UART

Specification

This UART controller tutorial is based on this implementation.

This implementation is characterized by:

ClockDivider/Parity/StopBit/DataLength configs are set by the component inputs.
RXD input is filtered by using a sampling window of N samples and a majority vote.

Interfaces of this UartCtrl are:

Name	Type	Description
config	UartCtrlConfig	Give all configurations to the controller
write	Stream[Bits]	Port used by the system to give transmission order to the controller
read	Flow[Bits]	Port used by the controller to notify the system about a successfully received frame
uart	Uart	Uart interface with rxd / txd

Data structures

Before implementing the controller itself we need to define some data structures.

Controller construction parameters

Name	Type	Description
dataWidthMax	Int	Maximum number of data bits that could be sent using a single UART frame
clockDividerWidth	Int	Number of bits that the clock divider has
preSamplingSize	Int	Number of samples to drop at the beginning of the sampling window
samplingSize	Int	Number of samples use at the middle of the window to get the filtered RXD value
postSamplingSize	Int	Number of samples to drop at the end of the sampling window

To make the implementation easier let’s assume that preSamplingSize + samplingSize + postSamplingSize is always a power of two.

Instead of adding each construction parameters (generics) to UartCtrl one by one, we can group them inside a class that will be used as single parameter of UartCtrl.

case class UartCtrlGenerics( dataWidthMax: Int = 8,
                             clockDividerWidth: Int = 20, // baudrate = Fclk / rxSamplePerBit / clockDividerWidth
                             preSamplingSize: Int = 1,
                             samplingSize: Int = 5,
                             postSamplingSize: Int = 2) {
  val rxSamplePerBit = preSamplingSize + samplingSize + postSamplingSize
  assert(isPow2(rxSamplePerBit))
  if ((samplingSize % 2) == 0)
    SpinalWarning(s"It's not nice to have a odd samplingSize value (because of the majority vote)")
}

UART bus

Let’s define a UART bus without flow control.

case class Uart() extends Bundle with IMasterSlave {
  val txd = Bool
  val rxd = Bool

  override def asMaster(): Unit = {
    out(txd)
    in(rxd)
  }
}

UART configuration enums

Let’s define parity and stop bit enumerations.

object UartParityType extends SpinalEnum(sequancial) {
  val NONE, EVEN, ODD = newElement()
}

object UartStopType extends SpinalEnum(sequancial) {
  val ONE, TWO = newElement()
  def toBitCount(that : T) : UInt = (that === ONE) ? U"0" | U"1"
}

UartCtrl configuration Bundles

Let’s define Bundles that will be used as IO elements to setup UartCtrl.

case class UartCtrlFrameConfig(g: UartCtrlGenerics) extends Bundle {
  val dataLength = UInt(log2Up(g.dataWidthMax) bit) //Bit count = dataLength + 1
  val stop       = UartStopType()
  val parity     = UartParityType()
}

case class UartCtrlConfig(g: UartCtrlGenerics) extends Bundle {
  val frame        = UartCtrlFrameConfig(g)
  val clockDivider = UInt (g.clockDividerWidth bit) //see UartCtrlGenerics.clockDividerWidth for calculation

  def setClockDivider(baudrate : Double,clkFrequency : Double = ClockDomain.current.frequency.getValue) : Unit = {
    clockDivider := (clkFrequency / baudrate / g.rxSamplePerBit).toInt
  }
}

Implementation

In UartCtrl, 3 things will be instantiated:

One clock divider that generates a tick pulse at the UART RX sampling rate.
One UartCtrlTx Component
One UartCtrlRx Component

UartCtrlTx

The interfaces of this Component are the following :

Name	Type	Description
configFrame	UartCtrlFrameConfig	Contains data bit width count and party/stop bits configurations
samplingTick	Bool	Time reference that pulses `rxSamplePerBit` times per UART baud
write	Stream[Bits]	Port used by the system to give transmission orders to the controller
txd	Bool	UART txd pin

Let’s define the enumeration that will be used to store the state of UartCtrlTx:

object UartCtrlTxState extends SpinalEnum {
  val IDLE, START, DATA, PARITY, STOP = newElement()
}

Let’s define the skeleton of UartCtrlTx:

class UartCtrlTx(g : UartCtrlGenerics) extends Component {
  import g._

  val io = new Bundle {
    val configFrame  = in(UartCtrlFrameConfig(g))
    val samplingTick = in Bool
    val write        = slave Stream (Bits(dataWidthMax bit))
    val txd          = out Bool
  }

