windozeuser said:
I'm talking about the high impedence inputs of logic gates. What is meant by "they are floating", and can anyone please explain a pull up and pull down resistor? Also, if someone could help to explain impedence better, I feel a little shaky on it's definition. I know it's the total opposition in an AC circuit taking in account Inductance, and capacitance. However, why is it still measured in ohms?
The posters above have your answer, but to help you understand further, consider these points:
1. a "floating" input is an input connected nowhere. Don't get it confused with a high impedance input. In simplest terms, impedance is resistance.
You can tell that an output is in an impedant state when an LED lights dimly when it is connected the right way between output and ground.
2. In digital terms, a pull-up and a pull-down define the default logic values for a given section. For example, if you connect a resistor between +ve and an input of an inverter (this is a pull-up resistor), that inverter has a default value of 1. If you instead connect a resistor between -ve and an input of the inverter (pull-down resistor), the default value is 0.
"Default" is the value that is assigned to the logic gate, should no input be defined. To override a default value, connect an input to +ve, or -ve.
3. When dealing with pull-up or pull-down resistors, you need to choose a wise value. Anything between 1K and 5K is ok. If you go too low, then current consumption of the circuit will be high when the logic input is the opposite of the default value, since the resistor is then connected between +ve and -ve. If on the other hand, the value is too high, the logic value might not be defined correctly.
In some cases, a logic "1" is defined between certain voltage levels, and a logic "0" is defined between another set of voltage levels.
I suggest a 1K pull-up or pull-down resistor because It works well with alot of logic gates.
Before you start taking everyones suggestion, you need to think about what you want to do with logic gates and why. In some cases, pull-up and pull-down resistors may not apply.