Retrieve the reset value of RegInit

Question

Is there a way to get the reset value of a RegInit by probing members of the type that is produces? I can see that a RegInit will return the type (e.g. UInt). For example I will have a register which I want to be controlled via a regmap

val myRWReg = RegInit(3.U(10.W))

// ...

node.regmap(
  0x0 -> Seq(RegField(10.W, myRWReg,
               RegFieldDesc("myRWReg", "A RW Register.", reset = myRWReg.init)))
)

.init is not an accessible member, but is shown here to imply it's what I'm looking for.

Is there a way to do this so that anytime a RegFieldDesc is used the reset description can match the hardware without having to keep another variable?

A workaround/solution is to make 3.U in my example a val that is changed based on any parameters and simply pass that, but wasn't sure if there was something else that could be used.

That sounds like a useful feature but does not exist. I think the extra `val` is the best option here. — Jack Koenig, Sep 21 '20 at 23:36
Ok fair enough. Thanks for the feedback! Would it be helpful to submit an enhancement request on the github repo? — l Steveo l, Sep 22 '20 at 12:57

score 1 · Accepted Answer · answered Mar 10 '21 at 19:18

I ended up coming up with something that may be useful for others. One of the things I wanted to also be able to do was not have to explicitly declare a Reg. I wanted this to be inferred by the connection. For example, I have a SW register that is driving an 8bit "port" on a Bundle. I could make a Reg but then I need to keep up with the size if it changes based on parameters.

object WavRWReg{
  
  def apply[T <: Data](connection : T, reset: T, name: String, desc: String = ""): RegField = {
    //val reg = RegInit(reset)
    val reg = RegInit(connection.cloneType, reset)
    
    reg.suggestName("swi_" + name)
    
    connection := reg
    
    //litValue returns the Bigint value
    val rf = RegField(reg.getWidth, reg.asUInt, RegFieldDesc(name, desc, access=RegFieldAccessType.RW , reset=Some(reset.litValue)))
    rf
  }
}

So this creates the Reginit to the specified reset val, and the width is inferred from the connection type. The reset value is also sent to the RegFieldDesc so I don't have to manually keep up with it in two places, keeping down on errors.

score 0 · Answer 2 · answered Oct 15 '20 at 17:44

you may also be interested in DescribedReg: https://github.com/chipsalliance/rocket-chip/blob/30e38e83a1a427bba5441afad83bde947fa75542/src/main/scala/regmapper/DescribedReg.scala#L7

It both builds the register and its description at the same time. An example usage: https://github.com/chipsalliance/rocket-chip/blob/30e38e83a1a427bba5441afad83bde947fa75542/src/main/scala/tile/BusErrorUnit.scala#L58

Retrieve the reset value of RegInit

2 Answers2