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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){
  //...
}