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README.md

VHD2VL v3.0

Vhd2vl is designed to translate synthesizable VHDL into Verilog 1995 or 2001. It does not support the full VHDL grammar - most of the testbench related features have been left out. See the examples and translated_examples directories for examples of what vhd2vl can do.

Vhd2vl does a pretty good job of translating, but you should ALWAYS test the generated Verilog, ideally by using a formal verification tool to compare it to the original VHDL!

A similar but more sophisticated effort is embedded in Icarus Verilog as vhdlpp, mostly by Maciej Suminski. If hands-free use of VHDL in a (Icarus) Verilog environment is the goal, that's probably a better tool. If you want to convert a bit of VHDL to Verilog, and will then maintain that Verilog as source, vhd2vl probably makes more sense, if for no other reason than it conserves comments. It's good that both options exist! You may find that your VHDL style is better accepted by one tool or the other.

The home page for (at least this version of) vhd2vl is http://doolittle.icarus.com/~larry/vhd2vl/

1.0 HOW TO BUILD AND INSTALL vhd2vl:

To build, just type make in the src directory.

This version of vhd2vl has been tested with

  • Debian 7 (Wheezy): gcc-4.7.2, bison-2.5, flex-2.5.35, glibc-2.13
  • Debian 8 (Jessie): gcc-4.9.2 or clang-3.5.0, bison-3.0.2, flex-2.5.39, glibc-2.19
  • Debian 9 (Stretch): gcc-6.3.0 or clang-3.8.1, bison-3.0.4, flex-2.6.1, glibc-2.24

It is also verified to work with recent tinycc from its git mob. This is portable C89/C99 code. It can be expected to work with any fairly recent version of the required tools.

To install, copy the resulting src/vhd2vl file to someplace in your $PATH, like $HOME/bin or /usr/local/bin.

2.0 HOW TO USE vhd2vl:

vhd2vl VHDL_file.vhd > translated_file.v

or

vhd2vl VHDL_file.vhd translated_file.v

The two are equivalent when everything works. The latter has some advantages when handling errors within a Makefile.

There are a few options available on the command line:

  • --debug turn ON debugging within the yacc (bison) parser
  • --src 1995|2001 to specify module declaration style
  • --quiet to avoid print vhd2vl header in translated_file.v

3.0 TROUBLESHOOTING:

If vhd2vl complains about a syntax error, this is usually due to a VHDL construct that vhd2vl cannot translate. Try commenting out the offending line, and running vhd2vl again. You can then edit the Verilog output file and manually translate the offending line of VHDL.

Comments in the middle of statements sometimes confuse vhd2vl. This is a "feature" of the logic that copies comments from VHDL to Verilog. If vhd2vl complains about a syntax error caused by a comment, just move that comment out of the middle of the statement and try again.

The grammar has rules that recognize common ways of writing clocked processes. Your code might contain clocked processes that do not match any of the templates in the grammar. This usually causes vhd2vl to complain about a clock'event expression in a process. If this happens, a minor rewrite of that process will let you work around the problem.

To test a copy of vhd2vl for regressions against the example code shipped, run make from this directory using a Bourne-style shell. If you have GHDL and/or iverilog installed, the example VHDL and Verilog code will be compiled -- and therefore syntax-checked -- with those tools.

4.0 VHDL PACKAGES

Vhd2vl does not understand VHDL package files. You might be able to work around that limitation with the following strategy:

  • Either by hand, or with a stupid script (I wrote mine in awk), break the package file into individual VHDL files, each named after the entity.
  • Use vhd2vl to convert each of those to Verilog. Test to make sure the conversions went OK.
  • Use iverilog's -y switch (or the eqivalent in your tool) to "find" those files as needed.

5.0 MISSING FEATURES AND KNOWN INCORRECT OUTPUT:

String types: awkward, because Verilog strings need predefined length.

Attribute: easy to parse, but I'm not sure what Verilog construct to turn it into. It smells like a parameter, not an (* attribute *).

Multiple actions in one process, as used in DDR logic?

Exit statement incompletely converted to disable statement.

Detection of indexed part select is limited. While it can correctly convert data(index*8+WIDTH-1 downto index*8) to data[index*8+WIDTH-1 -: WIDTH-1+1] it gets tripped on slightly more complex cases. The rule is that the larger expression must take the form smaller + offset or offset + smaller. Otherwise the output will be a direct transcription of the VHDL, which is not standard-conforming unless both ends of the range are constant.

Conversion functions (resize, to_unsigned, conv_integer) are parsed, but their semantics are ignored: resize(foo,n), to_unsigned(foo,n), and conv_integer(foo) are treated as equivalent to foo.

VHDL is case insensitive, vhd2vl is case retentive, and Verilog is case sensitive. If you're sloppy with case in the original VHDL, the resulting Verilog will have compile-time warnings or errors. See the comments about vhd2vl-2.1 in the changelog file.

Doesn't handle functions, procedures, or packages. See above for a possible way to handle packages.

Doesn't necessarily get clock edge sensitivities right if there is more than one clock in the list.

Totally broken handling of text in generic mappings, as Xilinx is wont to use for their primitives and wrappers.

Broken (invalid Verilog syntax for) initialization of process-scope variables.