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Connectable Operators

Table of Contents​

Terminology​

  • "Chisel type" - a Data that is not bound to hardware, i.e. not a component. (more details here).
    • E.g. UInt(3.W), new Bundle {..}, Vec(3, SInt(2.W)) are all Chisel types
  • "component" - a Data that is bound to hardware (IO, Reg, Wire, etc.)
    • E.g. Wire(UInt(3.W)) is a component, whose Chisel type is UInt(3.W)
  • Aggregate - a Chisel type or component that contains other Chisel types or components (i.e. Vec, Record, or Bundle)
  • Element - a Chisel type or component that does not contain other Chisel types or components (e.g. UInt, SInt, Clock, Bool etc.)
  • "member" - a Chisel type or component, or any of its children (could be an Aggregate or an Element)
    • E.g. Vec(3, UInt(2.W))(0) is a member of the parent Vec Chisel type
    • E.g. Wire(Vec(3, UInt(2.W)))(0) is a member of the parent Wire component
    • E.g. IO(Decoupled(Bool)).ready is a member of the parent IO component
  • "relative alignment" - whether two members of the same component or Chisel type are aligned/flipped, relative to one another
    • see section below for a detailed definition
  • "structural type check" - Chisel type A is structurally equivalent to Chisel type B if A and B have matching bundle field names and types (Record vs Vector vs Element), probe modifiers (probe vs nonprobe), vector sizes, Element types (UInt/SInt/Bool/Clock)
    • ignores relative alignment (flippedness)
  • "alignment type check" - a Chisel type A matches alignment with another Chisel type B if every member of A's relative alignment to A is the same as the structurally corresponding member of B's relative alignment to B.

Overview​

The Connectable operators are the standard way to connect Chisel hardware components to one another.

Note: For descriptions of the semantics for the previous operators, see Connection Operators.

All connection operators require the two hardware components (consumer and producer) to be structurally type equivalent.

The one exception to the structural type-equivalence rule is using the Connectable mechanism, detailed at this section towards the end of this document.

Aggregate (Record, Vec, Bundle) Chisel types can include data members which are flipped relative to one another. Due to this, there are many desired connection behaviors between two Chisel components. The following are the Chisel connection operators between a consumer c and producer p:

  • c := p (mono-direction): connects all p members to c; requires c and p to not have any flipped members
  • c :#= p (coercing mono-direction): connects all p members to c; regardless of alignment
  • c :<= p (aligned-direction): connects all aligned (non-flipped) c members from p
  • c :>= p (flipped-direction): connects all flipped p members from c
  • c :<>= p (bi-direction operator): connects all aligned c members from p; all flipped p members from c

These operators may appear to be a random collection of symbols; however, the characters are consistent between operators and self-describe the semantics of each operator:

  • : always indicates the consumer, or left-hand-side, of the operator.
  • = always indicates the producer, or right-hand-side, of the operator.
    • Hence, c := p connects a consumer (c) and a producer (p).
  • < always indicates that some members will be driven producer-to-consumer, or right-to-left.
    • Hence, c :<= p drives members in producer (p) to members in consumer (c).
  • > always indicates that some signals will be driven consumer-to-producer, or left-to-right.
    • Hence, c :>= p drives members in consumer (c) to members producer (p).
    • Hence, c :<>= p both drives members from p to c and from c to p.
  • # always indicates to ignore member alignment and to drive producer-to-consumer, or right-to-left.
    • Hence, c :#= p always drives members from p to c ignoring direction.

Note: in addition, an operator that ends in = has assignment-precedence, which means that x :<>= y + z will translate to x :<>= (y + z), rather than (x :<>= y) + z. This was not true of the <> operator and was a minor painpoint for users.

Alignment: Flipped vs Aligned​

A member's alignment is a relative property: a member is aligned/flipped relative to another member of the same component or Chisel type. Hence, one must always say whether a member is flipped/aligned with respect to (w.r.t) another member of that type (parent, sibling, child etc.).

