JTAG TAP

Introduction

Important

The goal of this page is to show the implementation of a JTAG TAP (a slave) by a non-conventional way.

Important

This implementation is not a simple one, it mix object oriented programming, abstract interfaces decoupling, hardware generation and hardware description.
Of course a simple JTAG TAP implementation could be done only with a simple hardware description, but the goal here is really to going forward and creating an very reusable and extensible JTAG TAP generator

Important

This page will not explains how JTAG work. A good tutorial could be find there.

One big difference between commonly used HDL and Spinal, is the fact that SpinalHDL allow you to define hardware generators/builders. It’s very different than describing hardware. Let’s take a look into the example bellow because the difference between generate/build/describing could seem “playing with word” or could be interpreted differently.

The example bellow is a JTAG TAP which allow the JTAG master to read switchs/keys inputs and write leds outputs. This TAP could also be recognized by a master by using the UID 0x87654321.

class SimpleJtagTap extends Component {
  val io = new Bundle {
    val jtag    = slave(Jtag())
    val switchs = in  Bits(8 bit)
    val keys    = in  Bits(4 bit)
    val leds    = out Bits(8 bit)
  }

  val tap = new JtagTap(io.jtag, 8)
  val idcodeArea  = tap.idcode(B"x87654321") (instructionId=4)
  val switchsArea = tap.read(io.switchs)     (instructionId=5)
  val keysArea    = tap.read(io.keys)        (instructionId=6)
  val ledsArea    = tap.write(io.leds)       (instructionId=7)
}

As you can see, a JtagTap is created but then some Generator/Builder functions (idcode,read,write) are called to create each JTAG instruction. This is what i call “Hardware generator/builder”, then these Generator/Builder are used by the user to describing an hardware. And there is the point, in commonly HDL you can only describe your hardware, which imply many donkey job.

This JTAG TAP tutorial is based on this implementation.

JTAG bus

First we need to define a JTAG bus bundle.

case class Jtag() extends Bundle with IMasterSlave {
  val tms = Bool
  val tdi = Bool
  val tdo = Bool

  override def asMaster() : Unit = {
    out(tdi, tms)
    in(tdo)
  }
}

As you can see this bus don’t contain the TCK pin because it will be provided by the clock domain.

JTAG state machine

Let’s define the JTAG state machine as explained here

object JtagState extends SpinalEnum {
  val RESET, IDLE,
      IR_SELECT, IR_CAPTURE, IR_SHIFT, IR_EXIT1, IR_PAUSE, IR_EXIT2, IR_UPDATE,
      DR_SELECT, DR_CAPTURE, DR_SHIFT, DR_EXIT1, DR_PAUSE, DR_EXIT2, DR_UPDATE = newElement()
}

class JtagFsm(jtag: Jtag) extends Area {
  import JtagState._
  val stateNext = JtagState()
  val state = RegNext(stateNext) randBoot()

  stateNext := state.mux(
    default    -> (jtag.tms ? RESET     | IDLE),           //RESET
    IDLE       -> (jtag.tms ? DR_SELECT | IDLE),
    IR_SELECT  -> (jtag.tms ? RESET     | IR_CAPTURE),
    IR_CAPTURE -> (jtag.tms ? IR_EXIT1  | IR_SHIFT),
    IR_SHIFT   -> (jtag.tms ? IR_EXIT1  | IR_SHIFT),
    IR_EXIT1   -> (jtag.tms ? IR_UPDATE | IR_PAUSE),
    IR_PAUSE   -> (jtag.tms ? IR_EXIT2  | IR_PAUSE),
    IR_EXIT2   -> (jtag.tms ? IR_UPDATE | IR_SHIFT),
    IR_UPDATE  -> (jtag.tms ? DR_SELECT | IDLE),
    DR_SELECT  -> (jtag.tms ? IR_SELECT | DR_CAPTURE),
    DR_CAPTURE -> (jtag.tms ? DR_EXIT1  | DR_SHIFT),
    DR_SHIFT   -> (jtag.tms ? DR_EXIT1  | DR_SHIFT),
    DR_EXIT1   -> (jtag.tms ? DR_UPDATE | DR_PAUSE),
    DR_PAUSE   -> (jtag.tms ? DR_EXIT2  | DR_PAUSE),
    DR_EXIT2   -> (jtag.tms ? DR_UPDATE | DR_SHIFT),
    DR_UPDATE  -> (jtag.tms ? DR_SELECT | IDLE)
  )
}

Note

The randBoot() on state make it initialized with a random state. It’s only for simulation purpose.

JTAG TAP

Let’s implement the core of the JTAG TAP, without any instruction, just the base manage the instruction register (IR) and the bypass.

class JtagTap(val jtag: Jtag, instructionWidth: Int) extends Area{
  val fsm = new JtagFsm(jtag)
  val instruction = Reg(Bits(instructionWidth bit))
  val instructionShift = Reg(Bits(instructionWidth bit))
  val bypass = Reg(Bool)

  jtag.tdo := bypass

  switch(fsm.state) {
    is(JtagState.IR_CAPTURE) {
      instructionShift := instruction
    }
    is(JtagState.IR_SHIFT) {
      instructionShift := (jtag.tdi ## instructionShift) >> 1
      jtag.tdo := instructionShift.lsb
    }
    is(JtagState.IR_UPDATE) {
      instruction := instructionShift
    }
    is(JtagState.DR_SHIFT) {
      bypass := jtag.tdi
    }
  }
}

Jtag instructions

Now that the JTAG TAP core is done, we can think about how to implement JTAG instructions by an reusable way.

