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Asynchronous combinational logic, help for assignment

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Asynchronous combinational logic, help for assignment

Postby welshboidai » Sat Dec 12, 2015 4:25 pm

Will adding gates to a asynchronous combinational logic circuit decrease the response time? I know that these types of circuits act instantaneously, but does that mean that any circuit containing x amount of gates respond at the same time? The question i have is for an assignment at which we are given 5 combinational logic circuits using NAND gates and the question: 2) which type of circuit will respond the quickest? I am a student and cant find the answer anywhere, hopefully you can help, thanks.
"Scientists study the world as it is, engineers create the world that has never been" - Theodore Von Karman
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Re: Asynchronous combinational logic, help for assignment

Postby pebe » Sun Dec 13, 2015 3:40 pm

Gate circuits don’t act instantaneously. Any gate contains a number of FET or bipolar elements in its signal path from input to output, and each of these elements has an inherent base/emitter or gate/source capacity. These capacities have to be charged or discharged as the signal changes state along the route, and that takes time. The result is that there is a very small time delay between when a gate input changes and when that change appears at the output. That is known as the propagation delay, and I think that is what your assignment is all about. If you Google ‘gates propagation delay wiki’ you will find an example there.

The data sheet for each type of gate will show its propagation delay. I don’t know the 5 combinations of gates you have been given, but if the signal has different path lengths in each combination, you only need to add up the propagation delays in each signal path to see which will respond quickest.

I hope that has helped.
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Re: Asynchronous combinational logic, help for assignment

Postby welshboidai » Sun Dec 13, 2015 4:49 pm

pebe wrote:The result is that there is a very small time delay between when a gate input changes and when that change appears at the output. That is known as the propagation delay


so asynchronous are only limited by propagation delays therefore dependent on how long it takes for a signal on its input to reach its output? all the circuits i am given consist of only NAND gates. so in theory the circuit with the least gates will respond the quickest right?
"Scientists study the world as it is, engineers create the world that has never been" - Theodore Von Karman
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Re: Asynchronous combinational logic, help for assignment

Postby welshboidai » Sun Dec 13, 2015 5:12 pm

good stuff thank you pebe. :D
"Scientists study the world as it is, engineers create the world that has never been" - Theodore Von Karman
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Re: Asynchronous combinational logic, help for assignment

Postby pebe » Sun Dec 13, 2015 7:26 pm

so asynchronous are only limited by propagation delays therefore dependent on how long it takes for a signal on its input to reach its output?

I don't really see the relevance of whether the signal is asynchronous or not.
all the circuits i am given consist of only NAND gates. so in theory the circuit with the least gates will respond the quickest right?

Could you post one of the circuits, in case there is some eventuality I have overlooked?
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Re: Asynchronous combinational logic, help for assignment

Postby welshboidai » Sun Dec 13, 2015 9:46 pm

i cannot upload a photo, it is j peg and wont allow. basically i have a NAND gate with inputs of A & B for the first circuit. second circuit i have a NAND gate with inputs of A & 1, third circuit i have 4 NAND gates the first two inputs are A & B and on the third gate i have the output of the first two gates along with C and then the fourth input is linked so the output of the third is the fourth gates input if that makes any sense. there are two more circuits but they consist of more gates. i think you nailed it on the first comment bud so im happy, the assignment is complete now :D
"Scientists study the world as it is, engineers create the world that has never been" - Theodore Von Karman
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