Clock domains

Introduction

In SpinalHDL, clock and reset signals can be combined to create a clock domain. Clock domains can be applied to some areas of the design and then all synchronous elements instantiated into those areas will then implicitly use this clock domain.

Clock domain application works like a stack, which means that if you are in a given clock domain you can still apply another clock domain locally.

Please note that a register captures its clock domain when the register is created, not when it is assigned. So please make sure to create them inside the desired ClockingArea.

Instantiation

The syntax to define a clock domain is as follows (using EBNF syntax):

ClockDomain(
  clock: Bool
  [,reset: Bool]
  [,softReset: Bool]
  [,clockEnable: Bool]
  [,frequency: IClockDomainFrequency]
  [,config: ClockDomainConfig]
)

This definition takes five parameters:

Argument	Description	Default
`clock`	Clock signal that defines the domain
`reset`	Reset signal. If a register exists which needs a reset and the clock domain doesn’t provide one, an error message will be displayed	null
`softReset`	Reset which infers an additional synchronous reset	null
`clockEnable`	The goal of this signal is to disable the clock on the whole clock domain without having to manually implement that on each synchronous element	null
`frequency`	Allows you to specify the frequency of the given clock domain and later read it in your design	UnknownFrequency
`config`	Specify the polarity of signals and the nature of the reset	Current config

An applied example to define a specific clock domain within the design is as follows:

val coreClock = Bool()
val coreReset = Bool()

// Define a new clock domain
val coreClockDomain = ClockDomain(coreClock, coreReset)

// Use this domain in an area of the design
val coreArea = new ClockingArea(coreClockDomain) {
  val coreClockedRegister = Reg(UInt(4 bits))
}

When an Area is not needed, it is also possible to apply the clock domain directly. Two syntaxes exist:

class Counters extends Component {
  val io = new Bundle {
    val enable = in Bool ()
    val freeCount, gatedCount, gatedCount2 = out UInt (4 bits)
  }
  val freeCounter = CounterFreeRun(16)
  io.freeCount := freeCounter.value

  val gatedClk = ClockDomain.current.readClockWire && io.enable
  val gated = ClockDomain(gatedClk, ClockDomain.current.readResetWire)

  // Here the "gated" clock domain is applied on "gatedCounter" and "gatedCounter2"
  val gatedCounter = gated(CounterFreeRun(16))
  io.gatedCount := gatedCounter.value
  val gatedCounter2 = gated on CounterFreeRun(16)
  io.gatedCount2 := gatedCounter2.value

  assert(gatedCounter.value === gatedCounter2.value, "gated count mismatch")
}

Configuration

In addition to constructor parameters, the following elements of each clock domain are configurable via a ClockDomainConfigclass:

Property	Valid values
`clockEdge`	`RISING`, `FALLING`
`resetKind`	`ASYNC`, `SYNC`, and `BOOT` which is supported by some FPGAs (where FF values are loaded by the bitstream)
`resetActiveLevel`	`HIGH`, `LOW`
`softResetActiveLevel`	`HIGH`, `LOW`
`clockEnableActiveLevel`	`HIGH`, `LOW`

class CustomClockExample extends Component {
  val io = new Bundle {
    val clk    = in Bool()
    val resetn = in Bool()
    val result = out UInt (4 bits)
  }

  // Configure the clock domain
  val myClockDomain = ClockDomain(
    clock  = io.clk,
    reset  = io.resetn,
    config = ClockDomainConfig(
      clockEdge        = RISING,
      resetKind        = ASYNC,
      resetActiveLevel = LOW
    )
  )

  // Define an Area which use myClockDomain
  val myArea = new ClockingArea(myClockDomain) {
    val myReg = Reg(UInt(4 bits)) init(7)

    myReg := myReg + 1

    io.result := myReg
  }
}

By default, a ClockDomain is applied to the whole design. The configuration of this default domain is:

Clock : rising edge
Reset : asynchronous, active high
No clock enable

This corresponds to the following ClockDomainConfig:

val defaultCC = ClockDomainConfig(
  clockEdge        = RISING,
  resetKind        = ASYNC,
  resetActiveLevel = HIGH
)

Internal clock

An alternative syntax to create a clock domain is the following:

ClockDomain.internal(
  name: String,
  [config: ClockDomainConfig,]
  [withReset: Boolean,]
  [withSoftReset: Boolean,]
  [withClockEnable: Boolean,]
  [frequency: IClockDomainFrequency]
)

This definition takes six parameters:

Argument	Description	Default
`name`	Name of clk and reset signal
`config`	Specify polarity of signals and the nature of the reset	Current config
`withReset`	Add a reset signal	true
`withSoftReset`	Add a soft reset signal	false
`withClockEnable`	Add a clock enable	false
`frequency`	Frequency of the clock domain	UnknownFrequency

The advantage of this approach is to create clock and reset signals with a known/specified name instead of an inherited one.

