Introduction

Briey is a little SoC designed for FPGA. It is available in the SpinalHDL library and some documentation could be find there

Its toplevel implementation is an interesting example, because it mix some design pattern that make it very easy to modify. Adding a new master or a new peripheral to the bus fabric could be done in the seconde.

This toplevel implementation could be consulted there : https://github.com/SpinalHDL/VexRiscv/blob/master/src/main/scala/vexriscv/demo/Briey.scala

There is the Briey toplevel hardware diagram :

Defining all IO

val io = new Bundle{
  //Clocks / reset
  val asyncReset = in Bool()
  val axiClk     = in Bool()
  val vgaClk     = in Bool()

  //Main components IO
  val jtag       = slave(Jtag())
  val sdram      = master(SdramInterface(IS42x320D.layout))

  //Peripherals IO
  val gpioA      = master(TriStateArray(32 bits))   //Each pin has it's individual output enable control
  val gpioB      = master(TriStateArray(32 bits))
  val uart       = master(Uart())
  val vga        = master(Vga(RgbConfig(5,6,5)))
}

Clock and resets

Briey has three clocks inputs :

axiClock
vgaClock
jtag.tck

And one reset input :

asyncReset

Which will finally give 5 ClockDomain (clock/reset couple) :

Name	Clock	Description
resetCtrlClockDomain	axiClock	Used by the reset controller, Flops of this clock domain are initialized by the FPGA bitstream
axiClockDomain	axiClock	Used by all component connected to the AXI and the APB interconnect
coreClockDomain	axiClock	The only difference with the axiClockDomain, is the fact that the reset could also be asserted by the debug module
vgaClockDomain	vgaClock	Used by the VGA controller backend as a pixel clock
jtagClockDomain	jtag.tck	Used to clock the frontend of the JTAG controller

Reset controller

First we need to define the reset controller clock domain, which has no reset wire, but use the FPGA bitstream loading to setup flipflops.

val resetCtrlClockDomain = ClockDomain(
  clock = io.axiClk,
  config = ClockDomainConfig(
    resetKind = BOOT
  )
)

Then we can define a simple reset controller under this clock domain.

val resetCtrl = new ClockingArea(resetCtrlClockDomain) {
  val axiResetUnbuffered  = False
  val coreResetUnbuffered = False

  //Implement an counter to keep the reset axiResetOrder high 64 cycles
  // Also this counter will automaticly do a reset when the system boot.
  val axiResetCounter = Reg(UInt(6 bits)) init(0)
  when(axiResetCounter =/= U(axiResetCounter.range -> true)){
    axiResetCounter := axiResetCounter + 1
    axiResetUnbuffered := True
  }
  when(BufferCC(io.asyncReset)){
    axiResetCounter := 0
  }

  //When an axiResetOrder happen, the core reset will as well
  when(axiResetUnbuffered){
    coreResetUnbuffered := True
  }

  //Create all reset used later in the design
  val axiReset  = RegNext(axiResetUnbuffered)
  val coreReset = RegNext(coreResetUnbuffered)
  val vgaReset  = BufferCC(axiResetUnbuffered)
}

Systems clock domains

Now that the reset controller is implemented, we can define clock domain for all part of Briey :

val axiClockDomain = ClockDomain(
  clock     = io.axiClk,
  reset     = resetCtrl.axiReset,
  frequency = FixedFrequency(50 MHz) //The frequency information is used by the SDRAM controller
)

val coreClockDomain = ClockDomain(
  clock = io.axiClk,
  reset = resetCtrl.coreReset
)

val vgaClockDomain = ClockDomain(
  clock = io.vgaClk,
  reset = resetCtrl.vgaReset
)

val jtagClockDomain = ClockDomain(
  clock = io.jtag.tck
)

Also all the core system of Briey will be defined into a axi clocked area :

val axi = new ClockingArea(axiClockDomain) {
  //Here will come the rest of Briey
}

Main components

Briey is constituted mainly by 4 main components :

One RISCV CPU
One SDRAM controller
One on chip memory
One JTAG controller

RISCV CPU

The RISCV CPU used in Briey as many parametrization possibilities :

val core = coreClockDomain {
  val coreConfig = CoreConfig(
    pcWidth = 32,
    addrWidth = 32,
    startAddress = 0x00000000,
    regFileReadyKind = sync,
    branchPrediction = dynamic,
    bypassExecute0 = true,
    bypassExecute1 = true,
    bypassWriteBack = true,
    bypassWriteBackBuffer = true,
    collapseBubble = false,
    fastFetchCmdPcCalculation = true,
    dynamicBranchPredictorCacheSizeLog2 = 7
  )

