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

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)
}
module Example(
  output [7:0] out_a,
  output       out_b
);
  assign out_a = 8'h8; // @[experimental-features.md 22:7]
  assign out_b = 1'h1; // @[experimental-features.md 22:7]
endmodule

Partial specification is allowed, defaulting any unconnected fields to 0 (regardless of type).

class Example2 extends RawModule {
  val out = IO(Output(new MyBundle))
  out := (new MyBundle).Lit(_.b -> true.B)
}
module Example2(
  output [7:0] out_a,
  output       out_b
);
  assign out_a = 8'h0;
  assign out_b = 1'h1; // @[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))
}
module Example3(
  output [7:0] out_a,
  output [7:0] out_b_foo
);
  assign out_a = 8'h7b; // @[experimental-features.md 63:7]
  assign out_b_foo = 8'h2a; // @[experimental-features.md 63:7]
endmodule

Vec literals are not yet supported.

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
Condition A.clip(B) A.wrap(B) A.squeeze(B)
A === B max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi)
A contains B max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi)
B contains A max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi)
A min < B min, A max in B max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi)
A min in B, A max > B max max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi) max(Alo, Blo), min(Ahi, Bhi)
A strictly less than B error error error
A strictly greater than B error error error

Applying binary point operators to an Interval

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

operation after operation binary point lower upper meaning
setBinaryPoint(2) aaa.bb 2 X X set the precision
shiftLeftBinaryPoint(2) a.aabbb 5 X X increase the precision
shiftRighBinaryPoint(2) aaaa.b 1 X X reduce the precision