Type Casting in Chisel: Converting UInt to Int

Question

I am working on a Chisel project and encountered an issue related to converting UInt to Int. I have tried using the litValue method, but it's giving me a None.get error when executing my code. I need guidance on how to correctly perform the conversion from UInt to Int and avoid the None.get error

val uintValue: UInt = // Some UInt value
val intValue: Int = uintValue.litValue() // This line gives None.get error

I tried to convert a UInt value to an Int using the .litValue() method.

I was expecting this code to successfully convert the UInt value to an Int and store it in the intValue variable. However, when executing the code, it resulted in a None.get error, and I couldn't obtain the desired integer value.

I need guidance on how to correctly perform the conversion from UInt to Int and avoid the None.get error.

Answer 1

Not sure what exactly you are trying to do. UInt is a chisel data type, mean while Int is a scala data type.

Chisel datatypes are used to specify the type of values held in state elements or flowing on wires. While hardware designs ultimately operate on vectors of binary digits, other more abstract representations for values allow clearer specifications and help the tools generate more optimal circuits.

Based on the discussion How to convert Chisel.UInt to scala Int? it seems that it doesn't make sense what you are trying to do

In general, this is not possible. A UInt specifies the type of a run-time signal which may only have a specific value during circuit emulation. If you want to convert a UInt value to a Scala Int during simulation, use the peek() method.

It also provides some code, but I guess it's a bit outdated due to the question was done in May 21, 2015. So, I'm gonna add another example from the repo schoeberl - chisel-examples

• setup build.sbt

scalaVersion := "2.12.13"

// Chisel 3.5
addCompilerPlugin("edu.berkeley.cs" % "chisel3-plugin" % "3.5.3" cross CrossVersion.full)
libraryDependencies += "edu.berkeley.cs" %% "chisel3" % "3.5.3"
libraryDependencies += "edu.berkeley.cs" %% "chiseltest" % "0.5.3"

• class Hello

import chisel3._

/**
 * The blinking LED component.
 */

class Hello extends Module {
  val io = IO(new Bundle {
    val led = Output(UInt(1.W))
  })
  val CNT_MAX = (50000000 / 2 - 1).U

  val cntReg = RegInit(0.U(32.W))
  val blkReg = RegInit(0.U(1.W))

  cntReg := cntReg + 1.U
  when(cntReg === CNT_MAX) {
    cntReg := 0.U
    blkReg := ~blkReg
  }
  io.led := blkReg
}

/**
 * An object extending App to generate the Verilog code.
 */
object Hello extends App {
  (new chisel3.stage.ChiselStage).emitVerilog(new Hello())
}

• test Hello (how to write a test using chiseltest)

import chiseltest._
import org.scalatest.flatspec.AnyFlatSpec

class HelloTest extends AnyFlatSpec with ChiselScalatestTester {
  behavior of "Hello"
  it should "pass" in {
    test(new Hello) { c => // 
      c.clock.setTimeout(0)
      var ledStatus = BigInt(-1)
      println("Start the blinking LED")
      for (_ <- 0 until 100) {
        c.clock.step(10000)
        val ledNow = c.io.led.peek().litValue
        val s = if (ledNow == 0) "o" else "*"
        if (ledStatus != ledNow) {
          System.out.println(s)
          ledStatus = ledNow
        }
      }
      println("\nEnd the blinking LED")
    }
  }
}

---

The error you got comes from scala. Here you have some articles that talk about that

• Nicolas Rinaudo - Do not call get on an Option

Reason

Some Options are Nones, and get deals with them by throwing an exception:

None.get
// java.util.NoSuchElementException: None.get
//    at scala.None$.get(Option.scala:366)
//    at repl.Session$App$$anonfun$1.apply(option_get.md:9)
//    at repl.Session$App$$anonfun$1.apply(option_get.md:9)

If you have a default value to provide for the None case, use getOrElse:

None.getOrElse(-1)
// res0: Int = -1

• Sukumaar Mane - Scala Option Some None

Scala’s solution in Scala’s way:

Make use of Option, Some, None.

But what it is ???

Option acts sort of like a container of a reference if a reference is not empty i.e. it is pointing to some memory location then it returns Some otherwise None.

If you look at the code of the function litValue from abstract class Data

  /**
    * Returns the literal value if this is a literal that is representable as bits, otherwise crashes.
    */
  def litValue: BigInt = litOption.get

As it was detailed in the previous articles shared, it's not a good practice to call Option.get directly. You can also have the method litOption

  /**
    * If this is a literal that is representable as bits, returns the value as a BigInt.
    * If not a literal, or not representable as bits (for example, is or contains Analog), returns None.
    */
  def litOption: Option[BigInt]

This one returns an Option[BigInt], which means you can take some an action using Pattern Matching or just return a default value using getOrElse.