### VGA显示及相关的Verilog代码

#### 如何驱动一个VGA显示器？

#### 从FPGA的管脚中创建VGA视频信号

#### 频率产生

#### 第一个视频发生器
<code verilog>
reg [9:0] CounterX;
reg [8:0] CounterY;
wire CounterXmaxed = (CounterX==767);

always @(posedge clk)
if(CounterXmaxed)
  CounterX <= 0;
else
  CounterX <= CounterX + 1;

always @(posedge clk)
if(CounterXmaxed)
    CounterY <= CounterY + 1;
</code>


<code verilog>
reg vga_HS, vga_VS;
always @(posedge clk)
begin
  vga_HS <= (CounterX[9:4]==0);   // active for 16 clocks
  vga_VS <= (CounterY==0);   // active for 768 clocks
end

</code>

<code verilog>
assign R = CounterY[3] | (CounterX==256);
assign G = (CounterX[5] ^ CounterX[6]) | (CounterX==256);
assign B = CounterX[4] | (CounterX==256);
</code>

#### 绘制一个有用的图片
<code verilog>
module pong(clk, vga_h_sync, vga_v_sync, vga_R, vga_G, vga_B);
input clk;
output vga_h_sync, vga_v_sync, vga_R, vga_G, vga_B;

wire inDisplayArea;
wire [9:0] CounterX;
wire [8:0] CounterY;

hvsync_generator syncgen(.clk(clk), .vga_h_sync(vga_h_sync), .vga_v_sync(vga_v_sync),
                            .inDisplayArea(inDisplayArea), .CounterX(CounterX), .CounterY(CounterY));

// Draw a border around the screen
wire border = (CounterX[9:3]==0) || (CounterX[9:3]==79) || (CounterY[8:3]==0) || (CounterY[8:3]==59);
wire R = border;
wire G = border;
wire B = border;

reg vga_R, vga_G, vga_B;
always @(posedge clk)
begin
  vga_R <= R & inDisplayArea;
  vga_G <= G & inDisplayArea;
  vga_B <= B & inDisplayArea;
end

endmodule
</code>

#### 绘制一个Paddle
<code verilog>
reg [8:0] PaddlePosition;
reg [2:0] quadAr, quadBr;
always @(posedge clk) quadAr <= {quadAr[1:0], quadA};
always @(posedge clk) quadBr <= {quadBr[1:0], quadB};

always @(posedge clk)
if(quadAr[2] ^ quadAr[1] ^ quadBr[2] ^ quadBr[1])
begin
  if(quadAr[2] ^ quadBr[1])
  begin
    if(~&PaddlePosition)        // make sure the value doesn't overflow
      PaddlePosition <= PaddlePosition + 1;
  end
  else
  begin
    if(|PaddlePosition)        // make sure the value doesn't underflow
      PaddlePosition <= PaddlePosition - 1;
  end
end
</code>

<code verilog>

wire border = (CounterX[9:3]==0) || (CounterX[9:3]==79) || (CounterY[8:3]==0) || (CounterY[8:3]==59);
wire paddle = (CounterX>=PaddlePosition+8) && (CounterX<=PaddlePosition+120) && (CounterY[8:4]==27);

wire R = border | (CounterX[3] ^ CounterY[3]) | paddle;
wire G = border | paddle;
wire B = border | paddle;
</code>

#### 绘制一个球
<code verilog>
reg [9:0] ballX;
reg [8:0] ballY;
reg ball_inX, ball_inY;

always @(posedge clk)
if(ball_inX==0) ball_inX <= (CounterX==ballX) & ball_inY; else ball_inX <= !(CounterX==ballX+16);

always @(posedge clk)
if(ball_inY==0) ball_inY <= (CounterY==ballY); else ball_inY <= !(CounterY==ballY+16);

wire ball = ball_inX & ball_inY;
</code>

<code verilog>
wire border = (CounterX[9:3]==0) || (CounterX[9:3]==79) || (CounterY[8:3]==0) || (CounterY[8:3]==59);
wire paddle = (CounterX>=PaddlePosition+8) && (CounterX<=PaddlePosition+120) && (CounterY[8:4]==27);
wire BouncingObject = border | paddle; // active if the border or paddle is redrawing itself

reg CollisionX1, CollisionX2, CollisionY1, CollisionY2;
always @(posedge clk) if(BouncingObject & (CounterX==ballX ) & (CounterY==ballY+ 8)) CollisionX1<=1;
always @(posedge clk) if(BouncingObject & (CounterX==ballX+16) & (CounterY==ballY+ 8)) CollisionX2<=1;
always @(posedge clk) if(BouncingObject & (CounterX==ballX+ 8) & (CounterY==ballY )) CollisionY1<=1;
always @(posedge clk) if(BouncingObject & (CounterX==ballX+ 8) & (CounterY==ballY+16)) CollisionY2<=1;
</code>

<code verilog>
reg UpdateBallPosition;       // active only once for every video frame
always @(posedge clk) UpdateBallPosition <= (CounterY==500) & (CounterX==0);

reg ball_dirX, ball_dirY;
always @(posedge clk)
if(UpdateBallPosition)
begin
  if(~(CollisionX1 & CollisionX2))        // if collision on both X-sides, don't move in the X direction
  begin
    ballX <= ballX + (ball_dirX ? -1 : 1);
    if(CollisionX2) ball_dirX <= 1; else if(CollisionX1) ball_dirX <= 0;
  end

  if(~(CollisionY1 & CollisionY2))        // if collision on both Y-sides, don't move in the Y direction
  begin
    ballY <= ballY + (ball_dirY ? -1 : 1);
    if(CollisionY2) ball_dirY <= 1; else if(CollisionY1) ball_dirY <= 0;
  end
end
</code>

<code verilog>
wire R = BouncingObject | ball | (CounterX[3] ^ CounterY[3]);
wire G = BouncingObject | ball;
wire B = BouncingObject | ball;

reg vga_R, vga_G, vga_B;
always @(posedge clk)
begin
  vga_R <= R & inDisplayArea;
  vga_G <= G & inDisplayArea;
  vga_B <= B & inDisplayArea;
end
</code>