We use the following example of a non-nested bundle Parent to let us state all of the alignment relationships between members of p.

import chisel3._
class Parent extends Bundle {
  val alignedChild = UInt(32.W)
  val flippedChild = Flipped(UInt(32.W))
}
class MyModule0 extends Module {
  val p = Wire(new Parent)
}

First, every member is always aligned with themselves:

  • p is aligned w.r.t p
  • p.alignedChild is aligned w.r.t p.alignedChild
  • p.flippedChild is aligned w.r.t p.flippedChild

Next, we list all parent/child relationships. Because the flippedChild field is Flipped, it changes its alignment relative to its parent.

  • p is aligned w.r.t p.alignedChild
  • p is flipped w.r.t p.flippedChild

Finally, we can list all sibling relationships:

  • p.alignedChild is flipped w.r.t p.flippedChild

The next example has a nested bundle GrandParent who instantiates an aligned Parent field and flipped Parent field.

import chisel3._
class GrandParent extends Bundle {
  val alignedParent = new Parent
  val flippedParent = Flipped(new Parent)
}
class MyModule1 extends Module {
  val g = Wire(new GrandParent)
}

Consider the following alignments between grandparent and grandchildren. An odd number of flips indicate a flipped relationship; even numbers of flips indicate an aligned relationship.

  • g is aligned w.r.t g.flippedParent.flippedChild
  • g is aligned w.r.t g.alignedParent.alignedChild
  • g is flipped w.r.t g.flippedParent.alignedChild
  • g is flipped w.r.t g.alignedParent.flippedChild

Consider the following alignment relationships starting from g.alignedParent and g.flippedParent. Note that whether g.alignedParent is aligned/flipped relative to g has no effect on the aligned/flipped relationship between g.alignedParent and g.alignedParent.alignedChild because alignment is only relative to the two members in question!:

  • g.alignedParent is aligned w.r.t. g.alignedParent.alignedChild
  • g.flippedParent is aligned w.r.t. g.flippedParent.alignedChild
  • g.alignedParent is flipped w.r.t. g.alignedParent.flippedChild
  • g.flippedParent is flipped w.r.t. g.flippedParent.flippedChild

In summary, a member is aligned or flipped w.r.t. another member of the hardware component. This means that the type of the consumer/producer is the only information needed to determine the behavior of any operator. Whether the consumer/producer is a member of a larger bundle is irrelevant; you ONLY need to know the type of the consumer/producer.

Input/Output​

Input(gen)/Output(gen) are coercing operators. They perform two functions: (1) create a new Chisel type that has all flips removed from all recursive children members (still structurally equivalent to gen but no longer alignment type equivalent), and (2) apply Flipped if Input, keep aligned (do nothing) if Output. E.g. if we imagine a function called cloneChiselTypeButStripAllFlips, then Input(gen) is structurally and alignment type equivalent to Flipped(cloneChiselTypeButStripAllFlips(gen)).

Note that if gen is a non-aggregate, then Input(nonAggregateGen) is equivalent to Flipped(nonAggregateGen).

Future work will refactor how these primitives are exposed to the user to make Chisel's type system more intuitive. See [https://github.com/chipsalliance/chisel3/issues/2643].

With this in mind, we can consider the following examples and detail relative alignments of members.

First, we can use a similar example to Parent but use Input/Output instead of Flipped. Because alignedChild and flippedChild are non-aggregates, Input is basically just a Flipped and thus the alignments are unchanged compared to the previous Parent example.

import chisel3._
class ParentWithOutputInput extends Bundle {
  val alignedCoerced = Output(UInt(32.W)) // Equivalent to just UInt(32.W)
  val flippedCoerced = Input(UInt(32.W))  // Equivalent to Flipped(UInt(32.W))
}
class MyModule2 extends Module {
  val p = Wire(new ParentWithOutputInput)
}

The alignments are the same as the previous Parent example:

  • p is aligned w.r.t p
  • p.alignedCoerced is aligned w.r.t p.alignedCoerced
  • p.flippedCoerced is aligned w.r.t p.flippedCoerced
  • p is aligned w.r.t p.alignedCoerced
  • p is flipped w.r.t p.flippedCoerced
  • p.alignedCoerced is flipped w.r.t p.flippedCoerced

The next example has a nested bundle GrandParent who instantiates an Output ParentWithOutputInput field and an Input ParentWithOutputInput field.

import chisel3._
class GrandParentWithOutputInput extends Bundle {
  val alignedCoerced = Output(new ParentWithOutputInput)
  val flippedCoerced = Input(new ParentWithOutputInput)
}
class MyModule3 extends Module {
  val g = Wire(new GrandParentWithOutputInput)
}

Remember that Output(gen)/Input(gen) recursively strips the Flipped of any recursive children. This makes every member of gen aligned with every other member of gen.

