Friday, March 24, 2017

Register a bundle in Chisel

I have been playing with more with Chisel lately.  An area I find lacking in the documentation is how to register a bundle (i.e. struct).  Most examples I have seen use simple registers with type UInt.

It took some trial and error, but here is an example that elaborates and synthesizes properly.

class DDR3Command extends Bundle {
  val casN = UInt(1.W)
  val rasN = UInt(1.W)
  val ba = UInt(3.W)

  // user function to specify defaults (sync reset)
  def defaults() = {
    casN := 1.U
    rasN := 1.U
    ba := 0.U

class MemoryController extends Module {
  val io = IO(new Bundle {
    val ddr3 = Output(new DDR3Command())

  // create a wire of the bundle and set it's default values (sync reset)
  val resetDDR3Cmd = Wire(new DDR3Command())

  val nextDDR3Cmd = Reg(new DDR3Command(), init=resetDDR3Cmd)

  // for example purposes, only update select fields
  when (true.B) { := 3.U
    nextDDR3Cmd.casN := 0.U
    //nextDDR3Cmd.rasN := 1.U

  io.ddr3 := nextDDR3Cmd