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Axi4

Introduction

The AXI4 is a high bandwidth bus defined by ARM.

Configuration and instanciation

First each time you want to create a AXI4 bus, you will need a configuration object. This configuration object is an Axi4Config and has following arguments :

Note : useXXX specify if the bus has XXX signal present.

Parameter name	Type	Default
addressWidth	Int
dataWidth	Int
idWidth	Int
userWidth	Int
useId	Boolean	true
useRegion	Boolean	true
useBurst	Boolean	true
useLock	Boolean	true
useCache	Boolean	true
useSize	Boolean	true
useQos	Boolean	true
useLen	Boolean	true
useLast	Boolean	true
useResp	Boolean	true
useProt	Boolean	true
useStrb	Boolean	true
useUser	Boolean	false

There is in short how the AXI4 bus is defined in the SpinalHDL library :

case class Axi4(config: Axi4Config) extends Bundle with IMasterSlave{
  val aw = Stream(Axi4Aw(config))
  val w  = Stream(Axi4W(config))
  val b  = Stream(Axi4B(config))
  val ar = Stream(Axi4Ar(config))
  val r  = Stream(Axi4R(config))

  override def asMaster(): Unit = {
    master(ar,aw,w)
    slave(r,b)
  }
}

There is a short example of usage :

val axiConfig = Axi4Config(
  addressWidth = 32,
  dataWidth    = 32,
  idWidth      = 4
)
val axiX = Axi4(axiConfig)
val axiY = Axi4(axiConfig)

when(axiY.aw.valid){
  //...
}

Variations

There is 3 other variation of the Axi4 bus :

Type	Description
Axi4ReadOnly	Only AR and R channels are present
Axi4WriteOnly	Only AW, W and B channels are present
Axi4Shared	This variation is a library initiative. It use 4 channels, W, B ,R and also a new one which is named AWR. The AWR channel can be used to transmit AR and AW transactions. To dissociate them, a signal write is present. The advantage of this Axi4Shared variation is to use less area, especially in the interconnect.

Functions and operators

Name	Return	Description
X >> Y		Connect X to Y. Able infer default values as specified in the AXI4 specification, and also to adapt some width in a safe manner.
X << Y		Do the reverse of the >> operator
X.toWriteOnly	Axi4WriteOnly	Return an Axi4WriteOnly bus drive by X
X.toReadOnly	Axi4ReadOnly	Return an Axi4ReadOnly bus drive by X