Experimental Features

Chisel has a number of new features that are worth checking out. This page is an informal list of these features and projects.

• FixedPoint
• Module Variants
• Bundle Literals
• Vec Literals
• Interval Type
• Loading Memories for simulation or FPGA initialization

FixedPoint

FixedPoint numbers are basic Data type along side of UInt, SInt, etc. Most common math and logic operations are supported. Chisel allows both the width and binary point to be inferred by the Firrtl compiler which can simplify circuit descriptions. See FixedPointSpec

Module Variants

The standard Chisel Module requires a val io = IO(...), the experimental package introduces several new ways of defining Modules

• BaseModule: no contents, instantiable
• BlackBox extends BaseModule
• UserDefinedModule extends BaseModule: this module can contain Chisel RTL. No default clock or reset lines. No default IO. - User should be able to specify non-io ports, ideally multiple of them.
• ImplicitModule extends UserModule: has clock, reset, and io, essentially current Chisel Module.
• RawModule: will be the user-facing version of UserDefinedModule
• Module: type-aliases to ImplicitModule, the user-facing version of ImplicitModule.

Bundle Literals

Bundle literals can be constructed via an experimental import:

import chisel3._
import chisel3.experimental.BundleLiterals._

class MyBundle extends Bundle {
  val a = UInt(8.W)
  val b = Bool()
}

class Example extends RawModule {
  val out = IO(Output(new MyBundle))
  out := (new MyBundle).Lit(_.a -> 8.U, _.b -> true.B)
}

// Generated by CIRCT firtool-1.40.0
module Example(	// <stdin>:3:10
  output [7:0] out_a,	// experimental-features.md:21:15
  output       out_b	// experimental-features.md:21:15
);

  assign out_a = 8'h8;	// <stdin>:3:10, experimental-features.md:22:7
  assign out_b = 1'h1;	// <stdin>:3:10, experimental-features.md:22:7
endmodule

Partial specification is allowed, which results in “invalidated fields” as described in Unconnected Wires. Note that this can be used with RegInit to construct partially reset registers as described in the Cookbook.

class Example2 extends RawModule {
  val out = IO(Output(new MyBundle))
  out := (new MyBundle).Lit(_.b -> true.B)
}

// Generated by CIRCT firtool-1.40.0
module Example2(	// <stdin>:3:10
  output [7:0] out_a,	// experimental-features.md:36:15
  output       out_b	// experimental-features.md:36:15
);

  assign out_a = 8'h0;	// <stdin>:3:10, experimental-features.md:37:7
  assign out_b = 1'h1;	// <stdin>:3:10, experimental-features.md:37:7
endmodule

Bundle literals can also be nested arbitrarily.

class ChildBundle extends Bundle {
  val foo = UInt(8.W)
}

class ParentBundle extends Bundle {
  val a = UInt(8.W)
  val b = new ChildBundle
}

class Example3 extends RawModule {
  val out = IO(Output(new ParentBundle))
  out := (new ParentBundle).Lit(_.a -> 123.U, _.b -> (new ChildBundle).Lit(_.foo -> 42.U))
}

// Generated by CIRCT firtool-1.40.0
module Example3(	// <stdin>:3:10
  output [7:0] out_a,	// experimental-features.md:62:15
               out_b_foo	// experimental-features.md:62:15
);

  assign out_a = 8'h7B;	// <stdin>:3:10, experimental-features.md:63:7
  assign out_b_foo = 8'h2A;	// <stdin>:3:10, experimental-features.md:63:7
endmodule

Vec Literals

Vec literals are very similar to Bundle literals and can be constructed via an experimental import. They can be constructed in two forms, with type and length inferred as in:

import chisel3._
import chisel3.experimental.VecLiterals._

class VecExample1 extends Module {
  val out = IO(Output(Vec(2, UInt(4.W))))
  out := Vec.Lit(0xa.U, 0xbb.U)
}

// Generated by CIRCT firtool-1.40.0
module VecExample1(	// <stdin>:3:10
  input        clock,	// <stdin>:4:11
               reset,	// <stdin>:5:11
  output [3:0] out_0,	// experimental-features.md:83:15
               out_1	// experimental-features.md:83:15
);

  assign out_0 = 4'hA;	// <stdin>:3:10, experimental-features.md:84:7
  assign out_1 = 4'hB;	// <stdin>:3:10, experimental-features.md:84:7
endmodule

or explicitly as in:

import chisel3._
import chisel3.experimental.VecLiterals._

class VecExample1a extends Module {
  val out = IO(Output(Vec(2, UInt(4.W))))
  out := Vec(2, UInt(4.W)).Lit(0 -> 1.U, 1 -> 2.U)
}

