My verilog VGA driver causes the screen to flicker (Basys2) - verilog

Question

I'm trying to recreate Adventure(1979) in Verilog and so far I have character movement, collision and map generation done. It didn't flicker that much before I separated the maps into modules now it flickers constantly. When I was looking up this issue, I found out that the clock on the Basys2 board is pretty noisy and could be the culprit. However, putting the maps into modules shouldn't have made it worse unless I messed something up. Any idea what happened?
Here's my map generator:
module map_generator(clk_vga, reset, CurrentX, CurrentY, HBlank, VBlank, playerPosX, playerPosY, mapData
);
input clk_vga;
input reset;
input [9:0]CurrentX;
input [8:0]CurrentY;
input HBlank;
input VBlank;
input [9:0]playerPosX;
input [8:0]playerPosY;
output [7:0]mapData;
reg [7:0]mColor;
reg [5:0]currentMap = 0;
wire [7:0]startCastle;
StartCastle StartCastle(
.clk_vga(clk_vga),
.CurrentX(CurrentX),
.CurrentY(CurrentY),
.mapData(startCastle)
);
always #(posedge clk_vga) begin
if(reset)begin
currentMap <= 0;
end
end
always #(posedge clk_vga) begin
if(HBlank || VBlank) begin
mColor <= 0;
end
else begin
if(currentMap == 4'b0000) begin
mColor[7:0] <= startCastle[7:0];
end
//Add more maps later
end
end
assign mapData[7:0] = mColor[7:0];
endmodule
Here's the startCastle:
module StartCastle(clk_vga, CurrentX, CurrentY, active, mapData);
input clk_vga;
input [9:0]CurrentX;
input [8:0]CurrentY;
input active;
output [7:0]mapData;
reg [7:0]mColor;
always #(posedge clk_vga) begin
if(CurrentY < 40) begin
mColor[7:0] <= 8'b11100000;
end
else if(CurrentX < 40) begin
mColor[7:0] <= 8'b11100000;
end
else if(~(CurrentX < 600)) begin
mColor[7:0] <= 8'b11100000;
end
else if((~(CurrentY < 440) && (CurrentX < 260)) || (~(CurrentY < 440) && ~(CurrentX < 380))) begin
mColor[7:0] <= 8'b11100000;
end else
mColor[7:0] <= 8'b00011100;
end
assign mapData = mColor;
endmodule
Here's the VGA driver which is connected to my top module:
module vga_driver(clk_50MHz, vs_vga, hs_vga, RED, GREEN, BLUE, HBLANK, VBLANK, CURX, CURY, COLOR, CLK_DATA, RESET);
input clk_50MHz;
output vs_vga;
output hs_vga;
output [2:0] RED;
output [2:0] GREEN;
output [1:0] BLUE;
output HBLANK;
output VBLANK;
reg VS = 0;
reg HS = 0;
input RESET;
//current client data
input [7:0] COLOR;
output CLK_DATA;
output [9:0] CURX;
output [8:0] CURY;
//##### Module constants (http://tinyvga.com/vga-timing/640x480#60Hz)
parameter HDisplayArea = 640; // horizontal display area
parameter HLimit = 800; // maximum horizontal amount (limit)
parameter HFrontPorch = 16; // h. front porch
parameter HBackPorch = 48; // h. back porch
parameter HSyncWidth = 96; // h. pulse width
parameter VDisplayArea = 480; // vertical display area
parameter VLimit = 525; // maximum vertical amount (limit)
parameter VFrontPorch = 10; // v. front porch
parameter VBackPorch = 33; // v. back porch
parameter VSyncWidth = 2; // v. pulse width
//##### Local variables
wire clk_25MHz;
reg [9:0] CurHPos = 0; //maximum of HLimit (2^10 - 1 = 1023)
reg [9:0] CurVPos = 0; //maximum of VLimit
reg HBlank_reg, VBlank_reg, Blank = 0;
reg [9:0] CurrentX = 0; //maximum of HDisplayArea
reg [8:0] CurrentY = 0; //maximum of VDisplayArea (2^9 - 1 = 511)
//##### Submodule declaration
clock_divider clk_div(.clk_in(clk_50MHz), .clk_out(clk_25MHz));
//shifts the clock by half a period (negates it)
//see timing diagrams for a better understanding of the reason for this
clock_shift clk_shift(.clk_in(clk_25MHz), .clk_out(CLK_DATA));
//simulate the vertical and horizontal positions
always #(posedge clk_25MHz) begin
if(CurHPos < HLimit-1) begin
CurHPos <= CurHPos + 1;
end
else begin
CurHPos <= 0;
if(CurVPos < VLimit-1)
CurVPos <= CurVPos + 1;
else
CurVPos <= 0;
end
if(RESET) begin
CurHPos <= 0;
CurVPos <= 0;
end
end
//##### VGA Logic (http://tinyvga.com/vga-timing/640x480#60Hz)
//HSync logic
always #(posedge clk_25MHz)
if((CurHPos < HSyncWidth) && ~RESET)
HS <= 1;
else
HS <= 0;
//VSync logic
always #(posedge clk_25MHz)
if((CurVPos < VSyncWidth) && ~RESET)
VS <= 1;
else
VS <= 0;
//Horizontal logic
always #(posedge clk_25MHz)
if((CurHPos >= HSyncWidth + HFrontPorch) && (CurHPos < HSyncWidth + HFrontPorch + HDisplayArea) || RESET)
HBlank_reg <= 0;
else
HBlank_reg <= 1;
//Vertical logic
always #(posedge clk_25MHz)
if((CurVPos >= VSyncWidth + VFrontPorch) && (CurVPos < VSyncWidth + VFrontPorch + VDisplayArea) || RESET)
VBlank_reg <= 0;
else
VBlank_reg <= 1;
//Do not output any color information when we are in the vertical
//or horizontal blanking areas. Set a boolean to keep track of this.
