electronics

In my previous post, I briefly discussed the implementation of a pseudo-pong game on my Mojo FPGA developer board.

I was able to implement the VHDL version that I linked to later in the post and it works much better.

The biggest challenge I had with it was converting the two-button input to a potentiometer input. It turns out all I had to do was convert an array to an integer as input to the paddle's left position.

Here is a link to the article describing the implementation that I used:

FPGA Pong From FPGACenter.com.

The VHDL was a big challenge for me because I had to learn it as I went along. It works just like Verilog, but the syntax is so different.

Also, I modified my User Constraints File to make the RGB an array for compatibility with this particular implementation.

Here are the new custom constraints:

NET "hsync" LOC = P50 | IOSTANDARD = LVTTL;  
NET "vsync" LOC = P51 | IOSTANDARD = LVTTL;  
NET "rgb<2>" LOC = P41 | IOSTANDARD = LVTTL;  
NET "rgb<1>" LOC = P40 | IOSTANDARD = LVTTL;  
NET "rgb<0>" LOC = P35 | IOSTANDARD = LVTTL;

Here is the code containing my changes (in the vga_control and img_gen modules):

vga_control:

library IEEE;  
use IEEE.STD_LOGIC_1164.ALL;  
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity vga_control is  
     Port ( clk : in STD_LOGIC;
                start : in STD_LOGIC;
                reset : in STD_LOGIC;
                paddle : IN STD_LOGIC_VECTOR(9 downto 0);
                rgb : out STD_LOGIC_VECTOR (2 downto 0);
                h_s : out STD_LOGIC;
                v_s : out STD_LOGIC);
end vga_control;

architecture Behavioral of vga_control is

COMPONENT img_gen  
       PORT( clk : IN std_logic;
                     x_control : IN std_logic_vector(9 downto 0);
                     paddle : IN std_logic_vector(9 downto 0);
                     y_control : IN std_logic_vector(9 downto 0);
                     video_on : IN std_logic; 
                     rgb : OUT std_logic_vector(2 downto 0) );
END COMPONENT;

COMPONENT sync_mod  
       PORT( clk : IN std_logic;
                     reset : IN std_logic;
                     start : IN std_logic; 
                     y_control : OUT std_logic_vector(9 downto 0);
                     x_control : OUT std_logic_vector(9 downto 0);
                     h_s : OUT std_logic;
                     v_s : OUT std_logic;
                     video_on : OUT std_logic );
END COMPONENT;

signal x,y:std_logic_vector(9 downto 0);  
signal video:std_logic;

begin  
          U1: img_gen PORT MAP( clk =>clk , x_control => x, paddle => paddle , 
                                                         y_control => y, video_on =>video , rgb => rgb );

           U2: sync_mod PORT MAP( clk => clk, reset => reset, start => start, y_control => y, x_control =>x ,
                                                             h_s => h_s , v_s => v_s, video_on =>video );
end Behavioral;

My changes are on lines 9, 20, and 41. This sends the output from the potentiometer hooked up to the ADC on the Mojo to the img_gen module where it's converted to position data for the paddle control.

Here's the modified img_gen code:

library IEEE;  
use IEEE.STD_LOGIC_1164.ALL;  
use IEEE.STD_LOGIC_ARITH.ALL;  
use IEEE.STD_LOGIC_UNSIGNED.ALL;


entity img_gen is  
     Port ( clk : in STD_LOGIC;
                x_control : in STD_LOGIC_VECTOR(9 downto 0);
                paddle : in STD_LOGIC_VECTOR(9 downto 0);
                y_control : in STD_LOGIC_VECTOR(9 downto 0);
                video_on : in STD_LOGIC;
                rgb : out STD_LOGIC_VECTOR(2 downto 0));
end img_gen;

architecture Behavioral of img_gen is

--wall
constant wall_l:integer :=10;--the distance between wall and left side of screen  
constant wall_t:integer :=10;--the distance between wall and top side of screen  
constant wall_k:integer :=10;--wall thickness  
signal wall_on:std_logic;  
signal rgb_wall:std_logic_vector(2 downto 0); 

--bar
signal bar_l, bar_l_next: integer:=100;  
constant bar_t:integer :=420;--the distance between bar and top side of screen  
constant bar_k:integer :=10;--bar thickness  
constant bar_w:integer:=120;--bar width  
constant bar_v:integer:=10;--velocity of the bar  
signal bar_on:std_logic;  
signal rgb_bar:std_logic_vector(2 downto 0); 

--ball
signal ball_l,ball_l_next:integer :=100;--the distance between ball and left side of screen  
signal ball_t,ball_t_next:integer :=100; --the distance between ball and top side of screen  
constant ball_w:integer :=20;--ball Height  
constant ball_u:integer :=20;--ball width  
constant x_v,y_v:integer:=3;-- horizontal and vertical speeds of the ball  
signal ball_on:std_logic;  
signal rgb_ball:std_logic_vector(2 downto 0); 

