kernel: TimingInfo to clamp -ve setup/edge-sensitive delays to zero

[yosys.git] / manual / CHAPTER_Overview.tex

diff --git a/manual/CHAPTER_Overview.tex b/manual/CHAPTER_Overview.tex
index 2feb0f1cbef8c0f17eb51d82eb2cde11a469579a..be37d8d39c5490a694db8d6a8c19b26978d6f1f3 100644 (file)
--- a/manual/CHAPTER_Overview.tex
+++ b/manual/CHAPTER_Overview.tex
@@ -234,6 +234,8 @@ An RTLIL::Wire object has the following properties:
 \item The wire name
 \item A list of attributes
 \item A width (buses are just wires with a width > 1)
+\item Bus direction (MSB to LSB or vice versa)
+\item Lowest valid bit index (LSB or MSB depending on bus direction)
 \item If the wire is a port: port number and direction (input/output/inout)
 \end{itemize}
 
@@ -246,6 +248,11 @@ This makes some aspects of RTLIL more complex but enables Yosys to be used for
 coarse grain synthesis where the cells of the target architecture operate on
 entire signal vectors instead of single bit wires.
 
+In Verilog and VHDL, busses may have arbitrary bounds, and LSB can have either
+the lowest or the highest bit index. In RTLIL, bit 0 always corresponds to LSB;
+however, information from the HDL frontend is preserved so that the bus will be
+correctly indexed in error messages, backend output, constraint files, etc.
+
 An RTLIL::Cell object has the following properties:
 
 \begin{itemize}
@@ -331,8 +338,9 @@ to update {\tt \textbackslash{}q}.
 An RTLIL::Process is a container for zero or more RTLIL::SyncRule objects and
 exactly one RTLIL::CaseRule object, which is called the {\it root case}.
 
-An RTLIL::SyncRule object contains an (optional) synchronization condition
-(signal and edge-type) and zero or more assignments (RTLIL::SigSig).
+An RTLIL::SyncRule object contains an (optional) synchronization condition (signal and edge-type) and zero or
+more assignments (RTLIL::SigSig). The {\tt always} synchronization condition is used to break combinatorial
+loops when a latch should be inferred instead.
 
 An RTLIL::CaseRule is a container for zero or more assignments (RTLIL::SigSig)
 and zero or more RTLIL::SwitchRule objects. An RTLIL::SwitchRule objects is a
@@ -350,6 +358,18 @@ and this bit is a one (the second ``1'').} for {\tt \textbackslash{}reset == 1}
 sets {\tt \$0\textbackslash{}q[0:0]} to the value of {\tt \textbackslash{}d} if {\tt
 \textbackslash{}enable} is active (lines $6 \dots 11$).
 
+A case can specify zero or more compare values that will determine whether it matches. Each of the compare values
+must be the exact same width as the control signal. When more than one compare value is specified, the case matches
+if any of them matches the control signal; when zero compare values are specified, the case always matches (i.e.
+it is the default case).
+
+A switch prioritizes cases from first to last: multiple cases can match, but only the first matched case becomes
+active. This normally synthesizes to a priority encoder. The {\tt parallel\_case} attribute allows passes to assume
+that no more than one case will match, and {\tt full\_case} attribute allows passes to assume that exactly one
+case will match; if these invariants are ever dynamically violated, the behavior is undefined. These attributes
+are useful when an invariant invisible to the synthesizer causes the control signal to never take certain
+bit patterns.
+
 The lines $13 \dots 16$  then cause {\tt \textbackslash{}q} to be updated whenever there is
 a positive clock edge on {\tt \textbackslash{}clock} or {\tt \textbackslash{}reset}.