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diff --git a/tex/vhdl.tex b/tex/vhdl.tex index 1854592..6fc5c37 100644 --- a/tex/vhdl.tex +++ b/tex/vhdl.tex @@ -1,11 +1,69 @@ -\section{VHSIC Hardware Description Language (\texttt{VHDL})} +\section[VHSIC Hardware Description Language (VHDL)]{ + VHSIC Hardware Description Language (VHDL) + % \texttt{VHDL}: Very high-speed integrated circuits program + % Hardware Description Language +} \newcommand{\ph}[1]{\textrm{\textit{#1}}} \newcommand{\reqph}[1]{\textrm{\textlangle\,\ph{#1}\,\textrangle}} \newcommand{\optionalph}[1]{\textrm{[\,\ph{#1}\,]}} +\newcommand{\vhdl}[1]{\lstinline[language=vhdl]{#1}} + +\subsection{Basic syntax and identifiers} +In VHDL an identifier is a case insensitive string composed of +\texttt{A-Z a-z 0-9 \_} that +\begin{itemize} + \item is not a keyword, + \item does not start with a number or \texttt{\_}, + \item does not have two or more \texttt{\_} in a row. +\end{itemize} +Expressions are terminated by a semicolon \texttt{;}. +Two dashes in a row cause the rest of the line to be interpreted as a comment. +\begin{lstlisting}[language=vhdl] +expression; -- comment +\end{lstlisting} + \subsection{Entities and Architectures} +In VHDL the concept of \emph{entity} describes a black box of which only +inputs and outputs are known. The internals of an entity are described through +an \emph{architecture}. There can be multiple architectures for a single entity. + +\begin{center} + \ttfamily + \begin{tikzpicture} + \node[ + rectangle, draw = black, thick, fill = gray!20!white, + minimum width = 4.5cm, minimum height = 4cm, + ] (entity) {}; + + \node[anchor = south west] at (entity.north west) {Entity}; + + \foreach \x in {1,2,3}{ + \node[ + rectangle, draw = black, thick, fill = white, + minimum width = 3.5cm, minimum height = .75cm, + ] (arch\x) at ($(entity.north) + (0, -1 * \x)$) {Architecture \x}; + } + + \foreach \x in {1,...,4}{ + \draw[thick, <-] + ($(entity.north west) - (0, .75 * \x)$) + node (pinl\x) {} to ++(-.75, 0); + \draw[thick, ->] + ($(entity.north east) - (0, .75 * \x)$) + to ++(.75, 0) node (pinr\x) {} ; + } + + \node[right] at (pinr1) {Pin}; + \end{tikzpicture} +\end{center} + +Entities are declared with \vhdl{port()} that may contain a list of pins. Pins +have a mode that can be \vhdl{in} input (only LHS), \vhdl{out} output (only +RHS), \vhdl{inout} bidirectional or \vhdl{buffer} that can stay both on LHS and +RHS. The usage of the latter is discourareged in favour of an internal signal. \begin{lstlisting}[language=vhdl] entity `\reqph{name}` is port( @@ -14,6 +72,8 @@ entity `\reqph{name}` is end `\reqph{name}`; \end{lstlisting} +Architectures are normally named after the design model, example are +\texttt{behavioral}, \texttt{structural}, \texttt{selective}, etc. \begin{lstlisting}[language=vhdl] architecture `\reqph{name}` of `\reqph{entity}` is -- declare used variables, signals and component types @@ -22,21 +82,245 @@ begin end `\optionalph{name}`; \end{lstlisting} -\subsection{Declaration} +\subsection{Electric types and Libraries} +VHDL provides some types such as +\begin{itemize} + \item \vhdl{boolean} true or false, + \item \vhdl{bit} 0 or 1, + \item \vhdl{bit_vector} one dimensional array of bits, + \item \vhdl{integer} 32-bit binary representation of a value. +\end{itemize} +From external libraries other types are available: +\begin{itemize} + \item \vhdl{std_logic} advanced logic with 9 states, + \item \vhdl{std_ulogic} +\end{itemize} +The above are from the \vhdl{ieee.std_logic_1164} library, and can take the +values described in the following table. +\begin{center} + \begin{tabularx}{\linewidth}{>{\ttfamily}c l X} + \toprule + Value & Meaning & Usage \\ + \midrule + U & Uninitialized & In the simulator \\ + X & Undefined & Simulator sees a bus conflict \\ + 0 & Force to 0 & Low state of outputs \\ + 1 & Force to 1 & High state of outputs \\ + Z & High Impedance & Three state ports \\ + W & Weak Unknown & Simulator sees weak a bus conflict \\ + L & Weak Low & Open source outputs with pull-down resistor \\ + H & Weak High & Open drain output with pull-up resistor \\ + - & Don't care & Allow minimization \\ + \bottomrule + \end{tabularx} +\end{center} +%% TODO: copy conflict resolutiontable + +\subsection{Declarations} \label{sec:declarations} +Before a \vhdl{begin} -- \vhdl{end} block, there is usually a list of declarations. +A self evident examples are \emph{constants}. +\begin{lstlisting}[language=vhdl] +constant `\reqph{name}` : `\reqph{type}` := `\reqph{value}`; +\end{lstlisting} + +Next, \emph{signals} and \emph{variables}. Signals is are wires, they can only be +connected and do not have an initial state. Variables can be assigned like in +software, but can cause the synthesization of an unwanted D-Latch. + \begin{lstlisting}[language=vhdl] signal `\reqph{name}`, `\optionalph{name, \ldots}` : `\reqph{type}`; -variable `\reqph{name}`, `\optionalph{name, \ldots}` : `\reqph{type}`; +variable `\reqph{name}`, `\optionalph{name}`, `\optionalph{\ldots}` : `\reqph{type}`; +variable `\reqph{name}` : `\reqph{type}` := `\reqph{expression}`; +\end{lstlisting} + +For the hierarchical designs, when external entities are used, they must be +declared as components. The \vhdl{port()} expression must match the entity +declaration. +\begin{lstlisting}[language=vhdl] component `\reqph{entity name}` is port( - `\ph{}` + `\optionalph{list of pins}` ); end component; \end{lstlisting} +It is possible to create custom types, usually to create state machines. +\begin{lstlisting}[language=vhdl] +type `\reqph{name}` is (`\reqph{identifier}`, `\reqph{identifier}`, `\ph{\ldots}`); +\end{lstlisting} + \subsection{Concurrent Area} +\begin{center} + \ttfamily + \begin{tikzpicture}[ + node distance = 1mm, + pin/.style = { + draw = black, fill = black, circle, + inner sep = 0pt, minimum size = 2mm, + }, + component/.style = { + draw = black, thick, fill = white, rectangle, + minimum width = 18mm, minimum height = 12mm, + align = center, + }, + ] + + \node[ + draw = black, rectangle, fill = gray!20!white, thick, + minimum width = .75\linewidth, minimum height = 4cm, + ] (arch) {}; + + \node[anchor = south west] at (arch.north west) {Architecture}; + + \node[pin] (clk) at ($(arch.north west) - (0,1)$) {}; + \node[pin] (a) at ($(clk) - (0,1)$) {}; + \node[pin] (b) at ($(a) - (0,1)$) {}; + \node[pin] (y) at ($(arch.north east) - (0,1)$) {}; + + \node[left = of clk] {clk}; + \node[left = of a] {a}; + \node[left = of b] {b}; + \node[right = of y] {y}; + + \node[component] (c1) at ($(clk) + (2,-.2)$) {Process}; + \node[component] (c2) at ($(c1) + (.2,-1.8)$) {Component\\ Entity}; + \node[ + component, minimum width = 0mm, minimum height = 0mm, + ] (c3) at ($(c1) + (2.4,-.2)$) {Logic\\ Gate}; + + \draw[thick] + (clk) to[out = 0, in = 180] ($(c1.west) + (0,.2)$) + (a) to[out = 0, in = 180] ($(c1.west) - (0,.2)$) + (b) to[out = 0, in = 180] (c2.west) + + (c1.east) to[out = 0, in = 180] ($(c3.west) + (0,.2)$) + (c2.east) to[out = 0, in = 180] ($(c3.west) - (0,.2)$) + (c3.east) to[out = 0, in = 180] (y) + ; + + \end{tikzpicture} +\end{center} +In the architecture between \vhdl{begin} and \vhdl{end}, the expressions +are \emph{not} read sequentially, everything happens at the same time. +Statements inside the concurrent area optionally have a label. \begin{lstlisting}[language=vhdl] -`\optionalph{label}`: `\reqph{signal}` <= `\ph{Expression}` +`\optionalph{label}`: `\reqph{concurrent statement}`; +\end{lstlisting} +In the concurrent area signals, components and processes can be used