always #(posedge clk_25MHz)
if((HBlank_reg || VBlank_reg) && ~RESET)
Blank <= 1;
else
Blank <= 0;
//Keep track of the current "real" X position. This is the actual current X
//pixel location abstracted away from all the timing details
always #(posedge clk_25MHz)
if(HBlank_reg && ~RESET)
CurrentX <= 0;
else
CurrentX <= CurHPos - HSyncWidth - HFrontPorch;
//Keep track of the current "real" Y position. This is the actual current Y
//pixel location abstracted away from all the timing details
always #(posedge clk_25MHz)
if(VBlank_reg && ~RESET)
CurrentY <= 0;
else
CurrentY <= CurVPos - VSyncWidth - VFrontPorch;
assign CURX = CurrentX;
assign CURY = CurrentY;
assign VBLANK = VBlank_reg;
assign HBLANK = HBlank_reg;
assign hs_vga = HS;
assign vs_vga = VS;
//Respects VGA Blanking areas
assign RED = (Blank) ? 3'b000 : COLOR[7:5];
assign GREEN = (Blank) ? 3'b000 : COLOR[4:2];
assign BLUE = (Blank) ? 2'b00 : COLOR[1:0];
endmodule
clk_div:
module clock_divider(clk_in, clk_out);
input clk_in;
output clk_out;
reg clk_out = 0;
always #(posedge clk_in)
clk_out <= ~clk_out;
endmodule
clk_shift:
module clock_shift(clk_in, clk_out);
input clk_in;
output clk_out;
assign clk_out = ~clk_in;
endmodule

Answer 1

I'm posting this as an answer because I cannot put a photo in a comment.
Is this what your design looks like?
My only guess ATM is that you might have misplaced some ports during instantiation of vga_driver and/or map_generator (if you used the old style instantiation). Nevertheless, I'm going to check VGA timmings, as I can see a strange vertical line at the left of the screen, as if the hblank interval was visible.
By the way: I've changed the way you generate the display. You use regs for HS, VS, etc, which get updated the next clock cycle. I treat display generation as a FSM, so outputs come from combinational blocks triggered by certain values (or range of values) from the counters. Besides, I start horizontal and vertical counters so position (0,0) measured in pixel coordinates in the screen actually maps to values (0,0) from horizontal and vertical counters, so no arithmetic needed.
