Examples in Chisel that make use of MuxCase

Question

How do I implement a 4:1 Mux in chisel without using 2:1 Muxes? Is there a way where we can select one of the inputs of the N inputs by having something like Mux(sel, A,B,C,D.......N) where N can be taken in as a parameter? I am aware of the MuxCase in chisel but I am yet to find an example that makes use of MuxCase, any sort of documentation or example regarding this is greatly appreciated. Thank You.

Answer 1

There are a couple of usages in rocket-chip in MultiWidthFifo.scala

It's pretty straightforward. It takes a default value for what happens if none of the supplied conditions is true, otherwise it looks through a sequence of tuples, where each tuple of the form (bool condition, result) often written condition -> result. The return value is the result from the first boolean condition that is true.

Here is a toy example of a module that passes the number of input bools into a module which then uses that value to construct a sequence of mux cases.

class UsesMuxCase(numCases: Int) extends Module {
  val io = IO(new Bundle {
    val output = Output(UInt(10.W))
    val inputs = Input(Vec(numCases, Bool()))
  })

  val cases = io.inputs.zipWithIndex.map { case (bool, index) =>
    bool -> index.U(10.W)
  }
  io.output := MuxCase(0.U(10.W), cases)
}