--refreshing(1/60)
signal refresh_reg,refresh_next:integer;  
constant refresh_constant:integer:=830000;  
signal refresh_tick:std_logic;

--ball animation
signal xv_reg,xv_next:integer:=3;--variable of the horizontal speed  
signal yv_reg,yv_next:integer:=3;--variable of the vertical speed

--x,y pixel cursor
signal x,y:integer range 0 to 650;

--mux
signal vdbt:std_logic_vector(3 downto 0);

--buffer
signal rgb_reg,rgb_next:std_logic_vector(2 downto 0);

begin

--x,y pixel cursor
x <= conv_integer(x_control);  
y <= conv_integer(y_control );

--refreshing
process(clk)  
begin  
     if clk'event and clk='1' then
          refresh_reg<=refresh_next; 
     end if;
end process;  
refresh_next <= 0 when refresh_reg= refresh_constant else  
refresh_reg+1;  
refresh_tick <= '1' when refresh_reg = 0 else  
                           '0';
--register part
process(clk)  
begin  
     if clk'event and clk='1' then
         ball_l <= ball_l_next;
         ball_t <= ball_t_next;
         xv_reg <= xv_next;
         yv_reg <= yv_next;
         bar_l <= bar_l_next;
      end if;
end process;

--bar animation
process(refresh_tick,paddle)  
begin

    if refresh_tick= '1' then
       bar_l_next <= conv_integer(paddle);
    end if;
end process;

--ball animation
process(refresh_tick,ball_l,ball_t,xv_reg,yv_reg)  
begin  
     ball_l_next <=ball_l;
     ball_t_next <=ball_t;
     xv_next <= xv_reg;
     yv_next <= yv_reg;
     if refresh_tick = '1' then
        if ball_t > 400 and ball_l > (bar_l -ball_u) and ball_l < (bar_l +120) then --the ball hits the bar
           yv_next <= -y_v ;
       elsif ball_t < 35 then--The ball hits the wall
           yv_next <= y_v;
       end if;
       if ball_l < 10 then --The ball hits the left side of the screen
          xv_next <= x_v;
       elsif ball_l> 600 then 
          xv_next <= -x_v ; --The ball hits the right side of the screen
       end if; 
       ball_l_next <= ball_l + xv_reg;
       ball_t_next <= ball_t + yv_reg; 
    end if;
end process;

--wall object
wall_on <= '1' when x > wall_l and x < (640-wall_l) and y> wall_t and y < (wall_t+ wall_k) else  
                      '0'; 
rgb_wall <= "000";--Black  
--bar object
bar_on <= '1' when x > bar_l and x < (bar_l+bar_w) and y> bar_t and y < (bar_t+ bar_k) else  
                    '0'; 
rgb_bar <= "001";--blue

--ball object
ball_on <= '1' when x > ball_l and x < (ball_l+ball_u) and y> ball_t and y < (ball_t+ ball_w) else  
                     '0'; 
rgb_ball <= "010"; --Green

--buffer
process(clk)  
begin  
     if clk'event and clk = '1' then
         rgb_reg <= rgb_next;
     end if;
end process;

--mux
vdbt<=video_on & wall_on & bar_on &ball_on;  
with vdbt select  
     rgb_next <= "100" when "1000",--Background of the screen is red 
     rgb_wall when "1100",
     rgb_wall when "1101",
     rgb_bar when "1010",
     rgb_bar when "1011",
     rgb_ball when "1001",
      "000" when others;
--output
rgb <= rgb_reg;

end Behavioral;

My changes are on lines 10 and 95 to make the potentiometer control the paddle.

I recently found an old VGA monitor on the side of the road and it inspired me to see if I could get my Mojo FPGA board to output a VGA signal. I took the monitor apart and wired its VGA cable directly to my Mojo.

I read up on the VGA specification and learned that getting it to work was as easy as adding resistors to the Red, Green, and Blue outputs.

I decided to try my hand at implementing the PONG game that is outlined on fpga4fun.com.

There were some differences between that FPGA that they were using and my Mojo board, so I had to go through some trial-and-error.

For instance, the Verilog code that they have on the site depends on a 25MHz clock and my Mojo uses a 50MHz clock. I tried various ways of dividing the clock without success.

I finally found a great example using a 50MHz clock, but it wasn't quite right because it used 8-bits to encode the RGB values, 3 bits for red and green, and 2 bits for blue. I couldn't get this to work, so I found the code that it was based on and found 1 bit per color implementation. I was finally able to get a signal.

The rest of the time was spent implementing the Pong code from the original article. However, the article used a mouse input. I wanted to use a regular potentiometer (dial) to make it more like the "good old days". So, I wired up a pot to the Mojo and altered the Verilog to use it.