  // Provide one clockDivider.tick each rxSamplePerBit pulses of io.samplingTick
  // Used by the stateMachine as a baud rate time reference
  val clockDivider = new Area {
    val counter = Reg(UInt(log2Up(rxSamplePerBit) bits)) init(0)
    val tick = False
    ..
  }

  // Count up each clockDivider.tick, used by the state machine to count up data bits and stop bits
  val tickCounter = new Area {
    val value = Reg(UInt(Math.max(dataWidthMax, 2) bit))
    def reset() = value := 0
    ..
  }

  val stateMachine = new Area {
    import UartCtrlTxState._

    val state = RegInit(IDLE)
    val parity = Reg(Bool)
    val txd = True
    ..
    switch(state) {
      ..
    }
  }

  io.txd := RegNext(stateMachine.txd) init(True)
}

And here is the complete implementation:

class UartCtrlTx(g : UartCtrlGenerics) extends Component {
  import g._

  val io = new Bundle {
    val configFrame  = in(UartCtrlFrameConfig(g))
    val samplingTick = in Bool
    val write        = slave Stream (Bits(dataWidthMax bit))
    val txd          = out Bool
  }

  // Provide one clockDivider.tick each rxSamplePerBit pulse of io.samplingTick
  // Used by the stateMachine as a baud rate time reference
  val clockDivider = new Area {
    val counter = Reg(UInt(log2Up(rxSamplePerBit) bits)) init(0)
    val tick = False
    when(io.samplingTick) {
      counter := counter - 1
      tick := counter === 0
    }
  }

  // Count up each clockDivider.tick, used by the state machine to count up data bits and stop bits
  val tickCounter = new Area {
    val value = Reg(UInt(Math.max(dataWidthMax, 2) bit))
    def reset() = value := 0

    when(clockDivider.tick) {
      value := value + 1
    }
  }

  val stateMachine = new Area {
    import UartCtrlTxState._

    val state = RegInit(IDLE)
    val parity = Reg(Bool)
    val txd = True

    when(clockDivider.tick) {
      parity := parity ^ txd
    }

    io.write.ready := False
    switch(state) {
      is(IDLE){
        when(io.write.valid && clockDivider.tick){
          state := START
        }
      }
      is(START) {
        txd := False
        when(clockDivider.tick) {
          state := DATA
          parity := io.configFrame.parity === UartParityType.ODD
          tickCounter.reset()
        }
      }
      is(DATA) {
        txd := io.write.payload(tickCounter.value)
        when(clockDivider.tick) {
          when(tickCounter.value === io.configFrame.dataLength) {
            io.write.ready := True
            tickCounter.reset()
            when(io.configFrame.parity === UartParityType.NONE) {
              state := STOP
            } otherwise {
              state := PARITY
            }
          }
        }
      }
      is(PARITY) {
        txd := parity
        when(clockDivider.tick) {
          state := STOP
          tickCounter.reset()
        }
      }
      is(STOP) {
        when(clockDivider.tick) {
          when(tickCounter.value === toBitCount(io.configFrame.stop)) {
            state := io.write.valid ? START | IDLE
          }
        }
      }
    }
  }

  io.txd := RegNext(stateMachine.txd, True)
}

UartCtrlRx

The interfaces of this Component are the following:

Name	Type	Description
configFrame	UartCtrlFrameConfig	Contains data bit width and party/stop bits configurations
samplingTick	Bool	Time reference that pulses `rxSamplePerBit` times per UART baud
read	Flow[Bits]	Port used by the controller to notify the system about a successfully received frame
rxd	Bool	UART rxd pin, not synchronized with the current clock domain

Let’s define the enumeration that will be used to store the state of UartCtrlTx:

object UartCtrlRxState extends SpinalEnum {
  val IDLE, START, DATA, PARITY, STOP = newElement()
}

Let’s define the skeleton of the UartCtrlRx :

class UartCtrlRx(g : UartCtrlGenerics) extends Component {
  import g._
  val io = new Bundle {
    val configFrame  = in(UartCtrlFrameConfig(g))
    val samplingTick = in Bool
    val read         = master Flow (Bits(dataWidthMax bit))
    val rxd          = in Bool
  }

