How do I use the reset connection on a flip-flop?

Question

I'm happy to use Circuitikz to draw my flip-flops. But, I'm having trouble connecting to an active-low bubble on the bottom side of the flip-flop.

As I understand it, there are two anchors on the bottom of the flip-flop: down and bdown. Neither is on the bottom of the bubble, where you'd naturally want to connect. I've come up with an ugly hack, but there's got to be a "right" way.

Here's an MWE:

\documentclass[tikz]{standalone}
\usepackage{tikz}
\usepackage[RPvoltages]{circuitikz}
\tikzset{flipflop DQ/.style={flipflop, scale=.7,
         flipflop def={t1=D, t6=Q, c3=1,
         clock wedge size=.3, font=\normalsize}
}}
\tikzset{flipflop DQBR/.style={flipflop, scale=.7, % Has D, Q, QBar and Reset
         flipflop def={t1=D, t6=Q, c3=1, t4=\ctikztextnot{Q}, n4=1, nd=1,
         clock wedge size=.3, font=\normalsize},
}}
\tikzset{flipflop DQBP/.style={flipflop, scale=.7, % Has D, Q, QBar and Preset
         flipflop def={t1=D, t6=Q, c3=1, t4=\ctikztextnot{Q}, n4=1, nu=1,
         clock wedge size=.3, font=\normalsize}
}}
\begin{document}


\begin{circuitikz}
  %\draw[help lines] (-10,-3) grid (5,10);
  \node[flipflop DQBR] (ff0) at (0,3) {};
  \node[flipflop DQBR] (ff1) at (3,3) {};
  \node[flipflop DQBR] (ff2) at (6,3) {};
  \node[flipflop DQBP] (ff3) at (0,0) {};
  \node[flipflop DQ]   (ff4) at (3,0) {};
  \node[flipflop DQ]   (ff5) at (6,0) {};
  
  \draw (ff0.bdown) -- (ff3.up);  % Line overwrites bubble
  \draw (ff1.down) -- (ff4.up);
  \draw (ff2-Nd.south) -- (ff5.bup);  % Ugly-ness to get line not to 
     %draw over top of the bubble.
  
\end{circuitikz}

\end{document}

And the results: Note that the left connection overwrites the bubble, the center one misses, and only the right one connects nicely. Contrast that with the flip-flop on the left bottom, where the line connecting to the up anchor intersects cleanly with the bubble.

Answer 1

First of all --- I think you found a bug. I will look into it as soon as I can. But probably the bug is not what you think --- you shouldn't have the bottom pin on your DQBP, and you should have the reset pin on your DQBR flip-flop.

(side note --- I suggest you use Q and the not ball, or \ctikztextnot{Q} without the not ball. People can think that there is a double negation there).

Second of all, the solution you found for the connection to the ball is the correct one, not an ugly hack --- it is explained in the manual (bottom of page 130), when you have 0 length external pins --- in your case, you have the same situation due to the bug.

The theory is that when you say nd=1 (or nu=1) to have the negated ball, the external pin is drawn also. When drawing the external pin, the pin/ball overlap is taken into account in the shape.

What you really want here is having the pins, so the first and second rows work naturally. As a workaround, add a blank label to the pin for DQBR and DQBP.

DQ has no upper or down pins, so you have to use the border anchors.

\documentclass[border=4pt]{standalone}
\usepackage{tikz}
\usepackage[RPvoltages]{circuitikz}
\tikzset{flipflop DQ/.style={flipflop, scale=.7,
         flipflop def={t1=D, t6=Q, c3=1,
         clock wedge size=.3, font=\normalsize}
}}
\tikzset{flipflop DQBR/.style={flipflop, scale=.7, % Has D, Q, QBar and Reset
         flipflop def={t1=D, t6=Q, c3=1, t4=\ctikztextnot{Q}, n4=1, nd=1, td=~,
         clock wedge size=.3, font=\normalsize},
}}
\tikzset{flipflop DQBP/.style={flipflop, scale=.7, % Has D, Q, QBar and Preset
         flipflop def={t1=D, t6=Q, c3=1, t4=\ctikztextnot{Q}, n4=1, nu=1, tu=~,
         clock wedge size=.3, font=\normalsize}
}}
\begin{document}


\begin{circuitikz}
  \node[flipflop DQBR] (ff0) at (0,3) {0};
  \node[flipflop DQBR] (ff1) at (3,3) {1};
  \node[flipflop DQBR] (ff2) at (6,3) {2};
  \node[flipflop DQBP] (ff3) at (0,0) {3};
  \node[flipflop DQ]   (ff4) at (3,0) {4};
  \node[flipflop DQ]   (ff5) at (6,0) {5};

  \draw (ff0.down) -- (ff3.up);
  \draw (ff1.down) -- (ff4.bup);
  \draw (ff2.down) -- (ff5.bup);

\end{circuitikz}

\end{document}

---

PD the bug is just a silly typo (I can say that because I wrote it!)

diff --git a/tex/pgfcircmultipoles.tex b/tex/pgfcircmultipoles.tex
index 3454f6d..82b156c 100644
--- a/tex/pgfcircmultipoles.tex
+++ b/tex/pgfcircmultipoles.tex
@@ -1406,7 +1406,7 @@
                 \ifx\@@tmp\@@x\else\edef\@@or{1}\fi
                 \edef\@@tmp{\ctikzvalof{multipoles/flipflop/cd}}
                 \ifnum\@@tmp>0\edef\@@or{1}\fi
-                \edef\@@tmp{\ctikzvalof{multipoles/flipflop/nu}}
+                \edef\@@tmp{\ctikzvalof{multipoles/flipflop/nd}}
                 \ifnum\@@tmp>0\edef\@@or{1}\fi
                 % \typeout{TEST\space\@@tmp\space\@@x}
                 \ifnum\@@or>0

with the patch in (it will be fixed in v1.3.2), your original code (with the connection in red to show them better) works almost out of the box:

\documentclass[border=4pt]{standalone}
\usepackage{tikz}
\usepackage[RPvoltages]{circuitikz}
\tikzset{flipflop DQ/.style={flipflop, scale=.7,
         flipflop def={t1=D, t6=Q, c3=1,
         clock wedge size=.3, font=\normalsize}
}}
\tikzset{flipflop DQBR/.style={flipflop, scale=.7, % Has D, Q, QBar and Reset
         flipflop def={t1=D, t6=Q, c3=1, t4=\ctikztextnot{Q}, n4=1, nd=1,
         clock wedge size=.3, font=\normalsize},
}}
\tikzset{flipflop DQBP/.style={flipflop, scale=.7, % Has D, Q, QBar and Preset
         flipflop def={t1=D, t6=Q, c3=1, t4=\ctikztextnot{Q}, n4=1, nu=1,
         clock wedge size=.3, font=\normalsize}
}}
\begin{document}


\begin{circuitikz}
  \node[flipflop DQBR] (ff0) at (0,3) {0};
  \node[flipflop DQBR] (ff1) at (3,3) {1};
  \node[flipflop DQBR] (ff2) at (6,3) {2};
  \node[flipflop DQBP] (ff3) at (0,0) {3};
  \node[flipflop DQ]   (ff4) at (3,0) {4};
  \node[flipflop DQ]   (ff5) at (6,0) {5};

  \draw[red] (ff0.down) -- (ff3.up);
  \draw[red] (ff1.down) -- (ff4.bup);
  \draw[red] (ff2.down) -- (ff5.bup);

\end{circuitikz}
\end{document}

Sorry for the horrible antialiasing, blame okular