Consider the following alignments between grandparent and grandchildren. Because alignedCoerced and flippedCoerced are aligned with all their recursive members, they are fully aligned. Thus, only their alignment to g influences grandchildren alignment:

  • g is aligned w.r.t g.alignedCoerced.alignedCoerced
  • g is aligned w.r.t g.alignedCoerced.flippedCoerced
  • g is flipped w.r.t g.flippedCoerced.alignedCoerced
  • g is flipped w.r.t g.flippedCoerced.flippedCoerced

Consider the following alignment relationships starting from g.alignedCoerced and g.flippedCoerced. Note that whether g.alignedCoerced is aligned/flipped relative to g has no effect on the aligned/flipped relationship between g.alignedCoerced and g.alignedCoerced.alignedCoerced or g.alignedCoerced.flippedCoerced because alignment is only relative to the two members in question! However, because alignment is coerced, everything is aligned between g.alignedCoerced/g.flippedAligned and their children:

  • g.alignedCoerced is aligned w.r.t. g.alignedCoerced.alignedCoerced
  • g.alignedCoerced is aligned w.r.t. g.alignedCoerced.flippedCoerced
  • g.flippedCoerced is aligned w.r.t. g.flippedCoerced.alignedCoerced
  • g.flippedCoerced is aligned w.r.t. g.flippedCoerced.flippedCoerced

In summary, Input(gen) and Output(gen) recursively coerce children alignment, as well as dictate gen's alignment to its parent bundle (if it exists).

Connecting components with fully aligned members​

Mono-direction connection operator (:=)​

For simple connections where all members are aligned (non-flipped) w.r.t. one another, use :=:

import chisel3._
class FullyAlignedBundle extends Bundle {
  val a = Bool()
  val b = Bool()
}
class Example0 extends RawModule {
  val incoming = IO(Flipped(new FullyAlignedBundle))
  val outgoing = IO(new FullyAlignedBundle)
  outgoing := incoming
}

This generates the following Verilog, where each member of incoming drives every member of outgoing:

// Generated by CIRCT firtool-1.94.0
module Example0(
  input  incoming_a,
         incoming_b,
  output outgoing_a,
         outgoing_b
);

  assign outgoing_a = incoming_a;
  assign outgoing_b = incoming_b;
endmodule

You may be thinking "Wait, I'm confused! Isn't incoming flipped and outgoing aligned?" -- Noo! Whether incoming is aligned with outgoing makes no sense; remember, you only evaluate alignment between members of the same component or Chisel type. Because components are always aligned to themselves, outgoing is aligned to outgoing, and incoming is aligned to incoming, there is no problem. Their relative flippedness to anything else is irrelevant.

Connecting components with mixed alignment members​

Aggregate Chisel types can include data members which are flipped relative to one another; in the example below, alignedChild and flippedChild are aligned/flipped relative to MixedAlignmentBundle.

import chisel3._
class MixedAlignmentBundle extends Bundle {
  val alignedChild = Bool()
  val flippedChild = Flipped(Bool())
}

Due to this, there are many desired connection behaviors between two Chisel components. First we will introduce the most common Chisel connection operator, :<>=, useful for connecting components with members of mixed-alignments, then take a moment to investigate a common source of confusion between port-direction and connection-direction. Then, we will explore the remainder of the Chisel connection operators.