// Generated by CIRCT firtool-1.40.0
module VecExample1a(	// <stdin>:3:10
  input        clock,	// <stdin>:4:11
               reset,	// <stdin>:5:11
  output [3:0] out_0,	// experimental-features.md:104:15
               out_1	// experimental-features.md:104:15
);

  assign out_0 = 4'h1;	// <stdin>:3:10, experimental-features.md:105:7
  assign out_1 = 4'h2;	// <stdin>:3:10, experimental-features.md:105:7
endmodule

The following examples all use the explicit form. With the explicit form partial specification is allowed, which results in “invalidated fields” as described in Unconnected Wires. Note that this can be used with RegInit to construct partially reset registers as described in the Cookbook.

class VecExample2 extends RawModule {
  val out = IO(Output(Vec(4, UInt(4.W))))
  out := Vec(4, UInt(4.W)).Lit(0 -> 1.U, 3 -> 7.U)
}

// Generated by CIRCT firtool-1.40.0
module VecExample2(	// <stdin>:3:10
  output [3:0] out_0,	// experimental-features.md:119:15
               out_1,	// experimental-features.md:119:15
               out_2,	// experimental-features.md:119:15
               out_3	// experimental-features.md:119:15
);

  assign out_0 = 4'h1;	// <stdin>:3:10, experimental-features.md:120:7
  assign out_1 = 4'h0;	// <stdin>:3:10, experimental-features.md:120:7
  assign out_2 = 4'h0;	// <stdin>:3:10, experimental-features.md:120:7
  assign out_3 = 4'h7;	// <stdin>:3:10, experimental-features.md:120:7
endmodule

Registers can be initialized from Vec literals

class VecExample3 extends Module {
  val out = IO(Output(Vec(4, UInt(8.W))))
  val y = RegInit(
    Vec(4, UInt(8.W)).Lit(0 -> 0xAB.U(8.W), 1 -> 0xCD.U(8.W), 2 -> 0xEF.U(8.W), 3 -> 0xFF.U(8.W))
  )
  out := y
}

// Generated by CIRCT firtool-1.40.0
module VecExample3(	// <stdin>:3:10
  input        clock,	// <stdin>:4:11
               reset,	// <stdin>:5:11
  output [7:0] out_0,	// experimental-features.md:134:15
               out_1,	// experimental-features.md:134:15
               out_2,	// experimental-features.md:134:15
               out_3	// experimental-features.md:134:15
);

  assign out_0 = 8'hAB;	// <stdin>:3:10, :9:19
  assign out_1 = 8'hCD;	// <stdin>:3:10, :10:19
  assign out_2 = 8'hEF;	// <stdin>:3:10, :11:19
  assign out_3 = 8'hFF;	// <stdin>:3:10, :12:19
endmodule

Vec literals can also be nested arbitrarily.

class VecExample5 extends RawModule {
  val out = IO(Output(Vec(2, new ChildBundle)))
  out := Vec(2, new ChildBundle).Lit(
    0 -> (new ChildBundle).Lit(_.foo -> 42.U),
    1 -> (new ChildBundle).Lit(_.foo -> 7.U)
  )
}

// Generated by CIRCT firtool-1.40.0
module VecExample5(	// <stdin>:3:10
  output [7:0] out_0_foo,	// experimental-features.md:152:15
               out_1_foo	// experimental-features.md:152:15
);

  assign out_0_foo = 8'h2A;	// <stdin>:3:10, experimental-features.md:153:7
  assign out_1_foo = 8'h7;	// <stdin>:3:10, experimental-features.md:153:7
endmodule

Interval Type

Intervals are a new experimental numeric type that comprises UInt, SInt and FixedPoint numbers. It augments these types with range information, i.e. upper and lower numeric bounds. This information can be used to exercise tighter programmatic control over the ultimate widths of signals in the final circuit. The Firrtl compiler can infer this range information based on operations and earlier values in the circuit. Intervals support all the ordinary bit and arithmetic operations associated with UInt, SInt, and FixedPoint and adds the following methods for manipulating the range of a source Interval with the IntervalRange of target Interval

Clip – Fit the value source into the IntervalRange of target, saturate if out of bounds

The clip method applied to an interval creates a new interval based on the argument to clip, and constructs the necessary hardware so that the source Interval’s value will be mapped into the new Interval. Values that are outside the result range will be pegged to either maximum or minimum of result range as appropriate.