Once created, you have to assign the ClockDomain’s signals, as shown in the example below:

class InternalClockWithPllExample extends Component {
  val io = new Bundle {
    val clk100M = in Bool()
    val aReset  = in Bool()
    val result  = out UInt (4 bits)
  }
  // myClockDomain.clock will be named myClockName_clk
  // myClockDomain.reset will be named myClockName_reset
  val myClockDomain = ClockDomain.internal("myClockName")

  // Instantiate a PLL (probably a BlackBox)
  val pll = new Pll()
  pll.io.clkIn := io.clk100M

  // Assign myClockDomain signals with something
  myClockDomain.clock := pll.io.clockOut
  myClockDomain.reset := io.aReset || !pll.io.

  // Do whatever you want with myClockDomain
  val myArea = new ClockingArea(myClockDomain) {
    val myReg = Reg(UInt(4 bits)) init(7)
    myReg := myReg + 1

    io.result := myReg
  }
}

External clock

You can define a clock domain which is driven by the outside anywhere in your source. It will then automatically add clock and reset wires from the top level inputs to all synchronous elements.

ClockDomain.external(
  name: String,
  [config: ClockDomainConfig,]
  [withReset: Boolean,]
  [withSoftReset: Boolean,]
  [withClockEnable: Boolean,]
  [frequency: IClockDomainFrequency]
)

The arguments to the ClockDomain.external function are exactly the same as in the ClockDomain.internal function. Below is an example of a design using ClockDomain.external:

class ExternalClockExample extends Component {
  val io = new Bundle {
    val result = out UInt (4 bits)
  }

  // On the top level you have two signals  :
  //     myClockName_clk and myClockName_reset
  val myClockDomain = ClockDomain.external("myClockName")

  val myArea = new ClockingArea(myClockDomain) {
    val myReg = Reg(UInt(4 bits)) init(7)
    myReg := myReg + 1

    io.result := myReg
  }
}

Signal priorities in HDL generation

In the current version, reset and clock enable signals have different priorities. Their order is : asyncReset, clockEnable, syncReset and softReset.

Please be careful that clockEnable has a higher priority than syncReset. If you do a sync reset when the clockEnable is disabled (especially at the beginning of a simulation), the gated registers will not be reseted.

Here is an example:

val clockedArea = new ClockEnableArea(clockEnable) {
  val reg = RegNext(io.input) init False
}

It will generate VerilogHDL codes like:

always @(posedge clk) begin
  if(clockedArea_newClockEnable) begin
    if(!resetn) begin
      clockedArea_reg <= 1'b0;
    end else begin
      clockedArea_reg <= io_input;
    end
  end
end

If that behaviour is problematic, one workaround is to use a when statement as a clock enable instead of using the ClockDomain.enable feature. This is open for future improvements.

Context

You can retrieve in which clock domain you are by calling ClockDomain.current anywhere.

The returned ClockDomain instance has the following functions that can be called:

name	Description	Return
frequency.getValue	Return the frequency of the clock domain	Double
hasReset	Return if the clock domain has a reset signal	Boolean
hasSoftReset	Return if the clock domain has a soft reset signal	Boolean
hasClockEnable	Return if the clock domain has a clock enable signal	Boolean
readClockWire	Return a signal derived from the clock signal	Bool
readResetWire	Return a signal derived from the soft reset signal	Bool
readSoftResetWire	Return a signal derived from the reset signal	Bool
readClockEnableWire	Return a signal derived from the clock enable signal	Bool
isResetActive	Return True when the reset is active	Bool
isSoftResetActive	Return True when the soft reset is active	Bool
isClockEnableActive	Return True when the clock enable is active	Bool

An example is included below where a UART controller uses the frequency specification to set its clock divider:

val coreClockDomain = ClockDomain(coreClock, coreReset, frequency=FixedFrequency(100e6))

val coreArea = new ClockingArea(coreClockDomain) {
  val ctrl = new UartCtrl()
  ctrl.io.config.clockDivider := (coreClk.frequency.getValue / 57.6e3 / 8).toInt
}

Clock domain crossing

SpinalHDL checks at compile time that there are no unwanted/unspecified cross clock domain signal reads. If you want to read a signal that is emitted by another ClockDomain area, you should add the crossClockDomain tag to the destination signal as depicted in the following example:

//             _____                        _____             _____
//            |     |  (crossClockDomain)  |     |           |     |
//  dataIn -->|     |--------------------->|     |---------->|     |--> dataOut
//            | FF  |                      | FF  |           | FF  |
//  clkA   -->|     |              clkB -->|     |   clkB -->|     |
//  rstA   -->|_____|              rstB -->|_____|   rstB -->|_____|



