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nmigen-examples

I want to learn [n]Migen.

Verilog is awful, as you know. There are a few newer HDLs to explore. myHDL, Migen and nMigen, and SpinalHDL/Chisel.

Since I already know Python, it seems like nMigen is a good place to start. nMigen (new Migen) is a reimplementation of Migen, and I guess Migen is on its way to deprecation. OTOH, nMigen is far from mature.

I want to understand whether and how nMigen makes it possible to create abstractions that Verilog can't.

Extremely basic nMigen

nMigen is a Python library that implements a hardware description language (HDL). You describe hardware using nMigen's Python extensions, then nMigen can simulate your design, emit your design as Verilog, or synthesize it for an FPGA and optionally load it onto an FPGA dev board.

More complex arrangements are possible -- nMigen is a Python library, so it can be embedded into an arbitrary runtime. Check out the Glasgow project for an example that uses nMigen to automatically generate and load FPGA functions on the fly.

nMigen program structure

An nMigen design is organized as a tree of modules, just like Verilog. Each module has:

  • one or more clock domains
  • the comb pseudo-domain for combinatorial logic
  • zero or more submodules.

Modules are defined in classes derived from Elaboratable. Each Elaboratable has a method called elaborate, which constructs and initializes a Module. (I don't understand why they're called "elaboratable". The word does not connote anything useful to me.)

Within the elaborate method, you create the Module, attach any clock domains and submodules, and then add logic statements to the domains, including the combinatoric pseudo-domain, comb. By convention, the default clock domain is called sync. AFAIK there is nothing magical about sync -- you could use any identifier.

Each domain has a clock signal, an optional reset signal, and a set of logic statements. The logic statements are implicitly bound to the clock edge (rising edge by default); Instead of littering your code with Verilog's always @(posedge clk) ..., you litter your code with m.d.sync += .... nMigen automatically generates code to initialize signals on reset, and automatically gives signals a default reset value of zero.

So that's a higher level than Verilog. Clock and reset are implicit, and clock domains are explicit.

TBD: The Instantiate class.

nMigen's HDL

TBD: Start with the Value hierarchy, then explain operators, assignment, and control flow.

Explain that you need to understand Python operator overloading and context managers.

nMigen Simulation

Building in minimal simulation is trivial. Just drop this at the end of a source file.

from nmigen.cli import main
if __name__ == '__main__':
    x = MyModule()
    main(x, ports=x.ports)

Then you simulate that module like this.

$ nmigen mymodule.py simulate -v mymodule.vcd -w mymodule.gtkw -c 1000

You can also do more complex stuff building in a test bench that sets up specific conditions and looks for specific outputs. There is also a unit testing framework that I don't know much about. I think it lets you run a bunch of simulation runs on the same module.

nMigen also has Assert, Assume, and Cover keywords. They don't appear to be fully implemented yet.

nMigen and platforms

TBD.

Repository Organization

There are two main subdirectories. lib contains reusable modules. They do not have any platform dependencies. apps contains top-level modules that connect modules from lib together and bind them to the pins of a specific package.

All of the apps here target the iCEBreaker platform with various PMOD cards attached.

How to Use

For reusable modules in the lib directory, you can simulate a module or generate Verilog from it. See the README in that directory.

The apps directory has a subdirectory for each app. Each app creates a complete FPGA bitstream for one platform and downloads (uploads?) it. (All my apps are for the iCEBreaker FPGA at present.) There's only one way to use it -- run ./build.sh. See apps/README.md.

What's this nmigen command in the READMEs and build scripts?

That's a shell script I have in my path. It invokes python in the virtualenv where nmigen is installed. My implementation is complicated — you don't want to know. It's more or less equivalent to this, though.

#!/bin/sh
export VIRTUAL_ENV=/path/to/nmigen/virtualenv
export PATH="$VIRTUAL_ENV/bin:$PATH"
unset PYTHONHOME
python ${1+"$@"}

I recommend putting a script like that in your path.

Misc.

Enabling iCE40 DSP inference

By default, yosys does not infer DSP slices for multiplication. (i.e., it does not use DSPs to implement multiply ops.)

You can enable DSP inference by setting the NMIGEN_synth_opts environment variable to -dsp.

$ NMIGEN_synth_opts=-dsp nmigen my-app.py

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