Generates necessary hardware to clip values, values greater than range are set to range.high, values lower than range are set to range min.

Wrap – Fit the value source into the IntervalRange of target, wrapping around if out of bounds

The wrap method applied to an interval creates a new interval based on the argument to wrap, and constructs the necessary hardware so that the source Interval’s value will be mapped into the new Interval. Values that are outside the result range will be wrapped until they fall within the result range.

Generates necessary hardware to wrap values, values greater than range are set to range.high, values lower than range are set to range min.

Does not handle out of range values that are less than half the minimum or greater than twice maximum

Squeeze – Fit the value source into the smallest IntervalRange based on source and target.

The squeeze method applied to an interval creates a new interval based on the argument to clip, the two ranges must overlap behavior of squeeze with inputs outside of the produced range is undefined.

Generates no hardware, strictly a sizing operation

Range combinations

Applying binary point operators to an Interval

Consider a Interval with a binary point of 3: aaa.bbb

Loading Memories for simulation or FPGA initialization

Chisel supports multiple experimental methods for annotating memories to be loaded from a text file containing hex or binary data. When using verilog simulation it uses the $readmemh or $readmemb verilog extension.

Inline initialization with external file

Memories can be initialized by generating inline readmemh or readmemb statements in the output Verilog.

The function loadMemoryFromFileInline from chisel3.util.experimental allows the memory to be initialized by the synthesis software from the specified file. Chisel does not validate the file contents nor its location. Both the memory initialization file and the Verilog source should be accessible for the toolchain.

import chisel3._
import chisel3.util.experimental.loadMemoryFromFileInline

class InitMemInline(memoryFile: String = "") extends Module {
  val width: Int = 32
  val io = IO(new Bundle {
    val enable = Input(Bool())
    val write = Input(Bool())
    val addr = Input(UInt(10.W))
    val dataIn = Input(UInt(width.W))
    val dataOut = Output(UInt(width.W))
  })

  val mem = SyncReadMem(1024, UInt(width.W))
  // Initialize memory
  if (memoryFile.trim().nonEmpty) {
    loadMemoryFromFileInline(mem, memoryFile)
  }
  io.dataOut := DontCare
  when(io.enable) {
    val rdwrPort = mem(io.addr)
    when (io.write) { rdwrPort := io.dataIn }
      .otherwise    { io.dataOut := rdwrPort }
  }
}

The default is to use $readmemh (which assumes all numbers in the file are in ascii hex), but to use ascii binary there is an optional hexOrBinary argument which can be set to MemoryLoadFileType.Hex or MemoryLoadFileType.Binary. You will need to add an additional import.

SystemVerilog Bind Initialization

Chisel can also initialize memories by generating a SV bind module with readmemh or readmemb statements by using the function loadMemoryFromFile from chisel3.util.experimental.

import chisel3._
import chisel3.util.experimental.loadMemoryFromFile

class InitMemBind(val bits: Int, val size: Int, filename: String) extends Module {
  val io = IO(new Bundle {
    val nia = Input(UInt(bits.W))
    val insn = Output(UInt(32.W))
  })

  val memory = Mem(size, UInt(32.W))
  io.insn := memory(io.nia >> 2);
  loadMemoryFromFile(memory, filename)
}

Which generates the bind module:

module BindsTo_0_Foo(
  input         clock,
  input         reset,
  input  [31:0] io_nia,
  output [31:0] io_insn
);

initial begin
  $readmemh("test.hex", Foo.memory);
end
endmodule

bind Foo BindsTo_0_Foo BindsTo_0_Foo_Inst(.*);

Notes on files

There is no simple answer to where to put the hex or bin file with the initial contents. It’s probably best to create a resource directory somewhere and reference that through a full path or place the file beside the generated Verilog. Another option is adding the path to the memory file in the synthesis tool path. Because these files may be large, Chisel does not copy them.

Don’t forget there is no decimal option, so a 10 in an input file will be 16 decimal

See: ComplexMemoryLoadingSpec.scala and LoadMemoryFromFileSpec.scala for working examples.

Aggregate memories

Aggregate memories are supported and will be lowered to a single wide UInt. The memory loading file should be constructed to align with the final structure of the memory.