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I wanted to decode a quadrature rotary signal, so I started with the quadrature decoder code from opencores http://opencores.org/project,quadraturecount I was never able to get that code to count, even after ruling out other sources of problems So I modified the code in a way that made sense to myself, it turned out much shorter then the original and best of all it works correctly Yes, It's not enough that it works, I need outside confirmation that I did not do something bad. My question is, did I get lucky, and am I ignoring possible timing issues, race conditions? Under what conditions can I know that writing to a signal visible to outside circuits is safe? My head kind of hurts from thinking about it. Code follows, thanks library IEEE;use IEEE.STD_LOGIC_1164.ALL;use IEEE.STD_LOGIC_ARITH.ALL;use IEEE.STD_LOGIC_SIGNED.ALL;library work;--How we 'talk' to the outside world:entity QuadratureCounterPorts is Port ( clock : in std_logic; --system clock, i.e. 10MHz oscillator QuadA : in std_logic; --first input from quadrature device (i.e. optical disk encoder) QuadB : in std_logic; --second input from quadrature device (i.e. optical disk encoder) CounterValue : out std_logic_vector(23 downto 0); --just an example debuggin output setCount : in std_logic_vector(23 downto 0); writeEnable : in std_logic );end QuadratureCounterPorts;--What we 'do':architecture behave of QuadratureCounterPorts is signal Count : unsigned(23 downto 0); signal clockD : std_logic; signal Quad : std_logic_vector(1 downto 0); signal lastQuad : std_logic_vector(1 downto 0); function gray(a:std_logic_vector) return std_logic_vector isbegin if a="00" then return "00"; elsif a="10" then return "01"; elsif a="11" then return "10"; elsif a="01" then return "11"; end if; return "11";end function; begin --architecture QuadratureCounter -- do our actual work every clock cycle process(clock) begin --keep track of the counter if ( rising_edge(clock) ) then lastQuad <= Quad; Quad <= gray((0=>QuadA,1=>QuadB)); if((unsigned(Quad) = unsigned(lastQuad)+1)) then Count <= Count + 1; end if; if((unsigned(Quad) = unsigned(lastQuad)-1)) then Count <= Count - 1; end if; if(writeEnable = '1') then --Count <= unsigned(setCount); end if; end if; --clock'event CounterValue <= std_logic_vector(Count); end process; --(clock) end behave;