JTAG TAP class interface

First we need to define how an instruction could interact with the JTAG TAP core. We could of course directly take the JtagTap area, but it’s not very nice because is some situation the JTAG TAP core is provided by another IP (Altera virtual JTAG for example).

So let’s define a simple and abstract interface between the JTAG TAP core and instructions :

trait JtagTapAccess {
  def getTdi : Bool
  def getTms : Bool
  def setTdo(value : Bool) : Unit

  def getState : JtagState.T
  def getInstruction() : Bits
  def setInstruction(value : Bits) : Unit
}

Then let’s the JtagTap implement this abstract interface :

class JtagTap(val jtag: Jtag, ...) extends Area with JtagTapAccess{
  ...

  //JtagTapAccess impl
  override def getTdi: Bool = jtag.tdi
  override def setTdo(value: Bool): Unit = jtag.tdo := value
  override def getTms: Bool = jtag.tms

  override def getState: JtagState.T = fsm.state
  override def getInstruction(): Bits = instruction
  override def setInstruction(value: Bits): Unit = instruction := value
}

Base class

Let’s define a useful base class for JTAG instruction that provide some callback (doCapture/doShift/doUpdate/doReset) depending the selected instruction and the state of the JTAG TAP :

class JtagInstruction(tap: JtagTapAccess,val instructionId: Bits) extends Area {
  def doCapture(): Unit = {}
  def doShift(): Unit = {}
  def doUpdate(): Unit = {}
  def doReset(): Unit = {}

  val instructionHit = tap.getInstruction === instructionId

  Component.current.addPrePopTask(() => {
    when(instructionHit) {
      when(tap.getState === JtagState.DR_CAPTURE) {
        doCapture()
      }
      when(tap.getState === JtagState.DR_SHIFT) {
        doShift()
      }
      when(tap.getState === JtagState.DR_UPDATE) {
        doUpdate()
      }
    }
    when(tap.getState === JtagState.RESET) {
      doReset()
    }
  })
}

Note

About the Component.current.addPrePopTask(…) :
This allow you to call the given code at the end of the current component construction. Because of object oriented nature of JtagInstruction, doCapture, doShift, doUpdate and doReset should not be called before children classes construction (because children classes will use it as a callback to do some logic)

Read instruction

Let’s implement an instruction that allow the JTAG to read a signal.

class JtagInstructionRead[T <: Data](data: T) (tap: JtagTapAccess,instructionId: Bits)extends JtagInstruction(tap,instructionId) {
  val shifter = Reg(Bits(data.getBitsWidth bit))

  override def doCapture(): Unit = {
    shifter := data.asBits
  }

  override def doShift(): Unit = {
    shifter := (tap.getTdi ## shifter) >> 1
    tap.setTdo(shifter.lsb)
  }
}

Write instruction

Let’s implement an instruction that allow the JTAG to write a register (and also read its current value).

class JtagInstructionWrite[T <: Data](data: T) (tap: JtagTapAccess,instructionId: Bits) extends JtagInstruction(tap,instructionId) {
  val shifter,store = Reg(Bits(data.getBitsWidth bit))

  override def doCapture(): Unit = {
    shifter := store
  }
  override def doShift(): Unit = {
    shifter := (tap.getTdi ## shifter) >> 1
    tap.setTdo(shifter.lsb)
  }
  override def doUpdate(): Unit = {
    store := shifter
  }

  data.assignFromBits(store)
}

Idcode instruction

Let’s implement the instruction that return a idcode to the JTAG and also, when a reset occur, set the instruction register (IR) to it own instructionId.

class JtagInstructionIdcode[T <: Data](value: Bits)(tap: JtagTapAccess, instructionId: Bits)extends JtagInstruction(tap,instructionId) {
  val shifter = Reg(Bits(32 bit))

  override def doShift(): Unit = {
    shifter := (tap.getTdi ## shifter) >> 1
    tap.setTdo(shifter.lsb)
  }

  override def doReset(): Unit = {
    shifter := value
    tap.setInstruction(instructionId)
  }
}

User friendly wrapper

Let’s add some user friendly function to the JtagTapAccess to make instructions instantiation easier .

trait JtagTapAccess {
  ...

  def idcode(value: Bits)(instructionId: Bits) =
    new JtagInstructionIdcode(value)(this,instructionId)

  def read[T <: Data](data: T)(instructionId: Bits)   =
    new JtagInstructionRead(data)(this,instructionId)

  def write[T <: Data](data: T,  cleanUpdate: Boolean = true, readable: Boolean = true)(instructionId: Bits) =
    new JtagInstructionWrite[T](data,cleanUpdate,readable)(this,instructionId)
}

Usage demonstration

And there we are, we can now very easily create an application specific JTAG TAP without having to write any logic or any interconnections.

class SimpleJtagTap extends Component {
  val io = new Bundle {
    val jtag    = slave(Jtag())
    val switchs = in  Bits(8 bit)
    val keys    = in  Bits(4 bit)
    val leds    = out Bits(8 bit)
  }

  val tap = new JtagTap(io.jtag, 8)
  val idcodeArea  = tap.idcode(B"x87654321") (instructionId=4)
  val switchsArea = tap.read(io.switchs)     (instructionId=5)
  val keysArea    = tap.read(io.keys)        (instructionId=6)
  val ledsArea    = tap.write(io.leds)       (instructionId=7)
}

This way of doing things (Generating hardware) could also be applied to, for example, generating an APB/AHB/AXI bus slave.