Bi-direction connection operator (:<>=)​

For connections where you want 'bulk-connect-like-semantics' where the aligned members are driven producer-to-consumer and flipped members are driven consumer-to-producer, use :<>=.

class Example1 extends RawModule {
  val incoming = IO(Flipped(new MixedAlignmentBundle))
  val outgoing = IO(new MixedAlignmentBundle)
  outgoing :<>= incoming
}

This generates the following Verilog, where the aligned members are driven incoming to outgoing and flipped members are driven outgoing to incoming:

// Generated by CIRCT firtool-1.94.0
module Example1(
  input  incoming_alignedChild,
  output incoming_flippedChild,
         outgoing_alignedChild,
  input  outgoing_flippedChild
);

  assign incoming_flippedChild = outgoing_flippedChild;
  assign outgoing_alignedChild = incoming_alignedChild;
endmodule

Port-Direction Computation versus Connection-Direction Computation​

A common question is if you use a mixed-alignment connection (such as :<>=) to connect submembers of parent components, does the alignment of the submember to their parent affect anything? The answer is no, because alignment is always computed relative to what is being connected to, and members are always aligned with themselves.

In the following example connecting from incoming.alignedChild to outgoing.alignedChild, whether incoming.alignedChild is aligned with incoming is irrelevant because the :<>= only computes alignment relative to the thing being connected to, and incoming.alignedChild is aligned with incoming.alignedChild.

class Example1a extends RawModule {
  val incoming = IO(Flipped(new MixedAlignmentBundle))
  val outgoing = IO(new MixedAlignmentBundle)
  outgoing.alignedChild :<>= incoming.alignedChild // whether incoming.alignedChild is aligned/flipped to incoming is IRRELEVANT to what gets connected with :<>=
}

While incoming.flippedChild's alignment with incoming does not affect our operators, it does influence whether incoming.flippedChild is an output or input port of my module. A common source of confusion is to mistake the process for determining whether incoming.flippedChild will resolve to a Verilog output/input (the port-direction computation) with the process for determining how :<>= drives what with what (the connection-direction computation). While both processes consider relative alignment, they are distinct.

The port-direction computation always computes alignment relative to the component marked with IO. An IO(Flipped(gen)) is an incoming port, and any member of gen that is aligned/flipped with gen is an incoming/outgoing port. An IO(gen) is an outgoing port, and any member of gen that is aligned/flipped with gen is an outgoing/incoming port.

The connection-direction computation always computes alignment based on the explicit consumer/producer referenced for the connection. If one connects incoming :<>= outgoing, alignments are computed based on incoming and outgoing. If one connects incoming.alignedChild :<>= outgoing.alignedChild, then alignments are computed based on incoming.alignedChild and outgoing.alignedChild (and the alignment of incoming to incoming.alignedChild is irrelevant).

This means that users can try to connect to input ports of their module! If I write x :<>= y, and x is an input to the current module, then that is what the connection is trying to do. However, because input ports are not drivable from within the current module, Chisel will throw an error. This is the same error a user would get using a mono-directioned operator: x := y will throw the same error if x is an input to the current module. Whether a component is drivable is irrelevant to the semantics of any connection operator attempting to drive to it.

In summary, the port-direction computation is relative to the root marked IO, but connection-direction computation is relative to the consumer/producer that the connection is doing. This has the positive property that connection semantics are solely based on the Chisel structural type and its relative alignments of the consumer/producer (nothing more, nothing less).

Aligned connection operator (:<=)​

For connections where you want the aligned-half of 'bulk-connect-like-semantics' where the aligned members are driven producer-to-consumer and flipped members are ignored, use :<= (the "aligned connection").

class Example2 extends RawModule {
  val incoming = IO(Flipped(new MixedAlignmentBundle))
  val outgoing = IO(new MixedAlignmentBundle)
  incoming.flippedChild := DontCare // Otherwise FIRRTL throws an uninitialization error
  outgoing :<= incoming
}

This generates the following Verilog, where the aligned members are driven incoming to outgoing and flipped members are ignored:

// Generated by CIRCT firtool-1.94.0
module Example2(
  input  incoming_alignedChild,
  output incoming_flippedChild,
         outgoing_alignedChild,
  input  outgoing_flippedChild
);

  assign incoming_flippedChild = 1'h0;
  assign outgoing_alignedChild = incoming_alignedChild;
endmodule

Flipped connection operator (:>=)​

For connections where you want the flipped-half of 'bulk-connect-like-semantics' where the aligned members are ignored and flipped members are connected consumer-to-producer, use :>= (the "flipped connection", or "backpressure connection").