This is my version of VGA display generation:
module videosyncs (
input wire clk,
input wire [2:0] rin,
input wire [2:0] gin,
input wire [1:0] bin,
output reg [2:0] rout,
output reg [2:0] gout,
output reg [1:0] bout,
output reg hs,
output reg vs,
output wire [10:0] hc,
output wire [10:0] vc
);
/* http://www.abramovbenjamin.net/calc.html */
// VGA 640x480#60Hz,25MHz
parameter htotal = 800;
parameter vtotal = 524;
parameter hactive = 640;
parameter vactive = 480;
parameter hfrontporch = 16;
parameter hsyncpulse = 96;
parameter vfrontporch = 11;
parameter vsyncpulse = 2;
parameter hsyncpolarity = 0;
parameter vsyncpolarity = 0;
reg [10:0] hcont = 0;
reg [10:0] vcont = 0;
reg active_area;
assign hc = hcont;
assign vc = vcont;
always #(posedge clk) begin
if (hcont == htotal-1) begin
hcont <= 0;
if (vcont == vtotal-1) begin
vcont <= 0;
end
else begin
vcont <= vcont + 1;
end
end
else begin
hcont <= hcont + 1;
end
end
always #* begin
if (hcont>=0 && hcont<hactive && vcont>=0 && vcont<vactive)
active_area = 1'b1;
else
active_area = 1'b0;
if (hcont>=(hactive+hfrontporch) && hcont<(hactive+hfrontporch+hsyncpulse))
hs = hsyncpolarity;
else
hs = ~hsyncpolarity;
if (vcont>=(vactive+vfrontporch) && vcont<(vactive+vfrontporch+vsyncpulse))
vs = vsyncpolarity;
else
vs = ~vsyncpolarity;
end
always #* begin
if (active_area) begin
gout = gin;
rout = rin;
bout = bin;
end
else begin
gout = 3'h00;
rout = 3'h00;
bout = 2'h00;
end
end
endmodule
Which is instantiated by your vga_driver module, which becomes nothing but a wrapper for this module:
module vga_driver (
input wire clk_25MHz,
output wire vs_vga,
output wire hs_vga,
output wire [2:0] RED,
output wire [2:0] GREEN,
output wire [1:0] BLUE,
output wire HBLANK,
output wire VBLANK,
output [9:0] CURX,
output [8:0] CURY,
input [7:0] COLOR,
input wire RESET
);
assign HBLANK = 0;
assign VBLANK = 0;
videosyncs syncgen (
.clk(clk_25MHz),
.rin(COLOR[7:5]),
.gin(COLOR[4:2]),
.bin(COLOR[1:0]),
.rout(RED),
.gout(GREEN),
.bout(BLUE),
.hs(hs_vga),
.vs(vs_vga),
.hc(CURX),
.vc(CURY)
);
endmodule
Note that in map_generator, the first if statement in this always block will never be true. We can forget about it, as the VGA display module will blank RGB outputs when needed.
always #(posedge clk_vga) begin
if(HBlank || VBlank) begin //
mColor <= 0; // Never reached
end //
else begin //
if(currentMap == 4'b0000) begin
mColor[7:0] <= startCastle[7:0];
end
//Add more maps later
end
end
Using the same approach, I've converted the map generator module to be a combinational module. For example, for map 0 (the castle -without the castle, I see-) it is like this:
module StartCastle(
input wire [9:0] CurrentX,
input wire [8:0] CurrentY,
output wire [7:0] mapData
);
reg [7:0] mColor;
assign mapData = mColor;
always #* begin
if(CurrentY < 40) begin
mColor[7:0] <= 8'b11100000;
end
else if(CurrentX < 40) begin
mColor[7:0] <= 8'b11100000;
end
else if(~(CurrentX < 600)) begin
mColor[7:0] <= 8'b11100000;
end
else if((~(CurrentY < 440) && (CurrentX < 260)) || (~(CurrentY < 440) && ~(CurrentX < 380))) begin
mColor[7:0] <= 8'b11100000;
end else
mColor[7:0] <= 8'b00011100;
end
endmodule
Just a FSM whose output is the colour that goes in a pixel. The input being the coordinates of the current pixel.
So when it is time to display map 0, map_generator simply switches to it based upon the current value of currentMap
module map_generator (
input wire clk,
input wire reset,
input wire [9:0]CurrentX,
input wire [8:0]CurrentY,
input wire HBlank,
input wire VBlank,
input wire [9:0]playerPosX,
input wire [8:0]playerPosY,
output wire [7:0]mapData
);
reg [7:0] mColor;
assign mapData = mColor;
reg [5:0]currentMap = 0;
wire [7:0] castle_map;
StartCastle StartCastle(
.CurrentX(CurrentX),
.CurrentY(CurrentY),
.mapData(castle_map)
);
always #(posedge clk) begin
if(reset) begin
currentMap <= 0;
end
end
always #* begin
if(currentMap == 6'b000000) begin
mColor = castle_map;
end
//Add more maps later
end
endmodule
This may look like a lot of comb logic is generated and so glitches may happen. It's actually very fast, no noticeable glitches on screen, and you can use the actual current x and y coordinates to choose what to display on screen. Thus, no need for an inverted clock. My final version of your design has only one 25MHz clock.
By the way, you want to keep device dependent constructions away from your design, placing things like clock generators in separate modules that will be connected to your design in the top module, which should be the only device dependent module.
