Chisel模块中的条件端口

Question

I have a selectable feature which is not normally required. However to support this feature, some I/O ports should be added to the origin Module I/O port.

I am doing it in this way:

import Chisel._

class TestModule extends Module {

  class IOBundle extends Bundle {
    val i = Bool(INPUT)
    val o = Bool(OUTPUT)
  }

  class IOBundle_EXT extends IOBundle {
    val o_ext = Bool(OUTPUT)
  }

  val io = if(true) new IOBundle_EXT else new IOBundle;

  io.o := io.i
  io.o_ext := io.i

}

After running sbt "run TestModule --backend c --compile --test --genHarness", the compiler complains:

[error] xxxx/test/condi_port.scala:17: value o_ext is not a member of TestModule.this.IOBundle
[error]   io.o_ext := io.i
[error]      ^
[error] one error found
[error] (compile:compile) Compilation failed

So the if statement has no effect. val io is still assigned to IOBundle, rather than the extended IOBoundle_EXT, which makes no sense to me.

Answer 1

Chisel now supports Options in IO bundles.

As an example, I explored Options here ( https://github.com/ucb-bar/riscv-boom/commit/da6edcb4b7bec341e31a55567ee04c8a1431d659 ), but here's a summary:

class MyBundle extends Bundle
{
   val my_ext = if (SOME_SWITCH) Some(ExtBundle) else None
}

...

io.my_ext match
{
   case Some(b: ExtBundle) =>
      my_ext.a := Bool(false)
      ...
   case _ => require (!SOME_SWITCH)
}

It's incredibly verbose, but I was able to get it working even when doing bulk connects and hiding bundles within bundles, etc.

Answer 2

Even though the compiler could determine that only one result is possible (the expression is always true), the type system sets the type of the result equal to the greatest common subtype of the two possible (true or false) sub-expressions.

You can verify this trivially with the following:

scala> val result = if (true) 1 else "one"
result: Any = 1

Answer 3

Try this:

import Chisel._

class TestModule(val useEXT : Boolean) extends Module {

  class IOBundle extends Bundle {
    val i = Bool(INPUT)
    val o = Bool(OUTPUT)
  }

  class IOBundle_EXT extends IOBundle {
    val o_ext = Bool(OUTPUT)
  }

  val io = { 
    if(useEXT) { 
      val res = new IOBundle_EXT; res.o_ext := res.i; res 
    } else {
      new IOBundle }};

  io.o := io.i

}