Unconnected Wires

The Invalidate API (#645) adds support to Chisel for reporting unconnected wires as errors.

Prior to this pull request, Chisel automatically generated a firrtl is invalid for Module IO(), and each Wire() definition. This made it difficult to detect cases where output signals were never driven. Chisel now supports a DontCare element, which may be connected to an output signal, indicating that that signal is intentionally not driven. Unless a signal is driven by hardware or connected to a DontCare, Firrtl will complain with a "not fully initialized" error.

API

Output signals may be connected to DontCare, generating a is invalid when the corresponding firrtl is emitted.

class Out extends Bundle { 
  val debug = Bool()
  val debugOption = Bool()
}
val io = new Bundle { val out = new Out }

io.out.debug := true.B
io.out.debugOption := DontCare

This indicates that the signal io.out.debugOption is intentionally not driven and firrtl should not issue a "not fully initialized" error for this signal.

This can be applied to aggregates as well as individual signals:

import chisel3._
class ModWithVec extends Module {
  // ...
  val nElements = 5
  val io = IO(new Bundle {
    val outs = Output(Vec(nElements, Bool()))
  })
  io.outs <> DontCare
  // ...
}

class TrivialInterface extends Bundle {
  val in  = Input(Bool())
  val out = Output(Bool())
}

class ModWithTrivalInterface extends Module {
  // ...
  val io = IO(new TrivialInterface)
  io <> DontCare
  // ...
}

Determining the unconnected element

I have an interface with 42 wires. Which one of them is unconnected?

The firrtl error message should contain something like:

firrtl.passes.CheckInitialization$RefNotInitializedException:  @[:@6.4] : [module Router]  Reference io is not fully initialized.
   @[Decoupled.scala 38:19:@48.12] : node _GEN_23 = mux(and(UInt<1>("h1"), eq(UInt<2>("h3"), _T_84)), _GEN_2, VOID) @[Decoupled.scala 38:19:@48.12]
   @[Router.scala 78:30:@44.10] : node _GEN_36 = mux(_GEN_0.ready, _GEN_23, VOID) @[Router.scala 78:30:@44.10]
   @[Router.scala 75:26:@39.8] : node _GEN_54 = mux(io.in.valid, _GEN_36, VOID) @[Router.scala 75:26:@39.8]
   @[Router.scala 70:50:@27.6] : node _GEN_76 = mux(io.load_routing_table_request.valid, VOID, _GEN_54) @[Router.scala 70:50:@27.6]
   @[Router.scala 65:85:@19.4] : node _GEN_102 = mux(_T_62, VOID, _GEN_76) @[Router.scala 65:85:@19.4]
   : io.outs[3].bits.body <= _GEN_102

The first line is the initial error report. Successive lines, indented and beginning with source line information indicate connections involving the problematic signal. Unfortunately, if these are when conditions involving muxes, they may be difficult to decipher. The last line of the group, indented and beginning with a : should indicate the uninitialized signal component. This example (from the Router tutorial) was produced when the output queue bits were not initialized. The old code was:

  io.outs.foreach { out => out.noenq() }

which initialized the queue's valid bit, but did not initialize the actual output values. The fix was:

  io.outs.foreach { out =>
    out.bits := 0.U.asTypeOf(out.bits)
    out.noenq()
  }