APB3 definition

Introduction

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

Specification

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

Implementation

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 = {
    out(PADDR,PSEL,PENABLE,PWRITE,PWDATA)
    in(PREADY,PRDATA)
    if(config.useSlaveError) in(PSLVERROR)
  }
}

Usage

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){
  //...
}