Function

Introduction

The ways you can use Scala functions to generate hardware are radically different than VHDL/Verilog for many reasons:

  • You can instantiate registers, combinational logic, and components inside them.

  • You don’t have to play with process/@always blocks that limit the scope of assignment of signals.

  • Everything is passed by reference, which allows easy manipulation.
    For example, you can give a bus to a function as an argument, then the function can internally read/write to it. You can also return a Component, a Bus, or anything else from Scala and the Scala world.

RGB to gray

For example, if you want to convert a Red/Green/Blue color into greyscale by using coefficients, you can use functions to apply them:

// Input RGB color
val r, g, b = UInt(8 bits)

// Define a function to multiply a UInt by a Scala Float value.
def coef(value: UInt, by: Float): UInt = (value * U((255 * by).toInt, 8 bits) >> 8)

// Calculate the gray level
val gray = coef(r, 0.3f) + coef(g, 0.4f) + coef(b, 0.3f)

Valid Ready Payload bus

For instance, if you define a simple bus with valid, ready, and payload signals, you can then define some useful functions inside of it.

case class MyBus(payloadWidth: Int) extends Bundle with IMasterSlave {
  val valid   = Bool
  val ready   = Bool
  val payload = Bits(payloadWidth bits)

  // Define the direction of the data in a master mode
  override def asMaster(): Unit = {
    out(valid, payload)
    in(ready)
  }

  // Connect that to this
  def <<(that: MyBus): Unit = {
    this.valid   := that.valid
    that.ready   := this.ready
    this.payload := that.payload
  }

  // Connect this to the FIFO input, return the fifo output
  def queue(size: Int): MyBus = {
    val fifo = new MyBusFifo(payloadWidth, size)
    fifo.io.push << this
    return fifo.io.pop
  }
}

class MyBusFifo(payloadWidth: Int, depth: Int) extends Component {

  val io = new Bundle {
    val push = slave(MyBus(payloadWidth))
    val pop  = master(MyBus(payloadWidth))
  }

  val mem = Mem(Bits(payloadWidth bits), depth)

  // ...

}