Function of enable input in gates

[waqarrashid33]

what are the function of enable input in gates?


If in or gate we use it and it is at high and the other two inputs are at low then what will be out put...1/0?
Please clear me about the enable input

 

[vk6kro]

If either input of an OR gate is high, then the state of the other input doesn't matter. The output will be high.

To allow input pulses to pass from input to output of the gate, then the "enable" input has to be low.

This input pin is no different to any other pin on the gate input, but its function makes it an "enable" (or disble) pin.

 

[waqarrashid33]

when the enable is at low the gate/I.C will be disable and when the enable is at high the gate/IC will be disable...but told that To allow input pulses to pass from input to output of the gate, then the "enable" input has to be low...what is correct..if you have any external link about it then please provide it..

 

[Bobbywhy]

waqarrashid33, for a good explanation of the "enable" function of logic gates see:

http://www.allaboutcircuits.com/vol_4/chpt_3/8.html

 

[vk6kro]

This only applies to an OR gate. Other gates are different.

http://en.wikipedia.org/wiki/Or_gate

As you can see, with an OR gate, if either or both of the inputs are high, then the output will be high. If they are both LOW, the output will be LOW

So, if you make one of the inputs LOW, then the output will depend on what the other input does.
If it is HIGH, the output will be HIGH, and if it is LOW, the output will be LOW.

Now, if you make the enable input HIGH, what can happen?
If the other input is LOW the output will be HIGH.
If the other input is HIGH, the output will be HIGH.
So the output will always be HIGH.

So, even if you put pulses (ie HIGH, LOW, HIGH...) on the other input, none of them will get through to the output.

That is why they call it a gate. It can let stuff through or stop it.

So what use is all this?
If you made the enable input LOW for exactly 1 second, you could use another circuit to count how many pulses got through and display this number on a display of some sort.
You would then have a frequency counter.