// Implementation where clock and reset pins are given by components' IO
class CrossingExample extends Component {
  val io = new Bundle {
    val clkA = in Bool()
    val rstA = in Bool()

    val clkB = in Bool()
    val rstB = in Bool()

    val dataIn  = in Bool()
    val dataOut = out Bool()
  }

  // sample dataIn with clkA
  val area_clkA = new ClockingArea(ClockDomain(io.clkA,io.rstA)) {
    val reg = RegNext(io.dataIn) init(False)
  }

  // 2 register stages to avoid metastability issues
  val area_clkB = new ClockingArea(ClockDomain(io.clkB,io.rstB)) {
    val buf0   = RegNext(area_clkA.reg) init(False) addTag(crossClockDomain)
    val buf1   = RegNext(buf0)          init(False)
  }

  io.dataOut := area_clkB.buf1
}


// Alternative implementation where clock domains are given as parameters
class CrossingExample(clkA : ClockDomain,clkB : ClockDomain) extends Component {
  val io = new Bundle {
    val dataIn  = in Bool()
    val dataOut = out Bool()
  }

  // sample dataIn with clkA
  val area_clkA = new ClockingArea(clkA) {
    val reg = RegNext(io.dataIn) init(False)
  }

  // 2 register stages to avoid metastability issues
  val area_clkB = new ClockingArea(clkB) {
    val buf0   = RegNext(area_clkA.reg) init(False) addTag(crossClockDomain)
    val buf1   = RegNext(buf0)          init(False)
  }

  io.dataOut := area_clkB.buf1
}

In general, you can use 2 or more flip-flop driven by the destination clock domain to prevent metastability. The BufferCC(input: T, init: T = null, bufferDepth: Int = 2) function provided in spinal.lib._ will instantiate the necessary flip-flops (the number of flip-flops will depends on the bufferDepth parameter) to mitigate the phenomena.

class CrossingExample(clkA : ClockDomain,clkB : ClockDomain) extends Component {
  val io = new Bundle {
    val dataIn  = in Bool()
    val dataOut = out Bool()
  }

  // sample dataIn with clkA
  val area_clkA = new ClockingArea(clkA) {
    val reg = RegNext(io.dataIn) init(False)
  }

  // BufferCC to avoid metastability issues
  val area_clkB = new ClockingArea(clkB) {
    val buf1   = BufferCC(area_clkA.reg, False)
  }

  io.dataOut := area_clkB.buf1
}

Warning

The BufferCC function is only for signals of type Bit, or Bits operating as Gray-coded counters (only 1 bit-flip per clock cycle), and can not used for multi-bit cross-domain processes. For multi-bit cases, it is recommended to use StreamFifoCC for high bandwidth requirements, or use StreamCCByToggle to reduce resource usage in cases where bandwidth is not critical.

Special clocking Areas

Slow Area

A SlowArea is used to create a new clock domain area which is slower than the current one:

class TopLevel extends Component {

  // Use the current clock domain : 100MHz
  val areaStd = new Area {
    val counter = out(CounterFreeRun(16).value)
  }

  // Slow the current clockDomain by 4 : 25 MHz
  val areaDiv4 = new SlowArea(4) {
    val counter = out(CounterFreeRun(16).value)
  }

  // Slow the current clockDomain to 50MHz
  val area50Mhz = new SlowArea(50 MHz) {
    val counter = out(CounterFreeRun(16).value)
  }
}

def main(args: Array[String]) {
  new SpinalConfig(
    defaultClockDomainFrequency = FixedFrequency(100 MHz)
  ).generateVhdl(new TopLevel)
}

BootReset

clockDomain.withBootReset() could specify register’s resetkinde as boot. clockDomain.withSyncReset() could specify register’s resetkinde as Sync-reset.

class  Top extends Component {
    val io = new Bundle {
      val data = in Bits(8 bit)
      val a, b, c, d = out Bits(8 bit)
    }
    io.a  :=  RegNext(io.data) init 0
    io.b  :=  clockDomain.withBootReset()  on RegNext(io.data) init 0
    io.c  :=  clockDomain.withSyncReset()  on RegNext(io.data) init 0
    io.d  :=  clockDomain.withAsyncReset() on RegNext(io.data) init 0
}
SpinalVerilog(new Top)

ResetArea

A ResetArea is used to create a new clock domain area where a special reset signal is combined with the current clock domain reset:

class TopLevel extends Component {

  val specialReset = Bool()

  // The reset of this area is done with the specialReset signal
  val areaRst_1 = new ResetArea(specialReset, false) {
    val counter = out(CounterFreeRun(16).value)
  }

  // The reset of this area is a combination between the current reset and the specialReset
  val areaRst_2 = new ResetArea(specialReset, true) {
    val counter = out(CounterFreeRun(16).value)
  }
}

ClockEnableArea

A ClockEnableArea is used to add an additional clock enable in the current clock domain:

class TopLevel extends Component {

  val clockEnable = Bool()

  // Add a clock enable for this area
  val area_1 = new ClockEnableArea(clockEnable) {
    val counter = out(CounterFreeRun(16).value)
  }
}