VGA

Introduction

VGA interfaces are becoming an endangered species, but implementing a VGA controller is still a good exercise.

An explanation about the VGA protocol can be found here.

This VGA controller tutorial is based on this implementation.

Data structures

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

RGB color

First, we need a three channel color structure (Red, Green, Blue). This data structure will be used to feed the controller with pixels and also will be used by the VGA bus.

case class RgbConfig(rWidth : Int, gWidth : Int, bWidth : Int) {
  def getWidth = rWidth + gWidth + bWidth
}

case class Rgb(c: RgbConfig) extends Bundle {
  val r = UInt(c.rWidth bits)
  val g = UInt(c.gWidth bits)
  val b = UInt(c.bWidth bits)
}

VGA bus

io name	Driver	Description
vSync	master	Vertical synchronization, indicate the beginning of a new frame
hSync	master	Horizontal synchronization, indicate the beginning of a new line
colorEn	master	High when the interface is in the visible part
color	master	Carry the color, don’t care when colorEn is low

case class Vga(rgbConfig: RgbConfig) extends Bundle with IMasterSlave {
  val vSync = Bool()
  val hSync = Bool()

  val colorEn = Bool()
  val color   = Rgb(rgbConfig)

  override def asMaster() : Unit = this.asOutput()
}

This Vga Bundle uses the IMasterSlave trait, which allows you to create master/slave VGA interfaces using the following:

master(Vga(...))
slave(Vga(...))

VGA timings

The VGA interface is driven by using 8 different timings. Here is one simple example of a Bundle that is able to carry them.

case class VgaTimings(timingsWidth: Int) extends Bundle {
  val hSyncStart  = UInt(timingsWidth bits)
  val hSyncEnd    = UInt(timingsWidth bits)
  val hColorStart = UInt(timingsWidth bits)
  val hColorEnd   = UInt(timingsWidth bits)
  val vSyncStart  = UInt(timingsWidth bits)
  val vSyncEnd    = UInt(timingsWidth bits)
  val vColorStart = UInt(timingsWidth bits)
  val vColorEnd   = UInt(timingsWidth bits)
}

But this not a very good way to specify it because it is redundant for vertical and horizontal timings.

Let’s write it in a clearer way:

case class VgaTimingsHV(timingsWidth: Int) extends Bundle {
  val colorStart = UInt(timingsWidth bits)
  val colorEnd   = UInt(timingsWidth bits)
  val syncStart  = UInt(timingsWidth bits)
  val syncEnd    = UInt(timingsWidth bits)
}

case class VgaTimings(timingsWidth: Int) extends Bundle {
  val h = VgaTimingsHV(timingsWidth)
  val v = VgaTimingsHV(timingsWidth)
}

Then we can add some some functions to set these timings for specific resolutions and frame rates:

case class VgaTimingsHV(timingsWidth: Int) extends Bundle {
  val colorStart = UInt(timingsWidth bits)
  val colorEnd   = UInt(timingsWidth bits)
  val syncStart  = UInt(timingsWidth bits)
  val syncEnd    = UInt(timingsWidth bits)
}

case class VgaTimings(timingsWidth: Int) extends Bundle {
  val h = VgaTimingsHV(timingsWidth)
  val v = VgaTimingsHV(timingsWidth)

  def setAs_h640_v480_r60(): Unit = {
    h.syncStart := 96 - 1
    h.syncEnd := 800 - 1
    h.colorStart := 96 + 16 - 1
    h.colorEnd := 800 - 48 - 1
    v.syncStart := 2 - 1
    v.syncEnd := 525 - 1
    v.colorStart := 2 + 10 - 1
    v.colorEnd := 525 - 33 - 1
  }

  def setAs_h64_v64_r60(): Unit = {
    h.syncStart := 96 - 1
    h.syncEnd := 800 - 1
    h.colorStart := 96 + 16 - 1 + 288
    h.colorEnd := 800 - 48 - 1 - 288
    v.syncStart := 2 - 1
    v.syncEnd := 525 - 1
    v.colorStart := 2 + 10 - 1 + 208
    v.colorEnd := 525 - 33 - 1 - 208
  }
}

