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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