  // Implement the rxd sampling with a majority vote over samplingSize bits
  // Provide a new sampler.value each time sampler.tick is high
  val sampler = new Area {
    val syncroniser = BufferCC(io.rxd)
    val samples     = History(that=syncroniser,when=io.samplingTick,length=samplingSize)
    val value       = RegNext(MajorityVote(samples))
    val tick        = RegNext(io.samplingTick)
  }

  // Provide a bitTimer.tick each rxSamplePerBit
  // reset() can be called to recenter the counter over a start bit.
  val bitTimer = new Area {
    val counter = Reg(UInt(log2Up(rxSamplePerBit) bit))
    def reset() = counter := preSamplingSize + (samplingSize - 1) / 2 - 1)
    val tick = False
    ...
  }

  // Provide bitCounter.value that count up each bitTimer.tick, Used by the state machine to count data bits and stop bits
  // reset() can be called to reset it to zero
  val bitCounter = new Area {
    val value = Reg(UInt(Math.max(dataWidthMax, 2) bit))
    def reset() = value := 0
    ...
  }

  val stateMachine = new Area {
    import UartCtrlRxState._

    val state   = RegInit(IDLE)
    val parity  = Reg(Bool)
    val shifter = Reg(io.read.payload)
    ...
    switch(state) {
      ...
    }
  }
}

And here is the complete implementation:

class UartCtrlRx(g : UartCtrlGenerics) extends Component {
  import g._
  val io = new Bundle {
    val configFrame  = in(UartCtrlFrameConfig(g))
    val samplingTick = in Bool
    val read         = master Flow (Bits(dataWidthMax bit))
    val rxd          = in Bool
  }

  // Implement the rxd sampling with a majority vote over samplingSize bits
  // Provide a new sampler.value each time sampler.tick is high
  val sampler = new Area {
    val syncroniser = BufferCC(io.rxd)
    val samples     = History(that=syncroniser,when=io.samplingTick,length=samplingSize)
    val value       = RegNext(MajorityVote(samples))
    val tick        = RegNext(io.samplingTick)
  }

  // Provide a bitTimer.tick each rxSamplePerBit
  // reset() can be called to recenter the counter over a start bit.
  val bitTimer = new Area {
    val counter = Reg(UInt(log2Up(rxSamplePerBit) bit))
    def reset() = counter := preSamplingSize + (samplingSize - 1) / 2 - 1
    val tick = False
    when(sampler.tick) {
      counter := counter - 1
      when(counter === 0) {
        tick := True
      }
    }
  }

  // Provide bitCounter.value that count up each bitTimer.tick, Used by the state machine to count data bits and stop bits
  // reset() can be called to reset it to zero
  val bitCounter = new Area {
    val value = Reg(UInt(Math.max(dataWidthMax, 2) bit))
    def reset() = value := 0

    when(bitTimer.tick) {
      value := value + 1
    }
  }

  val stateMachine = new Area {
    import UartCtrlRxState._

    val state   = RegInit(IDLE)
    val parity  = Reg(Bool)
    val shifter = Reg(io.read.payload)

    //Parity calculation
    when(bitTimer.tick) {
      parity := parity ^ sampler.value
    }

    io.read.valid := False
    switch(state) {
      is(IDLE) {
        when(sampler.value === False) {
          state := START
          bitTimer.reset()
        }
      }
      is(START) {
        when(bitTimer.tick) {
          state := DATA
          bitCounter.reset()
          parity := io.configFrame.parity === UartParityType.ODD
          when(sampler.value === True) {
            state := IDLE
          }
        }
      }
      is(DATA) {
        when(bitTimer.tick) {
          shifter(bitCounter.value) := sampler.value
          when(bitCounter.value === io.configFrame.dataLength) {
            bitCounter.reset()
            when(io.configFrame.parity === UartParityType.NONE) {
              state := STOP
            } otherwise {
              state := PARITY
            }
          }
        }
      }
      is(PARITY) {
        when(bitTimer.tick) {
          state := STOP
          bitCounter.reset()
          when(parity =/= sampler.value) {
            state := IDLE
          }
        }
      }
      is(STOP) {
        when(bitTimer.tick) {
          when(!sampler.value) {
            state := IDLE
          }.elsewhen(bitCounter.value === toBitCount(io.configFrame.stop)) {
            state := IDLE
            io.read.valid := True
          }
        }
      }
    }
  }
  io.read.payload := stateMachine.shifter
}