Honestly, my code turned out to be spaghetti code because I haven't quite learned how to modularize Verilog to my satisfaction.

I was able to get the paddle working and responding to movement of the potentiometer, but for some reason, the code for the ball just didn't work in my implementation. I'm guessing it has to do with the timing. I still haven't been able to get the ball working, but I was pretty happy that the paddle works.

The Mojo wired to the VGA cable and the paddle "controller"

The paddle "controller" - a simple 10k potentiometer

Here is the Verilog definition for the Pong module:

`timescale 1ns / 1ps  
module pong(clk, red, green, blue, hsync, vsync, pot_sample);

// Input clk, 50 MHz Oscillator
input clk;  
input [9:0]pot_sample;

// VGA outputs
output red;  
output green;  
output blue;  
output hsync;  
output vsync;     

reg [9:0] hcount;     // VGA horizontal counter  
reg [9:0] vcount;     // VGA vertical counter  
reg [2:0] data;          // RGB data

wire hcount_ov;  
wire vcount_ov;  
wire inDisplayArea;  
wire hsync;  
wire vsync;  
reg  vga_clk;

// VGA mode parameters
parameter hsync_end   = 10'd95,  
   hdat_begin  = 10'd143,
   hdat_end  = 10'd783,
   hpixel_end  = 10'd799,
   vsync_end  = 10'd1,
   vdat_begin  = 10'd34,
   vdat_end  = 10'd514,
   vline_end  = 10'd524;


always @(posedge clk)  
begin  
 vga_clk = ~vga_clk;
end

always @(posedge vga_clk)  
begin  
 if (hcount_ov)
  hcount <= 10'd0;
 else
  hcount <= hcount + 10'd1;
end

assign hcount_ov = (hcount == hpixel_end);

always @(posedge vga_clk)  
begin  
 if (hcount_ov)
 begin
  if (vcount_ov)
   vcount <= 10'd0;
  else
   vcount <= vcount + 10'd1;
 end
end  
assign  vcount_ov = (vcount == vline_end);

////////////////////////////////////////////////////////////////
// Pong
////////////////////////////////////////////////////////////////
// Handle paddle position
reg [9:0] PaddlePosition;

always @(posedge vga_clk) begin

        PaddlePosition <= hdat_begin + pot_sample;

    if(0)
    begin
      if(0)
      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
end

wire paddle = (hcount >= PaddlePosition + 8) && (hcount <= PaddlePosition+120) && ((vcount <= vdat_end - 15) && (vcount > vdat_end - 25));

// Draw a border around the screen
wire border = ((hcount >=  hdat_begin) && (hcount <  hdat_begin + 10)) || ((hcount >  hdat_end - 10) && (hcount <=  hdat_end))  
    || ((vcount >= vdat_begin) && (vcount < vdat_begin + 10)) || ((vcount > vdat_end - 10) && (vcount <= vdat_end));

//////////////////////////////////////////////////////////////////
reg [9:0] ballX;  
reg [9:0] ballY;  
reg ball_inX, ball_inY;

always @(posedge vga_clk)  
if(ball_inX==0) ball_inX <= ((hcount>ballX) && (hcount<=ballX+16)) & ball_inY; else ball_inX <= 0;//!(hcount==ballX+16);

always @(posedge vga_clk)  
if(ball_inY==0) ball_inY <= ((vcount>ballY) && (vcount<=ballY+16)); else ball_inY <= 0;//!(vcount==ballY+16);

wire ball = ball_inX & ball_inY;

/////////////////////////////////////////////////////////////////
wire BouncingObject = border | paddle; // active if the border or paddle is redrawing itself

reg ResetCollision;  
always @(posedge vga_clk) ResetCollision <= (vcount==0) & (hcount==0);  // active only once for every video frame

reg CollisionX1, CollisionX2, CollisionY1, CollisionY2;  
always @(posedge vga_clk) if(ResetCollision) CollisionX1<=0; else if(BouncingObject & (hcount==ballX   ) & (vcount==ballY+ 8)) CollisionX1<=1;  
always @(posedge vga_clk) if(ResetCollision) CollisionX2<=0; else if(BouncingObject & (hcount==ballX+16) & (vcount==ballY+ 8)) CollisionX2<=1;  
always @(posedge vga_clk) if(ResetCollision) CollisionY1<=0; else if(BouncingObject & (hcount==ballX+ 8) & (vcount==ballY   )) CollisionY1<=1;  
always @(posedge vga_clk) if(ResetCollision) CollisionY2<=0; else if(BouncingObject & (hcount==ballX+ 8) & (vcount==ballY+16)) CollisionY2<=1;

/////////////////////////////////////////////////////////////////
wire UpdateBallPosition = ResetCollision;  // update the ball position at the same time that we reset the collision detectors

reg ball_dirX, ball_dirY;  
always @(posedge vga_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 

