Can I compute constants in software before Chisel begins designing hardware?

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I'm new to Chisel, and I was wondering if it's possible to calculate constants in software before Chisel begins designing any circuitry. For instance, I have a module which takes one parameter, myParameter, but from this parameter I'd like to derive more variables (constant1 and constant2) that would be later used to initialize registers.

class MyModule(myParameter: Int) extends Module {
    val io = IO(new Bundle{
        val in = Input(SInt(8.W))
        val out = Output(SInt(8.W))
    })

    val constant1 = 2 * myParameter
    val constant2 = 17 * myParameter

    val register1 = RegInit((constant1).U(8.W))
    val register2 = RegInit((constant2).U(8.W))
    //...
    //...
}

Is there a way to configure Chisel's functionality so that an instance of MyModule(2) will first evaluate all Scala vals in software: constant1 = 2 * 2 = 4 and constant2 = 17 * 2 = 34. Then proceed to instantiate and initialize registers register1 = RegInit(4.U(8.W)) and register2 = RegInit(34.U(8.W))?

chisel
1 Answers

I was wondering if it's possible to calculate constants in software before Chisel begins designing any circuitry

Unless I'm misunderstanding your question, this is, in fact, how Chisel works.

Fundamentally, Chisel is a Scala library where the execution of your compiled Scala code creates hardware. This means that any pure-Scala code in your Chisel only exists at elaboration time, that is, during execution of this Scala program (which we call a generator).

Now, values in your program are created in sequential order as defined by Scala (and more-or-less the same as any general purpose programming language). For example, io is defined before constant1 and constant2 so the Chisel object for io will be created before either constants are calculated, but this shouldn't really matter for the purposes of your question.

A common practice in Chisel is to create custom classes to hold parameters when you have a lot of them. In this case, you could do something similar like this:

// Note this doesn't extend anything, it's just a Scala class
// Also note myParameter is a val now, this makes it accessible outside the class
class MyParameters(val myParameter: Int) {
    val constant1 = 2 * myParameter
    val constant2 = 17 * myParameter
}

class MyModule(params: MyParameters) extends Module {
    val io = IO(new Bundle{
        val in = Input(SInt(8.W))
        val out = Output(SInt(8.W))
    })

    val register1 = RegInit((params.constant1).U(8.W))
    val register2 = RegInit((params.constant2).U(8.W))
    //...
    //...
}
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