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Build a 4-bit Johnson counter, sometimes also called a "twisted ring" counter. This is similar to a one-hot counter, except that the feedback bit is inverted, giving a sequence of values containing 1s as well as 0s:
0000
0001
0011
0111
1111
1110
1100
1000
This has the nice advantage that it encodes twice as many states as a one-hot counter for the same number of flip-flops plus an inverter.
When reset, the value should be 0000, and subsequent values should follow the pattern above.
library IEEE;
use IEEE.std_logic_1164.all;
entity johnson is
port(
clk : in std_logic;
reset : in std_logic;
count : out std_logic_vector(3 downto 0)
);
end johnson;
architecture synth of johnson is
begin
count <= "0000";
end;
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