  //The CPU has a systems of plugin which allow to add new feature into the core.
  //Those extension are not directly implemented into the core, but are kind of additive logic patch defined in a separated area.
  coreConfig.add(new MulExtension)
  coreConfig.add(new DivExtension)
  coreConfig.add(new BarrelShifterFullExtension)

  val iCacheConfig = InstructionCacheConfig(
    cacheSize =4096,
    bytePerLine =32,
    wayCount = 1,  //Can only be one for the moment
    wrappedMemAccess = true,
    addressWidth = 32,
    cpuDataWidth = 32,
    memDataWidth = 32
  )

  //There is the instanciation of the CPU by using all those construction parameters
  new RiscvAxi4(
    coreConfig = coreConfig,
    iCacheConfig = iCacheConfig,
    dCacheConfig = null,
    debug = true,
    interruptCount = 2
  )
}

On chip RAM

The instanciation of the AXI4 on chip RAM is very simple.

In fact it’s not an AXI4 but an Axi4Shared, which mean that a ARW channel replace the AR and AW ones. This solution use less area while being fully interoperable with full AXI4.

val ram = Axi4SharedOnChipRam(
  dataWidth = 32,
  byteCount = 4 kB,
  idWidth = 4     //Specify the AXI4 ID width.
)

SDRAM controller

First you need to define the layout and timings of your SDRAM device. On the DE1-SOC, the SDRAM device is an IS42x320D one.

object IS42x320D {
  def layout = SdramLayout(
    bankWidth   = 2,
    columnWidth = 10,
    rowWidth    = 13,
    dataWidth   = 16
  )

  def timingGrade7 = SdramTimings(
    bootRefreshCount =   8,
    tPOW             = 100 us,
    tREF             =  64 ms,
    tRC              =  60 ns,
    tRFC             =  60 ns,
    tRAS             =  37 ns,
    tRP              =  15 ns,
    tRCD             =  15 ns,
    cMRD             =   2,
    tWR              =  10 ns,
    cWR              =   1
  )
}

Then you can used those definition to parametrize the SDRAM controller instantiation.

val sdramCtrl = Axi4SharedSdramCtrl(
  axiDataWidth = 32,
  axiIdWidth   = 4,
  layout       = IS42x320D.layout,
  timing       = IS42x320D.timingGrade7,
  CAS          = 3
)

JTAG controller

The JTAG controller could be used to access memories and debug the CPU from an PC.

val jtagCtrl = JtagAxi4SharedDebugger(SystemDebuggerConfig(
  memAddressWidth = 32,
  memDataWidth    = 32,
  remoteCmdWidth  = 1,
  jtagClockDomain = jtagClockDomain
))

Peripherals

Briey integrate some peripherals :

GPIO
Timer
UART
VGA

GPIO

val gpioACtrl = Apb3Gpio(
  gpioWidth = 32
)

val gpioBCtrl = Apb3Gpio(
  gpioWidth = 32
)

Timer

The Briey timer module is constituted of :

One prescaler
One 32 bits timer
Three 16 bits timers

All of them are packed into the BrieyTimerCtrl component.

val timerCtrl = BrieyTimerCtrl()

UART controller

First we need to define a configuration for our UART controller :

val uartCtrlConfig = UartCtrlMemoryMappedConfig(
  uartCtrlConfig = UartCtrlGenerics(
    dataWidthMax      = 8,
    clockDividerWidth = 20,
    preSamplingSize   = 1,
    samplingSize      = 5,
    postSamplingSize  = 2
  ),
  txFifoDepth = 16,
  rxFifoDepth = 16
)

Then we can use it to instantiate the UART controller

val uartCtrl = Apb3UartCtrl(uartCtrlConfig)

VGA controller

First we need to define a configuration for our VGA controller :

val vgaCtrlConfig = Axi4VgaCtrlGenerics(
  axiAddressWidth = 32,
  axiDataWidth    = 32,
  burstLength     = 8,           //In Axi words
  frameSizeMax    = 2048*1512*2, //In byte
  fifoSize        = 512,         //In axi words
  rgbConfig       = RgbConfig(5,6,5),
  vgaClock        = vgaClockDomain
)

