\subsubsection{Gearbox}

\index{Gearbox@\te{Gearbox} (package)}

\label{Gearbox}

{\bf Package}


\begin{verbatim}
import Gearbox :: *
\end{verbatim}



{\bf Description} 

This package defines  FIFO-like converters that convert N-wide data to
and from 1-wide data at N-times the frequency. These converters
change the frequency and the data width, while the overall
data rate stays the same.  The data width on the fast side is always
1, while the data width on the slow side is N.  The converters are intended to
be used between clock 
domains with aligned edges for both types of clock domain crossings
(fast to slow and slow to fast). For example:
\begin{tabbing}
300 MHz at 8-bits  \hm \= converted to\hm \=  100 MHz at 24-bits \hm \=(fast
to slow)\\ 
100 MHz at 24-bits\>   converted to\>     300 MHz at 8-bits \>(slow to
fast)\\
\end{tabbing}
In both of these examples, the data type \te{a} is \te{Bit\#(8)} and 
N=3.


These modules are written in pure BSV using a style that utilzies only
\te{mkNullCrossingReg} to cross registered values between clock
domains.  Restricting the form of clock crossings is important to
ensure that the module preserves atomic semantics and also that it is
compatible with both Verilog and Bluesim backends.


{\bf Interfaces and methods}
\index{Gearbox@\te{Gearbox} (interface)}

The \te{Gearbox} interface provides the following methods: \te{enq},
\te{deq}, \te{first}, \te{notFull} and \te{notEmpty}. 



\begin{center}
\begin{tabular}{|p{.6in}|p{1.1 in}|p{3.6 in}|}
\hline
\multicolumn{3}{|c|}{Gearbox Interface}\\
\hline
Method Name & Type & Description\\
\hline
\hline 
\te{enq}&Action&Adds an entry to the converter of type \te{Vector\#(in,
a)}, where \te{a} is the datatype.  If the 
input is the fast domain then \te{in} = 1,  if the input is the slow
domain, \te{in} = N.\\
\hline
\te{deq}&Action&Removes the first entry from the converter.\\
\hline
\te{first}&\te{Vector\#(out, a)}&Returns the first entry from the
converter. If the output domain is the fast side, \te{out} = 1, if the
output domain is the slow side, \te{out} = N.\\
\hline
\te{notFull}&Bool&Returns a True value if there is space to enqueue an
entry into the FIFO.\\
\hline
\te{notEmpty}&Bool&Returns a True value if there is are elements in
the FIFO and you can dequeue from the FIFO.\\
\hline
\hline
\end{tabular}
\end{center}



\begin{verbatim}
interface Gearbox#(numeric type in, numeric type out, type a);
   method    Action          enq(Vector#(in, a) din);
   method    Action          deq();
   method    Vector#(out, a) first();
   method    Bool            notFull();
   method    Bool            notEmpty();
endinterface
\end{verbatim}



{\bf Modules}

The package provides two modules: \te{mkNto1Gearbox} for slow to fast
domain crossings, and  \te{mk1toNGearbox} to for
fast to slow domain crossings.  These are intended for use between
clock domains with aligned edges for both types of clock domain crossings.

{\bf Note:} for both modules the resets in the source and destination
domains (\te{sReset} and \te{dReset}) should be asserted together,
otherwise only half the unit will be in reset.

\index{mkNto1Gearbox@\te{mkNto1Gearbox} (module)}
\index[function]{Gearbox!mkNto1Gearbox}


With the \te{mkNto1Gearbox} module,  2xN elements of data storage are
provided,  grouped into 2 blocks of N elements each. Each block is
writable in the source (slow) domain and readable in the destination
(fast) domain. 

\begin{center}
\begin{tabular}{|p{.9 in}|p{4.4 in}|}
 \hline
&         \\
\te{mkNto1Gearbox}&Moves data from a slow domain to a fast domain,
changing the data width from a larger width to a smaller width.  The
data rate stays the same.  The width of the output is 1, the width of
the input is N.
 \\
\cline{2-2} 
& \begin{libverbatim}
module mkNto1Gearbox(Clock sClock, Reset sReset,
                     Clock dClock, Reset dReset,
                     Gearbox#(in, out, a) ifc)
   provisos(Bits#(a, a_sz), Add#(out, 0, 1), 
            Add#(out, z, in) );
\end{libverbatim} 
\\
\hline
\end{tabular}
\end{center}

\index{mk1toNGearbox@\te{mk1toNGearbox} (module)}
\index[function]{Gearbox!mk1toNGearbox}



With the \te{mk1toNGearbox} module, 2xN elements of data storage are
provided, grouped into 2 blocks of N elements each.
Each block is writable in the source (fast) domain
and readable in the destination (slow) domain.

\begin{center}
\begin{tabular}{|p{.9 in}|p{4.4 in}|}
 \hline
&         \\
\te{mk1toNGearbox} &  Moves data from a fast
domain to a slow  domain,
changing the data width from a smaller width to a larger width.  The
data rate stays the same.  The width of the input is 1, the width of
the output is N.
\\
\cline{2-2} 
& \begin{libverbatim}
module mk1toNGearbox(Clock sClock, Reset sReset,
                     Clock dClock, Reset dReset,
                     Gearbox#(in, out, a) ifc)
   provisos(Bits#(a, a_sz), Add#(in, 0, 1),
            Add#(in, z, out), Mul#(2, out, elements),
            Add#(1, w, elements), Add#(out, x, elements) );
\end{libverbatim} 
\\
\hline
\end{tabular}
\end{center}