Example: D flip-flops and shift register

Published 2009-08-01 | Author: Martin Scharrer

Example of a custom node shape for drawing D flip-flops. The shape is used to draw a serial shift register.

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D flip-flops and shift register

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% D flip-flops (DFFs) and shift register
% Author: Martin Scharrer



% Data Flip Flip (DFF) shape
  % The 'minimum width' and 'minimum height' keys, not the content, determine
  % the size
  % This is redundant, but makes some things easier:
  % Inherit from rectangle

  % Define same anchor a normal rectangle has
  \anchor{north}{\northeast \pgf@x=0pt}
  \anchor{east}{\northeast \pgf@y=0pt}
  \anchor{south}{\southwest \pgf@x=0pt}
  \anchor{west}{\southwest \pgf@y=0pt}
  \anchor{north east}{\northeast}
  \anchor{north west}{\northeast \pgf@x=-\pgf@x}
  \anchor{south west}{\southwest}
  \anchor{south east}{\southwest \pgf@x=-\pgf@x}
    \advance\pgf@x by -.5\wd\pgfnodeparttextbox%
    \advance\pgf@y by -.5\ht\pgfnodeparttextbox%
    \advance\pgf@y by +.5\dp\pgfnodeparttextbox%

  % Define anchors for signal ports
  % Draw the rectangle box and the port labels
    % Rectangle box
    % Angle (>) for clock input
    \pgf@xa=\pgf@x \pgf@ya=\pgf@y
    \pgf@xb=\pgf@x \pgf@yb=\pgf@y
    \pgf@xc=\pgf@x \pgf@yc=\pgf@y
    \pgfmathsetlength\pgf@x{1.6ex} % size depends on font size
    \advance\pgf@ya by \pgf@x
    \advance\pgf@xb by \pgf@x
    \advance\pgf@yc by -\pgf@x

    % Draw port labels
    \tikzset{flip flop/port labels} % Use font from this style







% Key to add font macros to the current font
\tikzset{add font/.code={\expandafter\def\expandafter\tikz@textfont\expandafter{\tikz@textfont#1}}} 

% Define default style for this node
\tikzset{flip flop/port labels/.style={font=\sffamily\scriptsize}}
\tikzset{every dff node/.style={draw,minimum width=2cm,minimum 
height=2.828427125cm,very thick,inner sep=1mm,outer sep=0pt,cap=round,add 



\begin{tikzpicture}[font=\sffamily,>=triangle 45]
  \def\N{7}  % Number of Flip-Flops minus one

  % Place FFs
  \foreach \m in {0,...,\N}
    \node [shape=dff] (DFF\m) at ($ 3*(\m,0) $) {Bit \#\m};

  % Connect FFs (Q1 with D1, etc.)
  \def\p{0}  % Used to save the previous number
  \foreach \m in {1,...,\N} { % Note that it starts with 1, not 0
    \draw [->] (DFF\p.Q) -- (DFF\m.D);

  % Connect and label data in- and output port
  \draw [<-] (DFF0.D) -- +(-1,0) node [anchor=east] {input} ;
  \draw [->] (DFF\N.Q) -- +(1,0) node [anchor=west] {output};

  % 'Reset' port label
  \path (DFF0) +(-2cm,+2cm) coordinate (temp)
    node [anchor=east] {reset};
  % Connect resets
  \foreach \m in {0,...,\N}
    \draw [->] (temp) -| (DFF\m.R);

  % 'Set' port label
  \path (DFF0) +(-2cm,-2cm) coordinate (temp)
    node [anchor=east] {set};
  % Connect sets
  \foreach \m in {0,...,\N}
    \draw [->] (temp) -| (DFF\m.S);

  % Clock port label
  \path (DFF0) +(-2cm,-2.5cm) coordinate (temp)
    node [anchor=east] {clock};
  \foreach \m in {0,...,\N}
    \draw [->] (temp) -| ($ (DFF\m.CLK) + (-5mm,0) $) --(DFF\m.CLK);

  % Clock port label
  \path (DFF0) +(-2cm,-3cm) coordinate (temp)
    node [anchor=east] {clock enable};
  \foreach \m in {0,...,\N}
    \draw [->] (temp) -| ($ (DFF\m.CE) + (-7.5mm,0) $) --(DFF\m.CE);



  • #1 Will, June 16, 2010 at 1:01 a.m.

    Thanks for the example!

    I have a question though.

    When computing the location of the 'text' anchor, you use pgfnodeparttextbox. The PGF manual says that this should be avoided:

    "The problem is that the box \pgfnodeparttextbox will most likely not have the correct size when the anchor is computed. After all, the anchor position might be recomputed at a time when several other nodes have been created."

    Taken from page 483 of Manual 2.0

    The code above seems to work fine, however, and I don't know any good alternative.

    Does anyone else know a good alternative which achieves the same functionality but without pgfnodeparttextbox?


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