Then we can use it to instantiate the VGA controller

val vgaCtrl = Axi4VgaCtrl(vgaCtrlConfig)

Bus interconnects

There is three interconnections components :

AXI4 crossbar
AXI4 to APB3 bridge
APB3 decoder

AXI4 to APB3 bridge

This bridge will be used to connect low bandwidth peripherals to the AXI crossbar.

val apbBridge = Axi4SharedToApb3Bridge(
  addressWidth = 20,
  dataWidth    = 32,
  idWidth      = 4
)

AXI4 crossbar

The AXI4 crossbar that interconnect AXI4 masters and slaves together is generated by using an factory. The concept of this factory is to create it, then call many function on it to configure it, and finaly call the build function to ask the factory to generate the corresponding hardware :

val axiCrossbar = Axi4CrossbarFactory()
// Where you will have to call function the the axiCrossbar factory to populate its configuration
axiCrossbar.build()

First you need to populate slaves interfaces :

//          Slave  -> (base address,  size) ,

axiCrossbar.addSlaves(
  ram.io.axi       -> (0x00000000L,   4 kB),
  sdramCtrl.io.axi -> (0x40000000L,  64 MB),
  apbBridge.io.axi -> (0xF0000000L,   1 MB)
)

Then you need to populate interconnections between slaves and masters :

//         Master -> List of slaves which are accessible

axiCrossbar.addConnections(
  core.io.i       -> List(ram.io.axi, sdramCtrl.io.axi),
  core.io.d       -> List(ram.io.axi, sdramCtrl.io.axi, apbBridge.io.axi),
  jtagCtrl.io.axi -> List(ram.io.axi, sdramCtrl.io.axi, apbBridge.io.axi),
  vgaCtrl.io.axi  -> List(            sdramCtrl.io.axi)
)

Then to reduce combinatorial path length and have a good design FMax, you can ask the factory to insert pipelining stages between itself a given master or slave :

Note

halfPipe / >> / << / >/-> in the following code are provided by the Stream bus library.

Some documentation could be find there. In short, it’s just some pipelining and interconnection stuff.

//Pipeline the connection between the crossbar and the apbBridge.io.axi
axiCrossbar.addPipelining(apbBridge.io.axi,(crossbar,bridge) => {
  crossbar.sharedCmd.halfPipe() >> bridge.sharedCmd
  crossbar.writeData.halfPipe() >> bridge.writeData
  crossbar.writeRsp             << bridge.writeRsp
  crossbar.readRsp              << bridge.readRsp
})

//Pipeline the connection between the crossbar and the sdramCtrl.io.axi
axiCrossbar.addPipelining(sdramCtrl.io.axi,(crossbar,ctrl) => {
  crossbar.sharedCmd.halfPipe()  >>  ctrl.sharedCmd
  crossbar.writeData            >/-> ctrl.writeData
  crossbar.writeRsp              <<  ctrl.writeRsp
  crossbar.readRsp               <<  ctrl.readRsp
})

APB3 decoder

The interconnection between the APB3 bridge and all peripherals is done via an APB3Decoder :

val apbDecoder = Apb3Decoder(
  master = apbBridge.io.apb,
  slaves = List(
    gpioACtrl.io.apb -> (0x00000, 4 kB),
    gpioBCtrl.io.apb -> (0x01000, 4 kB),
    uartCtrl.io.apb  -> (0x10000, 4 kB),
    timerCtrl.io.apb -> (0x20000, 4 kB),
    vgaCtrl.io.apb   -> (0x30000, 4 kB),
    core.io.debugBus -> (0xF0000, 4 kB)
  )
)

Misc

To connect all toplevel IO to components, the following code is required :

io.gpioA <> axi.gpioACtrl.io.gpio
io.gpioB <> axi.gpioBCtrl.io.gpio
io.jtag  <> axi.jtagCtrl.io.jtag
io.uart  <> axi.uartCtrl.io.uart
io.sdram <> axi.sdramCtrl.io.sdram
io.vga   <> axi.vgaCtrl.io.vga

And finally some connections between components are required like interrupts and core debug module resets

core.io.interrupt(0) := uartCtrl.io.interrupt
core.io.interrupt(1) := timerCtrl.io.interrupt

core.io.debugResetIn := resetCtrl.axiReset
when(core.io.debugResetOut){
  resetCtrl.coreResetUnbuffered := True
}