Home Why would an ALU's ouput change asynchronously in response to input changes?
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Why would an ALU's ouput change asynchronously in response to input changes?

Vol uptan
1#
Vol uptan Published in 2017-12-05 21:54:29Z

Wikipedia states "An ALU is a combinational logic circuit, meaning that its outputs will change asynchronously in response to input changes."

But, the definition of a combinational logic circuit is: "combinational logic ... where the output is a pure function of the present input only."

If the output of a combinational logic circuit is purely determined by present input, is Wikipedia stating that it's asynchronous because of the time it takes for an electrical signal to travel through the circuit (~light speed right?)?

Chris Dodd
2#
Chris Dodd Reply to 2017-12-07 17:23:27Z

The two statements say the same thing. The output is a pure function of the input, so the output will change ONLY when the input changes. It's asynchronous because there's no timing or clock involved -- it doesn't matter when the input changes, and each change in an input will result in corresponding changes in the output irrespective of clock signals.

Now for any real circuit, there will be a delay between input changes and output changes, and there may be jitter/settling issues, but that is a whole other problem.

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