I think NIOS and SOPC builder are great products. However, there are a couple things I find annoying regarding the generated Verilog code when it comes to compatibility with third party synthesis tools and simulators. They are:
- Support of specific synthesis tool only. The generated code have statements like:
//exemplar translate_off
Rather than:
//synthesis translate_off
- Mix of design and testbench code in the same file
The generated top level NIOS file contains the following structure:
module nios32 ...etc... endmodule
//exemplar translate_off
`include "/eda/sim_lib/220model.v" `include "cpu.v" ...
module test_bench ; ... endmodule //exemplar translate_on
I would prefer if the testbench was generated in a separate file (testbench.v or similar). This would ease simulation of the NIOS in my own environment. In this case I would simply not include testbench.v.
Sanity checks and assertions included in //synthesis translate_off/on pairs are of course ok.
- Use of `ifdef MODEL_TECH
Not a big issue I as I can do a +define+MODEL_TECH (even though it took me a while to figure this out) in my favorite simulator. However, I think more descriptive preprocessor variable names would be better, e.g.
//synthesis translate_off `ifdef NIOS_SIM_GEN_OPCODE assign opcode = ((((~instruction[15] && ~instruction[14] ... //synthesis translate_on
In some cases the simulation wont work if you don't define MODEL_TECH, e.g. like in:
`ifdef MODEL_TECH //////////////// SIMULATION-ONLY CONTENTS altsyncram the_altsyncram ( ...
I don't know the reason behind this structure since the synthesis tool wont touch this code. The `ifdef MODEL_TECH could have been left out.
- Use of include. The generated code will look like:
`include "/eda/sim_lib/altera_mf.v" `include "/eda/sim_lib/220model.v" `include "cpu.v"
The first two will cause problems for third party tools since they have no clue where to look. Further the path is absolute so I can't fix this in DC-FPGA by setting the search_path variable.
There is no need for include statements in this case. The filenames can be included on the command line or file input list in a synthesis tool or simulator.
It appears that my synthesis tool is processing the include statements even inside // synthesis translate_off (I guess the could be a synthesis translate_on in the include file), which results in an error when it hits the absolute path.
Are these issues present in NIOS-II?
Petter