Scala Chisel Ripple Carry Adder Syntax - scala

Question

im trying to design the following Ripple Carry Adder made of Fulladers. I tried a lot so far, but I'm struggling with Chisel Syntax. Could someone help me out and point out what I'm doing wrong? This is my Code below:
class RcaAdder(val n:Int) extends Module {
val io = IO(new Bundle {
val a = Input(UInt(n.W))
val b = Input(UInt(n.W))
val cin = Input(UInt(1.W))
val sum = Output(UInt(n.W))
val cout = Output(UInt(1.W))
})
//For loop
for(i <- 0 to n){
val fulladder = Module(new FullAdder())
fulladder.io.a := io.a(i)
fulladder.io.b := io.b(i)
if(i == 0){
fulladder.io.cin := io.cin
}else{
fulladder.io.cin := io.cout
}
io.cout := fulladder.io.cout
io.sum(i) := fulladder.io.sum
}
}
Which gets me the following error:
Exception in thread "main" chisel3.internal.ChiselException: Cannot reassign to read-only Bool(OpResult in RcaAdder)
I assume it has something to do with the " io.sum(i) := .. "
Please help me out! Thank you so much!

Answer 1

You are very close to getting it working. One problem you are having is that you cannot assign to a bit subset on the left hand side of :=. One way of getting around this is to create a Vec of UInt(1.W) and then use that as the RHS as a single as a single assignment. I think you have a problem with your ifs, I'd recommend using foldLeft instead of for because it provides a mechanism of accessing the previous elements. Put that all together and I think what you want is something like this.
class RcaAdder(n: Int) extends Module {
val io = IO(new Bundle {
val a = Input(UInt(n.W))
val b = Input(UInt(n.W))
val cin = Input(UInt(1.W))
val sum = Output(UInt(n.W))
val cout = Output(UInt(1.W))
})
val outBits = Wire(Vec(n, UInt(1.W)))
io.cout := (0 until n).foldLeft(io.cin) { case (carry, index) =>
val fullAdder = Module(new FullAdder)
fullAdder.io.a := io.a(index)
fullAdder.io.b := io.b(index)
fullAdder.io.cin := carry
outBits(index) := fullAdder.io.sum
fullAdder.io.cout. // This will be passed as carry to the next interation
}
io.sum := outBits.asUInt()
}
I've added a working test example here on scastie.
Good luck and welcome to Chisel