avoir bounce with button

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how can i write my vhdl code to avoid bounce?

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Draw a flowchart of the basic functionality, and then write a behavioural implementation of that.

Use a counter to generate a suitable interval at which to test the button's input signal. Test it a number of times to see if it persists. Once the signal has passed that test you can declare the button pressed (or released).

I hope that's something you can attempt to code yourself...

Cheers,

Alex