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parametrized netlist generation at scale

miwe

Dear Cadence Forum,

for an evaluation of different implementation variants of a design I am working on, I would like to use a hierarchical pPar parameter to select which cell I want to use for a simulation. As a simplified example, I'd like to use parameter pPar("driving_strength") to select an instance of a gate from a given set (i.e. standard cell library) of gates with differently sized output stages.

I would like to do analog transient simulations with that; up to now, I've been using spectre for this purpose.

What I have tried so far:

1. I have created a cellview with all possible cells being connected in parallel through "ideal switches" at the input and output. As ideal switches, I used "switch" from spectre (Cadende name "sp1tswitch", "sp2tswitch", ... from "analogLib") and configured the switch positions in the following manner:

(pPar(...) == expected_value) ? 1 : 0

This works, the results are as expected, but the spectre performance does not seem to scale well. For example, in a simple case with two parameters having four possible values each, simulating all sixteen possible combinations takes less than five minutes -- but when parameter one has a set of ten possible values and parameter two has a set of 20 possible values, even the simulation for one single point takes around 30 minutes.

2. The first approach left the unused branches disconnected, i.e. floating. In a second approach, I connected all unused inputs as well as outputs to a defined voltage (i.e. VDD or GND depending on use case), but did not notice a significant performance improvement.

3. I had a brief look at VerilogA and VerilogAMS to see whether I can use some "switch/case" or "if/else" for conditional instantitation of instances.

3a. As far as I am aware, VerilogA does not support conditional generation of instances -- or am I on the wrong track?

3b. For VerilogAMS, I generated the following code:

`include "constants.vams"
`include "disciplines.vams"

module VarDrivingStrengthGate ( in, out );
  input in;
  output out;
parameter real driving_strength = 0 ;

generate
  if (driving_strength == 1)
    STDCELL_GATE1 instanceGate(in, out);
  if (driving_strength == 2)
    STDCELL_GATE2 instanceGate(in, out);
endgenerate

endmodule

The module seems to be parsed correctly, I see that for example in the hierarchy editor that maps STDCELL_GATE1 and STDCELL_GATE2 to the correct cellviews. However, I could not get the simulation running. I tried:

3b.1 create "config" cellview, open with Hierarchy Editor, add Stop and View List (basically copy from spectre settings, add 'verilogams' to View List), select testbench schematic.

3b.2 create "adexl" cellview, create test, select "config" cellview to be simulated, select "spectreVerilog" as simulator.

3b.3 netlist creation aborts with an error, the logfile contains the following info:

\o spectreVerilog import netlist enabled
\o design has changed ... import netlist file deleted
\o partition...
\o *USRERR: During design partitioning, the partitioner failed to identify
\o "Model VarDrivingStrengthGate, lib MyLibrary, view verilogams" as analog, digital or mixed-signal.
\o This happens because view schematic is not included in either digital or analog stop-lists.
\o Verify the following lists in the stated order:
\o switch-list
\o stop-view list
\o analog stop-view list
\o digital stop-view list.
\e *Error* Failed to partition the design.
\e
\o       ...unsuccessful.
\o ERROR (ADE-3010): Cannot create and partition the design.

Questions

  • Am I on the right track using VerilogAMS "if/else" or "switch/case" statements to create pPar-dependent netlists?
  • What alternatives are there to create pPar-dependent netlists that scale well with the number of possible values?
  • If VerilogAMS is the way to go, can anyone give me a hint about what could have gone wrong in 3b?

I've spent quite some time on this issue and appreciate your help very much!

Best regards,
Michael Weiner

Cadence IC6.1.5-64b.500.132

Parents
  • Andrew Beckett

    Michael,

    This is not really a supported approach so it doesn't surprise me that you've hit numerous obstacles along the way. A few comments:

    1. sp1tswitch etc is not really intended for this. It's expected to be used as an analysis-dependent switch - and even that should be used with caution. In your case since you have all of your circuits present, the simulator is having to solve the entire circuit - and if you have some of the circuits that are essentially floating, that is probably going to lead to convergence difficulties (as well as having a bigger matrix to solve anyway).
    2. Doing view switching with parameters is an idea that we moved away from when we moved from 4.3.4 to 4.4 (in 1995/1996) when the hierarchy editor was introduced. The idea was that configuration of the design should be separate from the schematic hierarchy, because that way you can configure which views are used without needing to modify the schematics. 
    3. Spectre supports conditional inclusion of instances, but there's no netlister support for this. It would need spectre if statements in the netlist around the instances you want to include.
    4. Verilog A/AMS etc do not support conditional inclusion of instances
    5. An approach that you could take is to use the Add Config Sweep that was added in IC616 (ISR11, I think) - I'll outline that below

    For the Add Config Sweep, the idea is that you can add a new global variable in ADE XL which allows a view to be swept - effectively switching to use different views for a block in the simulation. Here's how you do it:

    First use the context (Right Mouse Button) menu over the global variables to add a new config sweep:

    Then a form will appear:

    List the views you want to sweep over (you can change this later)

    The Data View will now look like this:

    It's treated like another sweep variable - you can reference the variable in the corners UI too once you've created it here. You can also click on the variable value section and click on the "..." button when that appears to change the views (or just type in the list).

    This is much cleaner than trying to do it with pPar - I can't quite see why you'd want to do it that way anyway?

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Michael,

    This is not really a supported approach so it doesn't surprise me that you've hit numerous obstacles along the way. A few comments:

    1. sp1tswitch etc is not really intended for this. It's expected to be used as an analysis-dependent switch - and even that should be used with caution. In your case since you have all of your circuits present, the simulator is having to solve the entire circuit - and if you have some of the circuits that are essentially floating, that is probably going to lead to convergence difficulties (as well as having a bigger matrix to solve anyway).
    2. Doing view switching with parameters is an idea that we moved away from when we moved from 4.3.4 to 4.4 (in 1995/1996) when the hierarchy editor was introduced. The idea was that configuration of the design should be separate from the schematic hierarchy, because that way you can configure which views are used without needing to modify the schematics. 
    3. Spectre supports conditional inclusion of instances, but there's no netlister support for this. It would need spectre if statements in the netlist around the instances you want to include.
    4. Verilog A/AMS etc do not support conditional inclusion of instances
    5. An approach that you could take is to use the Add Config Sweep that was added in IC616 (ISR11, I think) - I'll outline that below

    For the Add Config Sweep, the idea is that you can add a new global variable in ADE XL which allows a view to be swept - effectively switching to use different views for a block in the simulation. Here's how you do it:

    First use the context (Right Mouse Button) menu over the global variables to add a new config sweep:

    Then a form will appear:

    List the views you want to sweep over (you can change this later)

    The Data View will now look like this:

    It's treated like another sweep variable - you can reference the variable in the corners UI too once you've created it here. You can also click on the variable value section and click on the "..." button when that appears to change the views (or just type in the list).

    This is much cleaner than trying to do it with pPar - I can't quite see why you'd want to do it that way anyway?

    Regards,

    Andrew.

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