Debouncing the Altera UP1 Board's Pushbutton

The following code will debounce the pushbutton by requiring that the (active-low) "rawbutton" input be asserted for 4095 clock periods before asserting the (active-high) "button" signal. At 8.333 MHz this adds about 500 microseconds of delay.

entity lab4 is
        port ( 
                rawbutton : in std_logic ;	-- un-debounced input
                clk : in std_logic 		-- 8.333 MHz ISA bus clock
                ) ;
end lab4 ;

architecture rtl of lab4 is
	signal rawbuttoncount, nextrawbuttoncount : unsigned(11 downto 0) ;
begin

	-- debounce button
	nextrawbuttoncount <=
		conv_unsigned(0,12) when rawbutton = '1' else
		rawbuttoncount when rawbuttoncount = 4095 else
		rawbuttoncount + 1 ;		

	button <=
		'1' when rawbuttoncount = 4095 else 
		'0' ;

	process(clk)
	begin
		if clk'event and clk ='1' then
			rawbuttoncount <= nextrawbuttoncount ;
		end if ;
	end process ;	

end rtl ;