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Sinus rom

Let’s imagine that you want to generate a sine wave and also have a filtered version of it (which is completely useless in practical, but let’s do it as an example).

Parameters name	Type	Description
resolutionWidth	Int	Number of bits used to represent numbers
sampleCount	Int	Number of samples in a sine period

IO name	Direction	Type	Description
sin	out	SInt(resolutionWidth bits)	Output which plays the sine wave
sinFiltred	out	SInt(resolutionWidth bits)	Output which plays the filtered version of the sine

So let’s define the Component:

class TopLevel(resolutionWidth : Int,sampleCount : Int) extends Component {
  val io = new Bundle {
    val sin = out SInt(resolutionWidth bits)
    val sinFiltred = out SInt(resolutionWidth bits)
  }
  // Here will come the logic implementation
}

To play the sine wave on the sin output, you can define a ROM which contain all samples of a sine period (tt could be just a quarter, but let’s do things by the simplest way).

Then you can read that ROM with an phase counter and this will generate your sine wave.

//Function used to generate the rom (later)
def sinTable = for(sampleIndex <- 0 until sampleCount) yield {
  val sinValue = Math.sin(2 * Math.PI * sampleIndex / sampleCount)
  S((sinValue * ((1<<resolutionWidth)/2-1)).toInt,resolutionWidth bits)
}

val rom =  Mem(SInt(resolutionWidth bits),initialContent = sinTable)
val phase = Reg(UInt(log2Up(sampleCount) bits)) init(0)
phase := phase + 1

io.sin := rom.readSync(phase)

Then to generate sinFiltred, you can for example use a first order low pass filter implementation:

io.sinFiltred := RegNext(io.sinFiltred  - (io.sinFiltred  >> 5) + (io.sin >> 5)) init(0)

Here is the complete code:

class TopLevel(resolutionWidth : Int,sampleCount : Int) extends Component {
  val io = new Bundle {
    val sin = out SInt(resolutionWidth bits)
    val sinFiltred = out SInt(resolutionWidth bits)
  }

  def sinTable = for(sampleIndex <- 0 until sampleCount) yield {
    val sinValue = Math.sin(2 * Math.PI * sampleIndex / sampleCount)
    S((sinValue * ((1<<resolutionWidth)/2-1)).toInt,resolutionWidth bits)
  }

  val rom =  Mem(SInt(resolutionWidth bits),initialContent = sinTable)
  val phase = Reg(UInt(log2Up(sampleCount) bits)) init(0)
  phase := phase + 1

  io.sin := rom.readSync(phase)
  io.sinFiltred := RegNext(io.sinFiltred  - (io.sinFiltred  >> 5) + (io.sin >> 5)) init(0)
}