Re: [Iverilog-devel] possible regression in 0.10 when simulating Lattice FF

[Martin W. <ic...@ma...>]

Galen Seitz wrote:
> I was hesitant to post a link to Lattice copyrighted code, as I wasn't sure 
> whether that was acceptable on the list.  That github project link contains 
> the entire simulation library for the XP2 family, so I've decided not to 
> worry about posting a link to a few files from the MachXO3 family.
> 
> <https://www.edaplayground.com/x/3EKT>
> 
> After running the simulation, from the EPWave window, click "Get Signals", 
> then select the ".ff" instance.  Click the "Append All" button.  These are 
> the ports of the .ff instance of the FD1P3DX module, which are the signals 
> I've been looking at.
> 
> If accessing EDA playground is a problem, let me know, and I will arrange 
> other access to the files.
> 
> My test design that demonstrates the problem consists of a testbench and 
> four files from the Lattice library.  GSR.v is a global reset, and should 
> be unused.  PUR.v is a power up reset which generates a 1 ns reset pulse 
> starting at time 0.  FD1P3DX.v is some sort of wrapper around the 
> UDFDL5E_UDP_X.v primitive.  When I grep the Lattice library I find that 
> there are six modules that instance this particular UDP primitive.
> 
> My testbench generates a reset signal that is 2 clocks long.  The D input 
> is tied to 0, and the enable is tied to 1.  Notifier is an internal signal 
> of FD1P3DX and is X, but that doesn't seem to impact the simulation.
> 
> With iverilog 0.10.0, the Q output is X at time 0 and remains X for the 
> entire simulation.  With any other simulator on EDA playground, Q is 0 at 
> time 0, and remains 0 for the entire simulation.  This includes iverilog 
> 0.9.7 and 0.9.6.

If you look inside the FD1P3DX module, the QB signal output from the UDP is 
correctly set to 0. The problem appears to be a race between the execution 
of the "initial Q=1'b0;" inside the UDP and the execution of the "always @ 
QB" inside the FD1P3DX module. If the initial statement is executed first, 
the "@ QB" will never trigger.