class Example3 extends RawModule {
  val incoming = IO(Flipped(new MixedAlignmentBundle))
  val outgoing = IO(new MixedAlignmentBundle)
  outgoing.alignedChild := DontCare // Otherwise FIRRTL throws an uninitialization error
  outgoing :>= incoming
}

This generates the following Verilog, where the aligned members are ignored and the flipped members are driven outgoing to incoming:

// Generated by CIRCT firtool-1.94.0
module Example3(
  input  incoming_alignedChild,
  output incoming_flippedChild,
         outgoing_alignedChild,
  input  outgoing_flippedChild
);

  assign incoming_flippedChild = outgoing_flippedChild;
  assign outgoing_alignedChild = 1'h0;
endmodule

Note: Astute observers will realize that c :<>= p is semantically equivalent to c :<= p followed by c :>= p.

Coercing mono-direction connection operator (:#=)​

For connections where you want to every producer member to always drive every consumer member, regardless of alignment, use :#= (the "coercion connection"). This operator is useful for initializing wires whose types contain members of mixed alignment.

import chisel3.experimental.BundleLiterals._
class Example4 extends RawModule {
  val w = Wire(new MixedAlignmentBundle)
  dontTouch(w) // So we see it in the output Verilog
  w :#= (new MixedAlignmentBundle).Lit(_.alignedChild -> true.B, _.flippedChild -> true.B)
}

This generates the following Verilog, where all members are driven from the literal to w, regardless of alignment:

// Generated by CIRCT firtool-1.94.0
module Example4();
  wire w_alignedChild = 1'h1;
  wire w_flippedChild = 1'h1;
endmodule

Note: Astute observers will realize that c :#= p is semantically equivalent to c :<= p followed by p :>= c (note p and c switched places in the second connection).

Another use case for :#= is for connecting a mixed-directional bundle to a fully-aligned monitor.

import chisel3.experimental.BundleLiterals._
class Example4b extends RawModule {
  val monitor = IO(Output(new MixedAlignmentBundle))
  val w = Wire(new MixedAlignmentBundle)
  dontTouch(w) // So we see it in the output Verilog
  w :#= DontCare
  monitor :#= w
}

This generates the following Verilog, where all members are driven from the literal to w, regardless of alignment:

// Generated by CIRCT firtool-1.94.0
module Example4b(
  output monitor_alignedChild,
         monitor_flippedChild
);

  wire w_alignedChild = 1'h0;
  wire w_flippedChild = 1'h0;
  assign monitor_alignedChild = w_alignedChild;
  assign monitor_flippedChild = w_flippedChild;
endmodule

Connectable​

Sometimes a user wants to connect Chisel components which are not type equivalent. For example, a user may want to hook up anonymous Record components who may have an intersection of their fields being equivalent, but cannot because they are not structurally equivalent. Alternatively, one may want to connect two types that have different widths.

Connectable is the mechanism to specialize connection operator behavior in these scenarios. For additional members which are not present in the other component being connected to, or for mismatched widths, or for always excluding a member from being connected to, they can be explicitly called out from the Connectable object, rather than trigger an error.

In addition, there are other techniques that can be used to address similar use cases including .viewAsSuperType, a static cast to a supertype (e.g. (x: T)), or creating a custom DataView. For a discussion about when to use each technique, please continue here.

This section demonstrates how Connectable specifically can be used in a multitude of scenarios.

Connecting Records​

One use case is to try to connect two Records; for matching members, they should be connected, but for unmatched members, the errors caused due to them being unmatched should be ignored. To accomplish this, use the other operators to initialize all Record members, then use :<>= with .waive to connect only the matching members.

