SystemVerilog Synthesis for Custom Target Library (using Cadence Genus Synthesis)

This walkthough illustrates the synthesis of a SystemVerilog behavioural model. The result is a Verilog sturctural model (gate level netlist) using gates from a standard library.

The Cadence Genus Synthesis program is used to convert a behavioural SystemVerilog description into structural Verilog.

Preparation

• Change to a new directory in which the synthesis will be run:

    cadteaching7<~>$  mkdir -p ~/design/cadence/genus/test
    cadteaching7<~>$  cd ~/design/cadence/genus/test
  

• Create directories for behavioural and gate level HDL files and copy the first behavioural file to be converted into the behavioural directory:

    cadteaching7<~/design/cadence/genus/test>$ mkdir behavioural gate_level
    cadteaching7<~/design/cadence/genus/test>$ cp <elsewhere>/<module>.sv behavioural/<module>.sv
  

Interactive Synthesis

• Invoke the graphical user interface to Cadence Genus Synthesis:

    cadteaching7<~/design/cadence/genus/test>$  genus_custom
  

  (if a Verilog design is to be synthesized use "genus_custom -v")

• Read in and elaborate the behavioural SystemVerilog file:

    genus@root:>  set_db init_hdl_search_path behavioural
    genus@root:>  read_hdl <module>.sv
    genus@root:>  elaborate <module>
    genus@root:>  check_design -unresolved
    genus@root:>  report sequential -hier
  

  (if a Verilog design is to be synthesized "read_hdl <module>.sv" will need to be replaced by "read_hdl <module>.v")

  After each individual command you should check for errors and warnings.

  The read_hdl operation checks the SystemVerilog file for syntax errors (although a SystemVerilog file which has been simulated with NC-Verilog should not should not give parsing errors you may get warnings or errors relating to unsynthesizable constructs).

  In the elaborate stage, further checking is done and further errors and warnings are flagged. In particular you may see a message such as "Warning : Referenced signal not in sensitivity list. This may cause simulation mismatches between the original and synthesized designs". This reminds you that sometimes a synthesis will complete but the resulting design is useless since it doesn't match the behavioural model. This situation can only be avoided by reading the warnings produced at each stage of the process.

  The check_design operation will check for unresolved references such as may occur if there is a module missing from your design. Note that you should specify multiple files for the read_hdl operation if your design contains hierarchy:

    genus@root:>  read_hdl <topmodule>.sv <submodule1>.sv <submodule2>.sv
    genus@root:>  elaborate <topmodule>
  

  If your design references a package, then the package file should be read in first:

    genus@root:>  read_hdl <package>.svh <topmodule>.sv <submodule1>.sv <submodule2>.sv
    genus@root:>  elaborate <topmodule>
  

  The report sequential operation will list all the sequential elements in the design. With a clocked circuit, most of these should be of type "flip-flop" with "asynchronous reset" other types may cause problems. In particular, "inferred latch" type elements are usually caused by design errors in your behavioural model. Possibly a latch has been inferred within a block which was intended to represent combinational logic. This will happen where the guidelines on using Procedural Blocks as Combinational Logic have been breached. See Hardware Modelling documentation for more detail.

  If necessary, correct the behavioural SystemVerilog file and then repeat the stages, starting with read_hdl. Repeat this process until no errors remain.

• Graphical User Interface

  Once the elaboration is complete, the graphical user interface window becomes active, allowing you to see the schematics that are generated. To view the schematic for a particular module, right-click over the module mane in the "Hierarchy" tab and select:

      Open-in -> Schematic Viewer (main)...
  

  After each synthesis stage, the schematics are updated to represent the changes made.

  Although advanced debugging is possible using this tool, its use will not be covered in this walkthrough.

• Synthesis

  1. Generic synthesis:

       genus@root:>  syn_generic
     

     Here synthesis is performed against a generic library; no attempt is made to map the design to available real cells.

     Failure at this stage is usually due to unsynthesizable SystemVerilog.

     You may need to consult the HDL modelling guidelines in the cadence documentation for more details.

  2. Set suitably lax timing constraints and map to gates from custom target library:

       genus@root:>  source default_time.tcl
       genus@root:>  syn_map
     

     Again, a failure here is likely to indicate problems with your behavioural model.

  3. Flatten and resynthesize:

       genus@root:>  ungroup -flatten -all
       genus@root:>  syn_opt
     

     Note that you could alternatively flatten the hierarchy before one of the earlier synthesis steps. Flattening the hierarchy at this late stage gives rise to an extra synthesis stage but can give better results.

• Statistics

  Following any synthesize -to_mapped stage, you can get a Quality of Results report:

    genus@root:>  report qor
  

  Information provided includes the number of gates (Leaf Instance Count) and the number of flip-flops (Sequential Instance Count). There is also an indication of the total cell area. Although the cell area information can be used to compare different designs, the actual value given is of limited use since it has not been customized for your own cell library.

• Write out the structural Verilog file:

    genus@root:>  write_hdl > gate_level/<module>.v
  

  here a .v suffix is used to indicate a Verilog file rather than a SystemVerilog file. Since the Verilog constructs used in this file should all be valid SystemVerilog constructs, a .sv suffix could be used instead.

• Exit Genus:

    genus@root:>  exit
  

Batch Synthesis

• If you are reasonably confident of performing the synthesis without errors, a much faster way to perform the synthesis is to use the -batch option to the genus_custom command:

    cadteaching7<~/design/cadence/genus/test>$  genus_custom -batch behavioural/<module>.sv gate_level/<module>.v
  

  or

    cadteaching7<~/design/cadence/genus/test>$  genus_custom -v -batch behavioural/<module>.v gate_level/<module>.v
  

• A log file, rc.log, stores the output of this command. You should use an editor to open this file and look for any errors/warnings etc. generated during synthesis.

Completion

• Always use simulation to confirm the success of the synthesis operation

  You will need a SystemVerilog library for the target cells; The default library is copied to the working directory by the genus_custom command. It is named

    fcdeCells.sv
  

  Assuming that you have a stimulus file <module>_stim.sv, simulation of the behavioural model might be performed using this command:

    cadteaching7<~/design/cadence/genus/test>$  xmverilog  <module>_stim.sv  behavioural/<module>.sv
  

  To perform the same simulation on the synthesized design you would need this modified command:

    cadteaching7<~/design/cadence/genus/test>$  xmverilog  -v fcdeCells.sv  +xmtimescale+1ns/10ps  <module>_stim.sv  gate_level/<module>.v
  

  Here the "-v fcdeCells.sv" option tells the simulator to look in the fcdeCells.sv library file for models of the basic gates, while the "+xmtimescale+1ns/10ps" option makes up for the fact that the synthesized gate_level/<module>.v Verilog file does not include the required "timeunit 1ns; timeprecision 10ps;" information that is found in the SystemVerilog files.

All Cadence documentation is available on-line via 'cdnshelp'. Unfortunately each Cadence install has a separate 'cdnshelp' installation. To get around this problem you can use the following command to invoke the appropriate 'cdnshelp' command for a particular cadence executable:

cdnshelp_for <cadence_executable>

For this tutorial use:

cdnshelp_for genus

On-line manuals of particular interest to this tutorial are:

GENUS...
Genus
Genus HDL Modelling Guide
Synthesizing Verilog Designs
Genus GUI Guide
Genus Command Reference

While the Genus HDL Modelling Guide gives comprehensive coverage of the issues, there is an older version which is probably easier to follow:

HDL Modelling in Encounter RTL Compiler

In particular the Modelling HDL Designs chapter gives details of suggested modelling styles.