rocket-chip/src/main/scala/jtag/JtagStateMachine.scala

// See LICENSE.jtag for license details.

package jtag

import util.{AsyncResetRegVec}
import chisel3._
import chisel3.util._

object JtagState {
  sealed abstract class State(val id: Int) {
    def U: UInt = id.U(State.width.W)
  }

  object State {
    import scala.language.implicitConversions

    implicit def toInt(x: State) = x.id
    implicit def toBigInt(x: State):BigInt = x.id

    // TODO: this could be automatically generated with macros and stuff
    val all: Set[State] = Set(
      TestLogicReset,
      RunTestIdle,
      SelectDRScan,
      CaptureDR,
      ShiftDR,
      Exit1DR,
      PauseDR,
      Exit2DR,
      UpdateDR,
      SelectIRScan,
      CaptureIR,
      ShiftIR,
      Exit1IR,
      PauseIR,
      Exit2IR,
      UpdateIR
    )
    val width = log2Up(all.size)
    def chiselType() = UInt(width.W)
  }

  // States as described in 6.1.1.2, numeric assignments from example in Table 6-3
  case object TestLogicReset extends State(15)  // no effect on system logic, entered when TMS high for 5 TCK rising edges
  case object RunTestIdle extends State(12)  // runs active instruction (which can be idle)
  case object SelectDRScan extends State(7)
  case object CaptureDR extends State(6)  // parallel-load DR shifter when exiting this state (if required)
  case object ShiftDR extends State(2)  // shifts DR shifter from TDI towards TDO, last shift occurs on rising edge transition out of this state
  case object Exit1DR extends State(1)
  case object PauseDR extends State(3)  // pause DR shifting
  case object Exit2DR extends State(0)
  case object UpdateDR extends State(5)  // parallel-load output from DR shifter on TCK falling edge while in this state (not a rule?)
  case object SelectIRScan extends State(4)
  case object CaptureIR extends State(14)  // parallel-load IR shifter with fixed logic values and design-specific when exiting this state (if required)
  case object ShiftIR extends State(10)  // shifts IR shifter from TDI towards TDO, last shift occurs on rising edge transition out of this state
  case object Exit1IR extends State(9)
  case object PauseIR extends State(11)  // pause IR shifting
  case object Exit2IR extends State(8)
  case object UpdateIR extends State(13)  // latch IR shifter into IR (changes to IR may only occur while in this state, latch on TCK falling edge)
}

/** The JTAG state machine, implements spec 6.1.1.1a (Figure 6.1)
  *
  * Usage notes:
  * - 6.1.1.1b state transitions occur on TCK rising edge
  * - 6.1.1.1c actions can occur on the following TCK falling or rising edge
  *
  *
  */
class JtagStateMachine extends Module(override_reset=Some(false.B)) {
  class StateMachineIO extends Bundle {
    val tms = Input(Bool())
    val currState = Output(JtagState.State.chiselType())

    val jtag_reset = Input(Bool())
  }
  val io = IO(new StateMachineIO)

  val nextState = Wire(JtagState.State.chiselType())

  val currStateReg = Module (new AsyncResetRegVec(w = JtagState.State.width,
    init = JtagState.State.toInt(JtagState.TestLogicReset)))

  currStateReg.clock := clock
  currStateReg.reset := io.jtag_reset
  currStateReg.io.en := true.B
  currStateReg.io.d  := nextState

  val currState = currStateReg.io.q

  switch (currState) {
    is (JtagState.TestLogicReset.U) {
      nextState := Mux(io.tms, JtagState.TestLogicReset.U, JtagState.RunTestIdle.U)
    }
    is (JtagState.RunTestIdle.U) {
      nextState := Mux(io.tms, JtagState.SelectDRScan.U, JtagState.RunTestIdle.U)
    }
    is (JtagState.SelectDRScan.U) {
      nextState := Mux(io.tms, JtagState.SelectIRScan.U, JtagState.CaptureDR.U)
    }
    is (JtagState.CaptureDR.U) {
      nextState := Mux(io.tms, JtagState.Exit1DR.U, JtagState.ShiftDR.U)
    }
    is (JtagState.ShiftDR.U) {
      nextState := Mux(io.tms, JtagState.Exit1DR.U, JtagState.ShiftDR.U)
    }
    is (JtagState.Exit1DR.U) {
      nextState := Mux(io.tms, JtagState.UpdateDR.U, JtagState.PauseDR.U)
    }
    is (JtagState.PauseDR.U) {
      nextState := Mux(io.tms, JtagState.Exit2DR.U, JtagState.PauseDR.U)
    }
    is (JtagState.Exit2DR.U) {
      nextState := Mux(io.tms, JtagState.UpdateDR.U, JtagState.ShiftDR.U)
    }
    is (JtagState.UpdateDR.U) {
      nextState := Mux(io.tms, JtagState.SelectDRScan.U, JtagState.RunTestIdle.U)
    }
    is (JtagState.SelectIRScan.U) {
      nextState := Mux(io.tms, JtagState.TestLogicReset.U, JtagState.CaptureIR.U)
    }
    is (JtagState.CaptureIR.U) {
      nextState := Mux(io.tms, JtagState.Exit1IR.U, JtagState.ShiftIR.U)
    }
    is (JtagState.ShiftIR.U) {
      nextState := Mux(io.tms, JtagState.Exit1IR.U, JtagState.ShiftIR.U)
    }
    is (JtagState.Exit1IR.U) {
      nextState := Mux(io.tms, JtagState.UpdateIR.U, JtagState.PauseIR.U)
    }
    is (JtagState.PauseIR.U) {
      nextState := Mux(io.tms, JtagState.Exit2IR.U, JtagState.PauseIR.U)
    }
    is (JtagState.Exit2IR.U) {
      nextState := Mux(io.tms, JtagState.UpdateIR.U, JtagState.ShiftIR.U)
    }
    is (JtagState.UpdateIR.U) {
      nextState := Mux(io.tms, JtagState.SelectDRScan.U, JtagState.RunTestIdle.U)
    }
  }

  io.currState := currState
}