to create +a logic. + +\subsubsection{Signal assignment and simple gates} +Signals are assigned using \vhdl{<=}. +\begin{lstlisting}[language=vhdl] +`\optionalph{label}`: `\reqph{signal}` <= `\reqph{expression}`; +\end{lstlisting} +Simple logic functions such as \vhdl{not}, \vhdl{and}, \vhdl{or}, \vhdl{xor}, +etc. can be used. +\begin{lstlisting}[language=vhdl] + y <= (a and s) or (b and not(s)); +\end{lstlisting} + +\subsubsection{Aggregates} +For vector types it is possible to create a value out of multiple signals. +\begin{lstlisting}[language=vhdl] +`\reqph{vector}` <= ( + `\reqph{index}` => `\reqph{source or value}`, + `\reqph{index}` => `\reqph{source or value}`, + `\optionalph{\tt others}` => `\reqph{source or value}` +); +\end{lstlisting} +\begin{lstlisting}[language=vhdl] +-- declaration +signal data : bit_vector(6 downto 0); +signal a, b : bit; +-- concurrent +data = (1 => a, 0 => b, others => '0') +\end{lstlisting} + +\subsubsection{Seiective and conditional assignment} +Higher level conditions can be written in two ways. +\begin{lstlisting}[language=vhdl] +-- using when +`\optionalph{label}:` y <= `\reqph{source}` when `\reqph{condition}` else + `\reqph{source}` when `\reqph{condition}` else + `\reqph{source}` when `\reqph{condition}`; + +-- using with +`\optionalph{label}`: with `\reqph{signal}` select `\reqph{dest}` <= + `\reqph{source}` when `\reqph{value}`, + `\reqph{source}` when `\reqph{value}`, + `\reqph{source}` when others; +\end{lstlisting} + +\subsubsection{Processes} +For more sophisticated logic, VHDL offers a way of writing sequential +statements called \emph{processes}. +\begin{lstlisting}[language=vhdl] +`\optionalph{label}:` process (`\optionalph{sensitivity list}`) +-- declarations +begin + -- sequential statements +end process; +\end{lstlisting} +Processes have a \emph{sensitivity list} that could also be empty. When a +signal in the sensitivity list changes state, the process is executed. In the +case of an empty sensitivity list, the process runs continuously. In the +declaration, everything from \S\ref{sec:declarations} applies. For the +sequential statements, the following applies: +\begin{itemize} + \item Neither selective (\vhdl{with}) nor conditional (\vhdl{when}) should be used, + as there because there are new sequential constructs (\vhdl{if} and \vhdl{case}). + \item Signal assignments (with \vhdl{<=}) change their value + \emph{only at the end of the process}. + \item Variables on the other hand change as soon as they are assigned (with \vhdl{:=}). +\end{itemize} +And for good practice: +\begin{itemize} + \item Before any \vhdl{if} or \vhdl{case} default values should be assigned. + \item Any signal on the RHS should be in the sensitivity list. + \item Processes with empty sensitivity lists should only be used for simulations. +\end{itemize} + +The sequential replacements for \vhdl{with} and \vhdl{when} are in the listings below. +\begin{lstlisting}[language=vhdl] +if `\reqph{condition}` then + -- sequential statements +elsif `\reqph{condition}` then + -- sequential statements +else + -- sequential statements +end if; +\end{lstlisting} +\begin{lstlisting}[language=vhdl] +case `\reqph{expression}` is + when `\reqph{choice}` => + -- sequential statements + when `\reqph{choice}` => + -- sequential statements + when others => + -- sequential statements +end case; +\end{lstlisting} + +Processes can detect \emph{events} of signals. Typically this is used for +clocks. +\begin{lstlisting}[language=vhdl] +process (clk) +begin + -- rising edge + if clk'event and clk = '1' then + ... end if; + if rising_edge(clk) then + ... end if; + + -- falling edge + if clk'event and clk = '0' then + ... end if; + if falling_edge(clk) then + ... end if; +end process; \end{lstlisting} +% vim:ts=2 sw=2 et: |