Note that none of .viewAsSuperType, static casts, nor a custom DataView helps this case because the Scala types are still Record.

import scala.collection.immutable.SeqMap

class Example9 extends RawModule {
  val abType = new Record { val elements = SeqMap("a" -> Bool(), "b" -> Flipped(Bool())) }
  val bcType = new Record { val elements = SeqMap("b" -> Flipped(Bool()), "c" -> Bool()) }

  val p = IO(Flipped(abType))
  val c = IO(bcType)

  DontCare :>= p
  c :<= DontCare

  c.waive(_.elements("c")):<>= p.waive(_.elements("a"))
}

This generates the following Verilog, where p.b is driven from c.b:

// Generated by CIRCT firtool-1.94.0
module Example9(
  output p_b,
  input  p_a,
  output c_c,
  input  c_b
);

  assign p_b = c_b;
  assign c_c = 1'h0;
endmodule

Defaults with waived connections​

Another use case is to try to connect two Records; for matching members, they should be connected, but for unmatched members, they should be connected to a default value. To accomplish this, use the other operators to initialize all Record members, then use :<>= with .waive to connect only the matching members.

import scala.collection.immutable.SeqMap

class Example10 extends RawModule {
  val abType = new Record { val elements = SeqMap("a" -> Bool(), "b" -> Flipped(Bool())) }
  val bcType = new Record { val elements = SeqMap("b" -> Flipped(Bool()), "c" -> Bool()) }

  val p = Wire(abType)
  val c = Wire(bcType)

  dontTouch(p) // So it doesn't get constant-propped away for the example
  dontTouch(c) // So it doesn't get constant-propped away for the example

  p :#= abType.Lit(_.elements("a") -> true.B, _.elements("b") -> true.B)
  c :#= bcType.Lit(_.elements("b") -> true.B, _.elements("c") -> true.B)

  c.waive(_.elements("c")) :<>= p.waive(_.elements("a"))
}

This generates the following Verilog, where p.b is driven from c.b, and p.a, c.b, and c.c are initialized to default values:

// Generated by CIRCT firtool-1.94.0
module Example10();
  wire p_a = 1'h1;
  wire c_c = 1'h1;
  wire c_b = 1'h1;
  wire p_b = c_b;
endmodule

Connecting types with optional members​

In the following example, we can use :<>= and .waive to connect two MyDecoupledOpts's, where only one has a bits member.

class MyDecoupledOpt(hasBits: Boolean) extends Bundle {
  val valid = Bool()
  val ready = Flipped(Bool())
  val bits = if (hasBits) Some(UInt(32.W)) else None
}
class Example6 extends RawModule {
  val in  = IO(Flipped(new MyDecoupledOpt(true)))
  val out = IO(new MyDecoupledOpt(false))
  out :<>= in.waive(_.bits.get) // We can know to call .get because we can inspect in.bits.isEmpty
}

This generates the following Verilog, where ready and valid are connected, and bits is ignored:

// Generated by CIRCT firtool-1.94.0
module Example6(
  input         in_valid,
  output        in_ready,
  input  [31:0] in_bits,
  output        out_valid,
  input         out_ready
);

  assign in_ready = out_ready;
  assign out_valid = in_valid;
endmodule

Always ignore errors caused by extra members (partial connection operator)​

The most unsafe connection is to connect only members that are present in both consumer and producer, and ignore all other members. This is unsafe because this connection will never error on any Chisel types.

To do this, you can use .waiveAll and static cast to Data:

class OnlyA extends Bundle {
  val a = UInt(32.W)
}
class OnlyB extends Bundle {
  val b = UInt(32.W)
}
class Example11 extends RawModule {
  val in  = IO(Flipped(new OnlyA))
  val out = IO(new OnlyB)

  out := DontCare

  (out: Data).waiveAll :<>= (in: Data).waiveAll
}

This generates the following Verilog, where nothing is connected:

// Generated by CIRCT firtool-1.94.0
module Example11(
  input  [31:0] in_a,
  output [31:0] out_b
);

  assign out_b = 32'h0;
endmodule

Connecting components with different widths​

Non-Connectable operators implicitly truncate if a component with a larger width is connected to a component with a smaller width. Connectable operators disallow this implicit truncation behavior and require the driven component to be equal or larger in width that the sourcing component.

If implicit truncation behavior is desired, then Connectable provides a squeeze mechanism which will allow the connection to continue with implicit truncation.

import scala.collection.immutable.SeqMap

class Example14 extends RawModule {
  val p = IO(Flipped(UInt(4.W)))
  val c = IO(UInt(3.W))

  c :<>= p.squeeze
}

This generates the following Verilog, where p is truncated prior to driving c:

// Generated by CIRCT firtool-1.94.0
module Example14(
  input  [3:0] p,
  output [2:0] c
);

  assign c = p[2:0];
endmodule

Excluding members from any operator on a Connectable​

If a user wants to always exclude a field from a connect, use the .exclude mechanism which will never connect the field (as if it didn't exist to the connection).