VGA Controller

Specification

io name	Direction	Description
softReset	in	Reset internal counters and keep the VGA interface inactive
timings	in	Specify VGA horizontal and vertical timings
pixels	slave	Stream of RGB colors that feeds the VGA controller
error	out	High when the pixels stream is too slow
frameStart	out	High when a new frame starts
vga	master	VGA interface

The controller does not integrate any pixel buffering. It directly takes them from the pixels Stream and puts them on the vga.color out at the right time. If pixels is not valid then error becomes high for one cycle.

Component and io definition

Let’s define a new VgaCtrl Component, which takes as RgbConfig and timingsWidth as parameters. Let’s give the bit width a default value of 12.

case class VgaCtrl(rgbConfig: RgbConfig, timingsWidth: Int = 12) extends Component {
  val io = new Bundle {
    val softReset = in Bool()
    val timings = in(VgaTimings(timingsWidth))
    val pixels = slave Stream Rgb(rgbConfig)

    val error = out Bool()
    val frameStart = out Bool()
    val vga = master(Vga(rgbConfig))
  }
...

Horizontal and vertical logic

The logic that generates horizontal and vertical synchronization signals is quite the same. It kind of resembles ~PWM~. The horizontal one counts up each cycle, while the vertical one use the horizontal synchronization signal as to increment.

Let’s define HVArea, which represents one ~PWM~ and then instantiate it two times: one for both horizontal and vertical synchronization.

case class VgaCtrl(rgbConfig: RgbConfig, timingsWidth: Int = 12) extends Component {
...
  case class HVArea(timingsHV: VgaTimingsHV, enable: Bool) extends Area {
    val counter = Reg(UInt(timingsWidth bits)) init 0

    val syncStart  = counter === timingsHV.syncStart
    val syncEnd    = counter === timingsHV.syncEnd
    val colorStart = counter === timingsHV.colorStart
    val colorEnd   = counter === timingsHV.colorEnd

    when(enable) {
      counter := counter + 1
      when(syncEnd) {
        counter := 0
      }
    }

    val sync    = RegInit(False) setWhen syncStart clearWhen syncEnd
    val colorEn = RegInit(False) setWhen colorStart clearWhen colorEnd

    when(io.softReset) {
      counter := 0
      sync    := False
      colorEn := False
    }
  }
  val h = HVArea(io.timings.h, True)
  val v = HVArea(io.timings.v, h.syncEnd)
...

As you can see, it’s done by using Area. This is to avoid the creation of a new Component which would have been much more verbose.

Interconnections

Now that we have timing generators for horizontal and vertical synchronization, we need to drive the outputs.

case class VgaCtrl(rgbConfig: RgbConfig, timingsWidth: Int = 12) extends Component {
...
  val colorEn = h.colorEn && v.colorEn
  io.pixels.ready := colorEn
  io.error := colorEn && ! io.pixels.valid

  io.frameStart := v.syncEnd

  io.vga.hSync := h.sync
  io.vga.vSync := v.sync
  io.vga.colorEn := colorEn
  io.vga.color := io.pixels.payload
...

Bonus

The VgaCtrl that was defined above is generic (not application specific). We can imagine a case where the system provides a Stream of Fragment of RGB, which means the system transmits pixels between start/end of picture indications.

In this case we can automatically manage the softReset input by asserting it when an error occurs, then wait for the end of the current pixels picture to deassert error.

Let’s add a function to VgaCtrl that can be called from the parent component to feed VgaCtrl by using this Stream of Fragment of RGB.

case class VgaCtrl(rgbConfig: RgbConfig, timingsWidth: Int = 12) extends Component {
...
  def feedWith(that : Stream[Fragment[Rgb]]): Unit = {
    io.pixels << that.toStreamOfFragment

    val error = RegInit(False)
    when(io.error) {
      error := True
    }
    when(that.isLast) {
      error := False
    }

    io.softReset := error
    when(error) {
      that.ready := True
    }
  }
}