DecoupledIO, EnqIO, DeqIO, and ready signal

[Raghu Prabhakar]

Hi all,

I'm trying to understand DecoupledIO. Here's a simple design:

class Dummy() extends Module {
  val io = IO(new Bundle {
    val in = Input(DecoupledIO(UInt(10.W)))

  })

}

The Verilog interface generated for this module is (correctly) as below:

module Dummy(

  input   clock,

  input   reset,

  output  io_in_ready,

  input   io_in_valid,

  input  [9:0] io_in_bits

); 

However, when I try to assign something to the 'ready' from within the module, it results in a Chisel compiler error:

io.in.ready := Bool(true) // Within the 'Dummy' module, causes the error below:

[error] (run-main-e) chisel3.internal.ChiselException: Connection between sink (chisel3.core.Bool@10) and source (chisel3.core.Bool@15) failed @: Sink is unwriteable by current module.

chisel3.internal.ChiselException: Connection between sink (chisel3.core.Bool@10) and source (chisel3.core.Bool@15) failed @: Sink is unwriteable by current module.

        at chisel3.internal.throwException$.apply(Error.scala:13)

        at chisel3.core.Data.connect(Data.scala:145)

        at chisel3.core.Data.$colon$eq(Data.scala:204)

        at example.Dummy.<init>(Dummy.scala:15)

        ...

1. Why is this assignment wrong? How should one use DecoupledIO?

2. I noticed that EnqIO, DeqIO, and DecoupledIO all generate the same interface. Are they all functionally equivalent?

Thanks,

Raghu

[Jim Lawson]

I suggest using the Decoupled factory method which wraps user-defined Data (typically a Bundle) with the DecoupledIO interface. Examples of such usage can be found in DecoupledAdder in the chisel-testers repo.

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[Raghu Prabhakar]

Cool, thanks. I changed the interface to be as below (following the example) and it works.

  val io = IO(new Bundle {                                                      

    val dec = Flipped(Decoupled(UInt(10.W)))

  })

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[neville]

I have a follow-up question regarding (2)

enqIO and decoupledIO seem to be different in my understanding. Can someone correct me if I am wrong?
enqIO when used for a queue of depth>1 uses ready signal to indicate whether the queue is still enqueuable. 

However, decoupledIO uses ready signal to indicate whether the valid data (which was sent by a producer) was accepted by the consumer or not.