it is floating. If u have some of the inputs/outputs unused on any IC,it is not good to leave them floating. pull them high/Low.
Most of CMOS chips need their inputs pulled high/low. they don't like floating inputs, but no problem with floating outputs :?: