title: “lowRISC Hardware Primitives”

Concepts

This directory contains basic building blocks to create a hardware design, called primitives. A primitive is described by its name, and has a well-defined list of ports and parameters.

Under the hood, primitives are slightly special, as they can have multiple implementations. In contrast to many other modules in a hardware design, primitives must often be implemented in technology-dependent ways. For example, a clock multiplexer for a Xilinx FPGA is implemented differently than one for a specific ASIC technology.

Not all primitives need to have multiple implementations.

• Primitives with a single, generic, implementation are normal SystemVerilog modules inside the hw/ip/prim/rtl directory. We call these primitives “technology-independent primitives”.
• Primitives with multiple implementations have only a FuseSoC core file in the hw/ip/prim directory. The actual implementations are in “technology libraries”. We call these primitives “technology-dependent primitives”.

Abstract primitives

Abstract primitives are wrappers around technology-dependent implementations of primitives, with the ability to select a specific implementation if needed.

In more technical terms, abstract primitives are SystemVerilog modules. The example below shows one.

`ifndef PRIM_DEFAULT_IMPL
  `define PRIM_DEFAULT_IMPL prim_pkg::ImplGeneric
`endif

module prim_pad_wrapper
#(
  parameter int unsigned AttrDw = 6
) (
  inout wire         inout_io, // bidirectional pad
  output logic       in_o,     // input data
  input              out_i,    // output data
  input              oe_i,     // output enable
  // additional attributes {drive strength, keeper, pull-up, pull-down, open-drain, invert}
  input [AttrDw-1:0] attr_i
);
  parameter prim_pkg::impl_e Impl = `PRIM_DEFAULT_IMPL;

  if (Impl == prim_pkg::ImplGeneric) begin : gen_generic
    prim_generic_pad_wrapper u_impl_generic (
      .*
    );
  end else if (Impl == prim_pkg::ImplXilinx) begin : gen_xilinx
    prim_xilinx_pad_wrapper u_impl_xilinx (
      .*
    );
  end else begin : gen_failure
    // TODO: Find code that works across tools and causes a compile failure
  end

endmodule

As seen from the source code snippet, abstract primitives have the following properties:

• They have an Impl parameter which can be set to choose a specific implementation of the primitive.
• The Impl parameter is set to a system-wide default determined by the PRIM_DEFAULT_IMPL define.
• All ports and parameters of the abstract primitive are forwarded to the implementations.

Technology libraries

Technology libraries collect implementations of primitives.

At least one technology library must exist: the generic technology library, which contains a pure-SystemVerilog implementation of the functionality. This library is commonly used for simulations and as functional reference. The generic technology library is contained in the hw/ip/prim_generic directory.

In addition to the implementation in the generic library, primitives may be implemented by as many other libraries as needed.

Technology libraries are referenced by their name.

Technology library discovery

In many cases, technology libraries contain vendor-specific code which cannot be shared widely or openly. Therefore, a FuseSoC looks for available technology libraries at build time, and makes all libraries it finds available.

The discovery is performed based on the agreed-on naming scheme for primitives.

• FuseSoC scans all libraries (e.g. as specified by its --cores-root command line argument) for cores.
• All cores with a name matching lowrisc:prim_TECHLIBNAME:PRIMNAME are considered. TECHLIBNAME is then added to the list of technology libraries.

After the discovery process has completed, a script (primgen) creates

• an abstract primitive (see above), and
• an entry in the prim_pkg package in the form of prim_pkg::ImplTechlibname to identify the technology library by its name.

User Guide

Use primitives

Primitives are normal SystemVerilog modules, and can be used as usual:

• instantiate it like a normal SystemVerilog module, and
• add a dependency in the FuseSoC core file.

Technology-dependent primitives have an additional parameter called Impl. Set this parameter to use a specific implementation of the primitive for this specific instance. For example:

prim_ram_2p #(
  .Width (TotalWidth),
  .Depth (Depth),
  // Force the use of the tsmc40lp technology library for this instance, instead
  // of using the build-time default.
  .Impl(prim_pkg::ImplTsmc40lp)
) u_mem (
  .clk_a_i    (clk_i),
  ...
)

Set the default technology library

If no specific technology library is chosen for an instantiated primitive the default library is used. The SystemVerilog define PRIM_DEFAULT_IMPL can be used to set the default for the whole design. Set this define to one of the enum values in prim_pkg.sv in the form prim_pkg::ImplTechlibname. Techlibname is the capitalized name of the technology library.

In the top-level FuseSoC core file the default technology library can be chosen like this:

# my_toplevel.core

# Declare filesets and other things (omitted)

parameters:
  # Make the parameter known to FuseSoC to enable overrides from the
  # command line. If not overwritten, use the generic technology library.
  PRIM_DEFAULT_IMPL:
    datatype: str
    paramtype: vlogdefine
    description: Primitives implementation to use, e.g. "prim_pkg::ImplGeneric".
    default: prim_pkg::ImplGeneric

targets:
  fpga_synthesis:
    filesets:
      - my_rtl_files
    parameters:
      # Use the xilinx technology library for this target by default.
      - PRIM_DEFAULT_IMPL=prim_pkg::ImplXilinx
    toplevel: my_toplevel

Create a technology library

To create a technology library follow these steps:

• Choose a name for the new technology library. Names are all lower-case. To ease sharing of technology libraries it is encouraged to pick a very specific name, e.g. tsmc40lp, and not asic.
• Copy the prim_generic folder into an arbitrary location (can be outside of this repository). Name the folder prim_YOURLIBRARYNAME.
• Replace the word generic everywhere with the name of your technology library. This includes
  • file and directory names (e.g. prim_generic_ram1p.sv becomes prim_tsmc40lp_ram1p.sv),
  • module names (e.g. prim_generic_ram1p becomes prim_tsmc40lp_ram1p), and
  • all other references (grep for it!).
• Implement all primitives. Replace the module body of the generic implementation with a technology-specific implementation as needed. Do not modify the list of ports or parameters in any way!

Implementation details

Technology-dependent primitives are implemented as a FuseSoC generator. The core of the primitive (e.g. lowrisc:prim:rom in prim/prim_rom.core) calls a FuseSoC generator. This generator is the script util/primgen.py. As input, the script receives a list of all cores found by FuseSoC anywhere in its search path. The script then looks through the cores FuseSoC discovered and extracts a list of technology libraries out of it. It then goes on to create the abstract primitive (copying over the list of parameters and ports from the generic implementation), and an associated core file, which depends on all technology-dependent libraries that were found.