Note that if a field matches in both producer and consumer, but only one is excluded, the other non-excluded field will still trigger an error; to fix this, use either .waive or .exclude.

import scala.collection.immutable.SeqMap

class BundleWithSpecialField extends Bundle {
  val foo = UInt(3.W)
  val special = Bool()
}
class Example15 extends RawModule {
  val p = IO(Flipped(new BundleWithSpecialField()))
  val c = IO(new BundleWithSpecialField())

  c.special := true.B // must initialize it

  c.exclude(_.special) :<>= p.exclude(_.special)
}

This generates the following Verilog, where the special field is not connected:

// Generated by CIRCT firtool-1.94.0
module Example15(
  input  [2:0] p_foo,
  input        p_special,
  output [2:0] c_foo,
  output       c_special
);

  assign c_foo = p_foo;
  assign c_special = 1'h1;
endmodule

Techniques for connecting structurally inequivalent Chisel types​

DataView and .viewAsSupertype create a view of the component that has a different Chisel type. This means that a user can first create a DataView of the consumer or producer (or both) so that the Chisel types are structurally equivalent. This is useful when the difference between the consumer and producer aren't super nested, and also if they have rich Scala types which encode their structure. In general, DataView is the preferred mechanism to use (if you can) because it maintains the most amount of Chisel information in the Scala type, but there are many instances where it doesn't work and thus one must fall back on Connectable.

Connectable does not change the Chisel type, but instead changes the semantics of the operator to not error on the waived members if they are dangling or unconnected. This is useful for when differences between the consumer and producer do not show up in the Scala type system (e.g. present/missing fields of type Option[Data], or anonymous Records) or are deeply nested in a bundle that is especially onerous to create a DataView.

Static casts (e.g. (x: T)) allows connecting components that have different Scala types, but leaves the Chisel type unchanged. Use this to force a connection to occur, even if the Scala types are different.

One may wonder why the operators require identical Scala types in the first place, if they can easily be bypassed. The reason is to encourage users to use the Scala type system to encode Chisel information as it can make their code more robust; however, we don't want to be draconian about it because there are times when we want to enable the user to "just connect the darn thing".

When all else fails one can always manually expand the connection to do what they want to happen, member by member. The down-side to this approach is its verbosity and that adding new members to a component will require updating the manual connections.

Things to remember about Connectable vs .viewAsSupertype/DataView vs static cast (e.g. (x: T)):

  • DataView and .viewAsSupertype will preemptively remove members that are not present in the new view which has a different Chisel type, thus DataView does affect what is connected
  • Connectable can be used to waive the error on members who end up being dangling or unconnected. Importantly, Connectable waives do not affect what is connected
  • Static cast does not remove extra members, thus a static cast does not affect what is connected

Connecting different sub-types of the same super-type, with colliding names​

In these examples, we are connecting MyDecoupled with MyDecoupledOtherBits. Both are subtypes of MyReadyValid, and both have a bits field of type UInt(32.W).

The first example will use .viewAsSupertype to connect them as MyReadyValid. Because it changes the Chisel type to omit both bits fields, the bits fields are unconnected.

import experimental.dataview._
class MyDecoupledOtherBits extends MyReadyValid {
  val bits = UInt(32.W)
}
class Example12 extends RawModule {
  val in  = IO(Flipped(new MyDecoupled))
  val out = IO(new MyDecoupledOtherBits)

  out := DontCare

  out.viewAsSupertype(new MyReadyValid) :<>= in.viewAsSupertype(new MyReadyValid)
}

Note that the bits fields are unconnected.

