This example will show the syntax to define an APB3 Bundle.


The specification from ARM could be interpreted as follows:

Signal Name Type Driver side Comment
PADDR UInt(addressWidth bits) Master Address in byte
PSEL Bits(selWidth) Master One bit per slave
PENABLE Bool Master -
PWRITE Bool Master -
PWDATA Bits(dataWidth bits) Master -
PREADY Bool Slave -
PRDATA Bits(dataWidth bits) Slave -
PSLVERROR Bool Slave Optional


This specification shows that the APB3 bus has multiple possible configurations. To represent that, we can define a configuration class in Scala:

case class Apb3Config(
  addressWidth  : Int,
  dataWidth     : Int,
  selWidth      : Int     = 1,
  useSlaveError : Boolean = true

Then we can define the APB3 Bundle which will be used to represent the bus in hardware:

case class Apb3(config: Apb3Config) extends Bundle with IMasterSlave {
  val PADDR      = UInt(config.addressWidth bit)
  val PSEL       = Bits(config.selWidth bits)
  val PENABLE    = Bool
  val PREADY     = Bool
  val PWRITE     = Bool
  val PWDATA     = Bits(config.dataWidth bit)
  val PRDATA     = Bits(config.dataWidth bit)
  val PSLVERROR  = if(config.useSlaveError) Bool else null

  override def asMaster(): Unit = {
    if(config.useSlaveError) in(PSLVERROR)


Here is a usage example of this definition:

val apbConfig = Apb3Config(
  addressWidth  = 16,
  dataWidth     = 32,
  selWidth      = 1,
  useSlaveError = false

val io = new Bundle{
  val apb = slave(Apb3(apbConfig))

io.apb.PREADY := True
when(io.apb.PSEL(0) && io.apb.PENABLE){