\documentclass{standalone}

\usepackage{tikz}
\usepackage{tikz-timing}
\usepackage{newpxtext}
\usepackage{newpxmath}

\begin{document}

\begin{tikztimingtable}[timing/d/background/.style={fill=white}]
  \small clk & 2L 3{6C} \\
  \small u\_en & 2D{u\_en} 18D{$\overline{\texttt{u\_en}}$}\\
  \small addr & 2U 18D{3} \\
  \small wr\_en & 2U 18L \\
  \small en & 8D{u\_en} 12D{$\overline{\texttt{u\_en}}$}\\
  \small d\_out & 8U 12D{0xDEADBEEF} \\
  \small r & 14U 6D{0xDEADBEEF} \\
  \extracode
  \node[help lines] at (2,2.25) {\tiny 1};
  \node[help lines] at (8,2.25) {\tiny 2};
  \node[help lines] at (14,2.25) {\tiny 3};
  \begin{pgfonlayer}{background}
    \vertlines[help lines]{2,8,14}
  \end{pgfonlayer}
\end{tikztimingtable}

\end{document}