Draw register map with TikZ

Question

I was wondering if there was a way to easily draw a register map (like the one below) in LaTeX, possibly in TikZ?

The closest I've gotten is just using the tabular environment and that's not even close to what I want, my result is listed below.

\begin{table}[h!]
\begin{tabular}{p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt} p{20pt}  >{\centering\arraybackslash}m{20pt}}
\centering 15 & \centering 14 & \centering 13 & \centering 12 & \centering 11 & \centering 10 & \centering 9 & \centering 8 & \centering 7 & \centering 6 & \centering 5 & \centering 4 & \centering 3 & \centering 2 & \centering 1 & 0\\\hline
\multicolumn{1}{|c|}{SEL} & 
\multicolumn{1}{c|}{SEL} & 
\multicolumn{1}{c|}{SEL} & 
\multicolumn{5}{|c|}{\cellcolor{gray!25}} & 
\multicolumn{1}{c|}{DIR} & 
\multicolumn{7}{c|}{PWM} \\\hline
\end{tabular}
\end{table}

Answer 1

Late to the question here, but this should be workable. I'm not certain if I have the right semantics for the different fields, but this should be relatively easy to adapt:

\documentclass{standalone}
\usepackage{tikz}

\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}

\newcommand{\bitrect}[2]{
  \begin{pgfonlayer}{foreground}
    \draw [thick] (0,0) rectangle (#1,1);
    \pgfmathsetmacro\result{#1-1}
    \foreach \x in {1,...,\result}
      \draw [thick] (\x,1) -- (\x, 0.8);
  \end{pgfonlayer}
%  \node [below left, align=right] at (0,0) {Type \\ Reset};
  \bitlabels{#1}{#2}
}
\newcommand{\rwbits}[3]{
  \draw [thick] (#1,0) rectangle ++(#2,1) node[pos=0.5]{#3};
  \pgfmathsetmacro\start{#1+0.5}
  \pgfmathsetmacro\finish{#1+#2-0.5}
%  \foreach \x in {\start,...,\finish}
%    \node [below, align=center] at (\x, 0) {R/W \\ 0};
}
\newcommand{\robits}[3]{
  \begin{pgfonlayer}{background}
    \draw [thick, fill=lightgray] (#1,0) rectangle ++(#2,1) node[pos=0.5]{#3};
  \end{pgfonlayer}
  \pgfmathsetmacro\start{#1+0.5}
  \pgfmathsetmacro\finish{#1+#2-0.5}
%  \foreach \x in {\start,...,\finish}
%    \node [below, align=center] at (\x, 0) {RO \\ 0};
}
\newcommand{\bitlabels}[2]{
  \foreach \bit in {1,...,#1}{
     \pgfmathsetmacro\result{#2}
     \node [above] at (\bit-0.5, 1) {\pgfmathprintnumber{\result}};
   }
}

\begin{document}
\begin{tikzpicture}
\bitrect{16}{16-\bit}
\rwbits{0}{1}{SEL}
\rwbits{1}{1}{SEL}
\rwbits{2}{1}{SEL}
\robits{3}{5}{}
\rwbits{8}{1}{DIR}
\rwbits{9}{7}{PWM}
\end{tikzpicture}
\end{document}

Other variations using this preamble include grouping the SEL bits similar to the PWM bits:

\begin{tikzpicture}
\bitrect{16}{16-\bit}
\rwbits{0}{3}{SEL}
\robits{3}{5}{}
\rwbits{8}{1}{DIR}
\rwbits{9}{7}{PWM}
\end{tikzpicture}

or the first example you gave:

\begin{tikzpicture}
% make sure to uncomment \foreach loops in \robits and \rwbits, and last node in \bitrect
% 16 bits, indices decreasing from 31...16
\bitrect{16}{32-\bit} % {length}{index label expression}
\rwbits{0}{3}{INTD} % {start bit}{length}{label}
\robits{3}{5}{reserved} % {start bit}{length}{label}
\rwbits{8}{3}{INTC}
\robits{11}{5}{reserved}
\end{tikzpicture}