UartCtrl

Let’s write UartCtrl that instantiates the UartCtrlRx and UartCtrlTx parts, generate the clock divider logic, and connect them to each other.

class UartCtrl(g : UartCtrlGenerics = UartCtrlGenerics()) extends Component {
  val io = new Bundle {
    val config = in(UartCtrlConfig(g))
    val write  = slave(Stream(Bits(g.dataWidthMax bit)))
    val read   = master(Flow(Bits(g.dataWidthMax bit)))
    val uart   = master(Uart())
  }

  val tx = new UartCtrlTx(g)
  val rx = new UartCtrlRx(g)

  //Clock divider used by RX and TX
  val clockDivider = new Area {
    val counter = Reg(UInt(g.clockDividerWidth bits)) init(0)
    val tick = counter === 0

    counter := counter - 1
    when(tick) {
      counter := io.config.clockDivider
    }
  }

  tx.io.samplingTick := clockDivider.tick
  rx.io.samplingTick := clockDivider.tick

  tx.io.configFrame := io.config.frame
  rx.io.configFrame := io.config.frame

  tx.io.write << io.write
  rx.io.read >> io.read

  io.uart.txd <> tx.io.txd
  io.uart.rxd <> rx.io.rxd
}

Simple usage

To synthesize a UartCtrl as 115200-N-8-1:

val uartCtrl: UartCtrl = UartCtrl(
  config = UartCtrlInitConfig(
    baudrate = 115200,
    dataLength = 7,  // 8 bits
    parity = UartParityType.NONE,
    stop = UartStopType.ONE
  )
)

If you are using txd pin only:

uartCtrl.io.uart.rxd := True  // High is the idle state for UART
txd := uartCtrl.io.uart.txd

On the contrary, if you are using rxd pin only:

val uartCtrl: UartCtrl = UartCtrl(
  config = UartCtrlInitConfig(
    baudrate = 115200,
    dataLength = 7,  // 8 bits
    parity = UartParityType.NONE,
    stop = UartStopType.ONE
  ),
  readonly = true
)

Example with test bench

Here is a top level example that does the followings things:

Instantiate UartCtrl and set its configuration to 921600 baud/s, no parity, 1 stop bit.
Each time a byte is received from the UART, it writes it on the leds output.
Every 2000 cycles, it sends the switches input value to the UART.

class UartCtrlUsageExample extends Component{
  val io = new Bundle{
    val uart = master(Uart())
    val switchs = in Bits(8 bits)
    val leds = out Bits(8 bits)
  }

  val uartCtrl = new UartCtrl()
  uartCtrl.io.config.setClockDivider(921600)
  uartCtrl.io.config.frame.dataLength := 7  //8 bits
  uartCtrl.io.config.frame.parity := UartParityType.NONE
  uartCtrl.io.config.frame.stop := UartStopType.ONE
  uartCtrl.io.uart <> io.uart

  //Assign io.led with a register loaded each time a byte is received
  io.leds := uartCtrl.io.read.toReg()

  //Write the value of switch on the uart each 2000 cycles
  val write = Stream(Bits(8 bits))
  write.valid := CounterFreeRun(2000).willOverflow
  write.payload := io.switchs
  write >-> uartCtrl.io.write
}


object UartCtrlUsageExample{
  def main(args: Array[String]) {
    SpinalVhdl(new UartCtrlUsageExample,defaultClockDomainFrequency=FixedFrequency(50e6))
  }
}

The following example is just a “mad one” but if you want to send a 0x55 header before sending the value of switches, you can replace the write generator of the preceding example by:

val write = Stream(Fragment(Bits(8 bits)))
write.valid := CounterFreeRun(4000).willOverflow
write.fragment := io.switchs
write.last := True
write.stage().insertHeader(0x55).toStreamOfFragment >> uartCtrl.io.write

Here you can get a simple VHDL testbench for this small UartCtrlUsageExample.

Bonus: Having fun with Stream

If you want to queue data received from the UART:

val uartCtrl = new UartCtrl()
val queuedReads = uartCtrl.io.read.toStream.queue(16)

If you want to add a queue on the write interface and do some flow control:

val uartCtrl = new UartCtrl()
val writeCmd = Stream(Bits(8 bits))
val stopIt = Bool
writeCmd.queue(16).haltWhen(stopIt) >> uartCtrl.io.write