\section{State Machines}
\begin{figure}[h]
  \centering
  \ttfamily
  \begin{tikzpicture}[
      node distance = 3mm,
      box/.style = {
        draw = black, thick, fill = gray!20!white,
        minimum width = 20mm, minimum height = 8mm,
      }
    ]

    % mealey
    \begin{scope}
      \node[box] (G) {\large G};
      \node[box, above = of G] (F) {\large F};
      \node[box, below = of G] (Z) {\large Z};

      \node[above = of F] {\large Mealey};

      \draw[very thick, ->, hsr-blue] (F.east) -- ++(1,0) node[right] {oup};
      \draw[very thick, ->, hsr-lakegreen] (G.east) -- ++(.5,0) |- (Z.east);
      \draw[very thick, ->] (Z.west) -- ++(-.5,0) |- ($(G.west) - (0,.25)$);
      \draw[very thick, ->] (G.west) ++ (-.5,-.25) |- ($(F.west) + (0,.25)$);

      \draw[very thick, ->, hsr-mauve] ($(G.west) + (-1.5,.25)$)
        node[left] {inp} -- ++(1.5,0);
      \draw[very thick, ->, hsr-mauve] ($(G.west) + (-1,.25)$) |- ($(F.west) - (0,.25)$);
    \end{scope}

    % moore
    \begin{scope}[yshift = -42mm]
      \node[box] (G) {\large G};
      \node[box, above = of G] (F) {\large F};
      \node[box, below = of G] (Z) {\large Z};

      \node[above = of F] {\large Moore};

      \draw[very thick, ->, hsr-blue] (F.east) -- ++(1,0) node[right] {oup};
      \draw[very thick, ->, hsr-lakegreen] (G.east) -- ++(.5,0) |- (Z.east);
      \draw[very thick, ->] (Z.west) -- ++(-.5,0) |- ($(G.west) - (0,.25)$);
      \draw[very thick, ->] (G.west) ++ (-.5,-.25) |- (F.west);

      \draw[very thick, ->, hsr-mauve] ($(G.west) + (-1.5,.25)$)
        node[left] {inp} -- ++(1.5,0);
    \end{scope}

    % Medwedjew
    \begin{scope}[yshift = -80mm]
      \node[box] (G) {\large G};
      \node[box, below = of G] (Z) {\large Z};

      \node[above = of G, yshift = 3mm] {\large Medwedjew};

      \draw[very thick, ->, hsr-lakegreen] (G.east) -- ++(.5,0) |- (Z.east);
      \draw[very thick, ->, hsr-blue] (Z.west) -- ++(-.5,0) |- ($(G.west) - (0,.25)$);
      \draw[very thick, ->, hsr-blue] (G.west) ++ (-.5,-.25) |- ($(G.east) + (1,.75)$)
        node[right] {oup};

      \draw[very thick, ->, hsr-mauve] ($(G.west) + (-1.5,.25)$)
        node[left] {inp} -- ++(1.5,0);
    \end{scope}
  \end{tikzpicture}
\end{figure}

\subsection{Encoding the state} \label{sec:fsm:encode}
For Mealey and Moore machines it is typical to write:
\begin{lstlisting}[language=vhdl]
type state_type is (st_rst, st_a, st_b, st_c, ...);
signal present_state, next_state : state_type;
\end{lstlisting}
The encoding of the state is left to the synthesizer or can be configured in
the graphical interface of the tool.  If a custom encoding is required
(Medwedjew), adding the following generates a custom encoding.
\begin{lstlisting}[language=vhdl]
attribute enum_encoding : string;
attribute enum_encoding of state_type:
  type is "0001 0010 0100 ...";
\end{lstlisting}
Or an equivalent approach is to use a vector subtype and constants.
\begin{lstlisting}[language=vhdl]
subtype state_type is bit_vector(3 downto 0);

constant st_rst : state_type := "0001";
constant st_a   : state_type := "0010";
constant st_b   : state_type := "0100";
...

signal present_state, next_state : state_type;
\end{lstlisting}

\subsection{Updating the state register (\texttt{Z})}
\begin{lstlisting}[language=vhdl]
register_logic: process (clk, rst)
begin
  -- asynchronous reset
  if rst = '1' then
    present_state <= st_rst;

  -- clock
  elsif rising_edge(clk) then
    present_state <= next_state;
  end if;
end process;
\end{lstlisting}

\subsection{Updating the state (\texttt{G})}
\begin{lstlisting}[language=vhdl]
next_state_logic:
process (present_state, `\optionalph{inputs}`)
begin
  -- default value
  next_state <= state_rst;

  case present_state is
    when st_rst =>
      -- reset state logic
      next_state <= `\reqph{state}`;

    when st_a =>
      -- logic using inputs
      next_state <= `\reqph{state}`;

    ...
    when others => null;
  end case;
end process;
\end{lstlisting}

\subsection{Updating the output (\texttt{F})}
Mealey
\begin{lstlisting}[language=vhdl]
output_logic:
process (present_state, `\reqph{inputs}`)
begin
  -- logic with state and inputs
  `\reqph{output}` <= `\reqph{expression}`;
end process;
\end{lstlisting}
Moore
\begin{lstlisting}[language=vhdl]
output_logic: process (present_state)
begin
  case present_state is
    when st_rst =>
      `\reqph{output}` <= `\reqph{value}`;

    ...
  end case;
end process;
\end{lstlisting}
Medwedjew
\begin{lstlisting}[language=vhdl]
output_logic: `\reqph{output}` <= present_state;
\end{lstlisting}