So, I've written a device-agnostic adventure module, which will contain the entire game:
module adventure (
input clk_vga,
input reset,
output vs_vga,
output hs_vga,
output [2:0] RED,
output [2:0] GREEN,
output [1:0] BLUE
);
wire HBLANK, VBLANK;
wire [7:0] COLOR;
wire [9:0] CURX;
wire [8:0] CURY;
wire [9:0] playerPosX = 10'd320; // no actually used in the design yet
wire [8:0] playerPosY = 9'd240; // no actually used in the design yet
vga_driver the_screen (.clk_25MHz(clk_vga),
.vs_vga(vs_vga),
.hs_vga(hs_vga),
.RED(RED),
.GREEN(GREEN),
.BLUE(BLUE),
.HBLANK(HBLANK),
.VBLANK(VBLANK),
.CURX(CURX),
.CURY(CURY),
.COLOR(COLOR)
);
map_generator the_mapper (.clk(clk_vga),
.reset(reset),
.CurrentX(CURX),
.CurrentY(CURY),
.HBlank(HBLANK),
.VBlank(VBLANK),
.playerPosX(playerPosX),
.playerPosY(playerPosY),
.mapData(COLOR)
);
endmodule
This module is not complete: it lacks inputs from joystick or any other input device to update player current position. For now, player current position is fixed.
The top level design (TLD) is written exclusively for the FPGA trainer you have. It is here where you need to generate proper clocks using your device's available resources, such as the DCM in Spartan 3/3E devices.
module tld_basys(
input wire clk_50MHz,
input wire RESET,
output wire vs_vga,
output wire hs_vga,
output wire [2:0] RED,
output wire [2:0] GREEN,
output wire [1:0] BLUE
);
wire clk_25MHz;
dcm_clocks gen_vga_clock (
.CLKIN_IN(clk_50MHz),
.CLKDV_OUT(clk_25MHz)
);
adventure the_game (.clk_vga(clk_25MHz),
.reset(RESET),
.vs_vga(vs_vga),
.hs_vga(hs_vga),
.RED(RED),
.GREEN(GREEN),
.BLUE(BLUE)
);
endmodule
The DCM generated clocks goes in this module (generated by the Xilinx Core Generator)
module dcm_clocks (CLKIN_IN,
CLKDV_OUT
);
input CLKIN_IN;
output CLKDV_OUT;
wire CLKFB_IN;
wire CLKFX_BUF;
wire CLKDV_BUF;
wire CLKIN_IBUFG;
wire CLK0_BUF;
wire GND_BIT;
assign GND_BIT = 0;
BUFG CLKDV_BUFG_INST (.I(CLKDV_BUF),
.O(CLKDV_OUT));
IBUFG CLKIN_IBUFG_INST (.I(CLKIN_IN),
.O(CLKIN_IBUFG));
BUFG CLK0_BUFG_INST (.I(CLK0_BUF),
.O(CLKFB_IN));
DCM_SP #(.CLKDV_DIVIDE(2.0), .CLKIN_DIVIDE_BY_2("FALSE"),
.CLKIN_PERIOD(20.000), .CLKOUT_PHASE_SHIFT("NONE"),
.DESKEW_ADJUST("SYSTEM_SYNCHRONOUS"), .DFS_FREQUENCY_MODE("LOW"),
.DLL_FREQUENCY_MODE("LOW"), .DUTY_CYCLE_CORRECTION("TRUE"),
.FACTORY_JF(16'hC080), .PHASE_SHIFT(0), .STARTUP_WAIT("FALSE") )
DCM_SP_INST (.CLKFB(CLKFB_IN),
.CLKIN(CLKIN_IBUFG),
.DSSEN(GND_BIT),
.PSCLK(GND_BIT),
.PSEN(GND_BIT),
.PSINCDEC(GND_BIT),
.RST(GND_BIT),
.CLKDV(CLKDV_BUF),
.CLKFX(),
.CLKFX180(),
.CLK0(CLK0_BUF),
.CLK2X(),
.CLK2X180(),
.CLK90(),
.CLK180(),
.CLK270(),
.LOCKED(),
.PSDONE(),
.STATUS());
endmodule
Although it is safe (for Xilinx devices) to use a simple clock divider as you did. If you fear that the synthesizer won't treat your divided clock as an actual clock, add a BUFG primitive to route the output from the divider to a global buffer so it can be used as a clock with no problems (see the module above for an example on how to do this).
As a final note, you may want to add more independency from the final device, by using 24-bit colours for your graphics. At the TLD, you will use the actual number of bits per colour component you really have, but if you move from the Basys2 with 8-bit colour trainer board to the, say, Nexys4 board, with 12-bit colour, you will automatically enjoy a richer output display.
Now, it looks like this (no vertical bars at the left, and colours seem to be more vibrant)