\subsubsection{GrayCounter}
\label{sec-GrayCounter}
%\index{Xilinx!GrayCounter (package)}

{\bf Package}
\index{GrayCounter (package)}

\begin{verbatim}
import GrayCounter :: * ;
\end{verbatim}




{\bf Description}

The GrayCounter package provides an interface and a  module to
implement a gray-coded counter with  methods for both
binary and Gray code.   This package is designed for use in the
\te{BRAMFIFO} module, Section \ref{sec-BRAMFIFO}.  Since BRAMs have
registered address inputs, the binary  outputs  are not registered.
The counter has two domains, source and destination.  Binary and Gray
code values are written in the source domain. Both types of values can
be read from  the source and the destination domains.

{\bf Types}

The GrayCounter package uses the type \te{Gray}, defined in the
Gray package, Section \ref{sec-Gray}.  The Gray package is
imported by the GrayCounter package.

{\bf Interfaces and Methods}

The GrayCounter package includes one interface, \te{GrayCounter}.

\begin{center}
\begin{tabular}{|p {1 in}|p{1 in}|p{3 in}|}
\hline
\multicolumn{3}{|c|}{GrayCounter Interface Methods}\\
\hline
Name & Type & Description\\
\hline
\hline 
\te{incr}  & Action &Increments the counter by 1\\
\hline
\te{decr} & Action &Decrements the counter by 1 \\
\hline
\te{sWriteBin}&Action&Writes a binary value into the counter in the
source domain.  \\
\hline
\te{sReadBin}  & \te{Bit\#(n)} &Returns a binary value from the
source domain of the counter. The output is not registered   \\
\hline
\te{sWriteGray}&Action&Writes a Gray code value into  the counter in
the source domain.  \\
\hline
\te{sReadGray}  & \te{Gray\#(n)} & Returns the Gray code value from
the source domain of the counter.  The output is registered.   \\
\hline
\te{dReadBin}  & \te{Bit\#(n)} & Returns the binary value from the
destination domain of the counter.  The output is not registered.   \\
\hline
\te{dReadGray}&\te{Gray\#(n)} &Returns the Gray code value from the
destination domain of the counter.  The output is registered.\\
\hline
\end{tabular}
\end{center}



\begin{verbatim}
interface GrayCounter#(numeric type n);
   method    Action      incr;
   method    Action      decr;
   method    Action      sWriteBin(Bit#(n) value);
   method    Bit#(n)     sReadBin;
   method    Action      sWriteGray(Gray#(n) value);
   method    Gray#(n)    sReadGray;
   method    Bit#(n)   	 dReadBin;
   method    Gray#(n)  	 dReadGray;
endinterface: GrayCounter
\end{verbatim}



{\bf Modules}

The module \te{mkGrayCounter} instantiates a Gray code counter with
methods for both binary and Gray code.

\index{mkGrayCounter@\te{mkGrayCounter} (module)}
\index[function]{GrayCounter!mkGrayCounter}

\begin{tabular}{|p{1.4 in}|p{4.2 in}|}
\hline
& \\
\te{mkGrayCounter} &Instantiates a Gray counter with an initial value
\te{initval}.\\
\cline{2-2}
& \begin{libverbatim}
module mkGrayCounter#(Gray#(n) initval, 
                      Clock dClk, Reset dRstN)
                     (GrayCounter#(n))
         provisos(Add#(1, msb, n));
\end{libverbatim}
\\
\hline
\end{tabular}