// Generated by CIRCT firtool-1.94.0
module Example12(
  input         in_valid,
  output        in_ready,
  input  [31:0] in_bits,
  output        out_valid,
  input         out_ready,
  output [31:0] out_bits
);

  assign in_ready = out_ready;
  assign out_valid = in_valid;
  assign out_bits = 32'h0;
endmodule

The second example will use a static cast and .waive(_.bits) to connect them as MyReadyValid. Note that because the static cast does not change the Chisel type, the connection finds that both consumer and producer have a bits field. This means that since they are structurally equivalent, they match and are connected. The waive(_.bits) does nothing, because the bits are neither dangling nor unconnected.

import experimental.dataview._
class Example13 extends RawModule {
  val in  = IO(Flipped(new MyDecoupled))
  val out = IO(new MyDecoupledOtherBits)

  out := DontCare

  out.waiveAs[MyReadyValid](_.bits) :<>= in.waiveAs[MyReadyValid](_.bits)
}

Note that the bits fields ARE connected, even though they are waived, as .waive just changes whether an error should be thrown if they are missing, NOT to not connect them if they are structurally equivalent. To always omit the connection, use .exclude on one side and either .exclude or .waive on the other side.

// Generated by CIRCT firtool-1.94.0
module Example13(
  input         in_valid,
  output        in_ready,
  input  [31:0] in_bits,
  output        out_valid,
  input         out_ready,
  output [31:0] out_bits
);

  assign in_ready = out_ready;
  assign out_valid = in_valid;
  assign out_bits = in_bits;
endmodule

Connecting sub-types to super-types by waiving extra members​

Note that in this example, it would be better to use .viewAsSupertype.

In the following example, we can use :<>= to connect a MyReadyValid to a MyDecoupled by waiving the bits member.

class MyReadyValid extends Bundle {
  val valid = Bool()
  val ready = Flipped(Bool())
}
class MyDecoupled extends MyReadyValid {
  val bits = UInt(32.W)
}
class Example5 extends RawModule {
  val in  = IO(Flipped(new MyDecoupled))
  val out = IO(new MyReadyValid)
  out :<>= in.waiveAs[MyReadyValid](_.bits)
}

This generates the following Verilog, where ready and valid are connected, and bits is ignored:

// Generated by CIRCT firtool-1.94.0
module Example5(
  input         in_valid,
  output        in_ready,
  input  [31:0] in_bits,
  output        out_valid,
  input         out_ready
);

  assign in_ready = out_ready;
  assign out_valid = in_valid;
endmodule

Connecting different sub-types​

Note that in this example, it would be better to use .viewAsSupertype.

Note that the connection operator requires the consumer and producer to be the same Scala type to encourage capturing more information statically, but they can always be cast to Data or another common supertype prior to connecting.

In the following example, we can use :<>= and .waiveAs to connect two different sub-types of MyReadyValid.

class HasBits extends MyReadyValid {
  val bits = UInt(32.W)
}
class HasEcho extends MyReadyValid {
  val echo = Flipped(UInt(32.W))
}
class Example7 extends RawModule {
  val in  = IO(Flipped(new HasBits))
  val out = IO(new HasEcho)
  out.waiveAs[MyReadyValid](_.echo) :<>= in.waiveAs[MyReadyValid](_.bits)
}

This generates the following Verilog, where ready and valid are connected, and bits and echo are ignored:

// Generated by CIRCT firtool-1.94.0
module Example7(
  input         in_valid,
  output        in_ready,
  input  [31:0] in_bits,
  output        out_valid,
  input         out_ready,
  input  [31:0] out_echo
);

  assign in_ready = out_ready;
  assign out_valid = in_valid;
endmodule

FAQ​

How do I connect two items as flexibly as possible (try your best but never error)​

Use .unsafe (both waives and allows squeezing of all fields).

class ExampleUnsafe extends RawModule {
  val in  = IO(Flipped(new Bundle { val foo = Bool(); val bar = Bool() }))
  val out = IO(new Bundle { val baz = Bool(); val bar = Bool() })
  out.unsafe :<>= in.unsafe // bar is connected, and nothing errors
}

How do I connect two items but don't care about the Scala types being equivalent?​

Use .as (upcasts the Scala type).

class ExampleAs extends RawModule {
  val in  = IO(Flipped(new Bundle { val foo = Bool(); val bar = Bool() }))
  val out = IO(new Bundle { val foo = Bool(); val bar = Bool() })
  // foo and bar are connected, although Scala types aren't the same
  out.as[Data] :<>= in.as[Data]
}