/////////////////////////////////////////////////////////////////

// display area calculation
assign inDisplayArea = ((hcount >= hdat_begin) && (hcount < hdat_end))  
     && ((vcount >= vdat_begin) && (vcount < vdat_end));

assign hsync = (hcount > hsync_end);  
assign vsync = (vcount > vsync_end);

reg vga_R, vga_G, vga_B;

always @(posedge vga_clk)  
begin  
  vga_R <= BouncingObject | ball;
  vga_G <= BouncingObject | ball;
  vga_B <= BouncingObject | ball;
end

assign red = (inDisplayArea) ?  vga_R : 0;  
assign green = (inDisplayArea) ?  vga_G : 0;  
assign blue = (inDisplayArea) ?  vga_B : 0;      


// generate "image"
always @(posedge vga_clk)  
begin  
  data <= (vcount[2:0] ^ hcount[2:0]); 
end

endmodule

Line #6 is the direct input of the potentiometer from the Mojo's on-board Analog to Digital Converter (ADC). Here is the top file showing how it's all wired together:

module mojo_top(  
    // 50MHz clock input
     input clk,
    // Input from reset button (active low)
    input rst_n,
    // cclk input from AVR, high when AVR is ready
    input cclk,
    // Outputs to the 8 onboard LEDs
    output[7:0]led,
    // AVR SPI connections
    output spi_miso,
    input spi_ss,
    input spi_mosi,
    input spi_sck,
    // AVR ADC channel select
    output [3:0] spi_channel,
    // Serial connections
    input avr_tx, // AVR Tx => FPGA Rx
    output avr_rx, // AVR Rx => FPGA Tx
    input avr_rx_busy, // AVR Rx buffer full
     output  red,
    output  green,
    output  blue,
    output hsync, vsync
    );

wire rst = ~rst_n; // make reset active high

// these signals should be high-z when not used
assign spi_miso = 1'bz;  
assign avr_rx = 1'bz;  
assign spi_channel = 4'bzzzz;

assign led = 8'b0;

wire [3:0] channel;  
wire new_sample;  
wire [9:0] sample;  
wire [3:0] sample_channel;  
wire [9:0] pot_sample;

avr_interface avr_interface (  
 .clk(clk),
 .rst(rst),
 .cclk(cclk),
 .spi_miso(spi_miso),
 .spi_mosi(spi_mosi),
 .spi_sck(spi_sck),
 .spi_ss(spi_ss),
 .spi_channel(spi_channel),
 .tx(avr_rx),
 .rx(avr_tx),
 .channel(channel),
 .new_sample(new_sample),
 .sample(sample),
 .sample_channel(sample_channel),
 .tx_data(8'h00),
 .new_tx_data(1'b0),
 .tx_busy(),
 .tx_block(avr_rx_busy),
 .rx_data(),
 .new_rx_data()
);

 input_capture input_capture (
 .clk(clk),
 .rst(rst),
 .channel(channel),
 .new_sample(new_sample),
 .sample(sample),
 .sample_channel(sample_channel),
 .sample_out(pot_sample)
);

pong pongBoard(  
    .clk(clk), 
    .red(red),
    .green(green),
    .blue(blue),
    .hsync(hsync),
    .vsync(vsync),
    .pot_sample(pot_sample)
);

endmodule

And, finally, here are the custom user constraints that I used to wire the output pins up to the VGA connector:

NET "hsync" LOC = P50 | IOSTANDARD = LVTTL;  
NET "vsync" LOC = P51 | IOSTANDARD = LVTTL;  
NET "red" LOC = P41 | IOSTANDARD = LVTTL;  
NET "green" LOC = P40 | IOSTANDARD = LVTTL;  
NET "blue" LOC = P35 | IOSTANDARD = LVTTL;

I'd really like to get the ball working with this. My scoring will be different than traditional Pong as I am not yet ready to tackle the "AI" of the computer player. I'd like the player to score 1 point each time the ball hits the paddle and lose a point each time the ball hits the bottom wall. If the ball hits the bottom wall while the score is 0, then the game is over. I'll have to add in a reset button, but that shouldn't be much trouble.

I just found another implementation of Pong in VHDL that I may try later. It divides the 50MHz clock into a 25MHz clock using a mod 2 counter. I discuss my implementation of that code in another post.

I haven't written any VHDL, yet, so I'm still trying to wrap my head around it. It was hard enough switching from Mojo's Lucid language to Verilog as Lucid made things a bit easier. However, I felt that since there were a lot more people out there using Verilog that I would be better off learning that. Now I'm learning that there are even more people using VHDL.

Lucid, Verilog, and VHDL are a far cry from C#, C, and JavaScript; three languages I'm very familiar with. This has been a very beneficial lesson in parallel "programming".