What I am attempting to do is create a VGA controller from a Lattice MachXO CPLD in Verilog.
The Problem
I am attempting to display the color red with a resolution of 640x480 @ 60Hz using a 25.175 MHz clock internal to the CPLD; however, when I plug the CPLD into a monitor I get an "Out of Range" message; no monitor I try can understand what resolution I would like.
What I've tried
I have simulated the code in ModelSim (pictures included), and everything appears to look good save for one issue. When I count the amount of time steps that have occurred from the during the V-Sync display zone (when H-Sync is drawing) and divided it by the frequency of H-Sync, I get 479 pulses -- one short of the 480 lines I should be drawing. I don't understand where this could be coming from as I've check my timings many times, and I suspect that this may be a symptom of the problem, but I'm not sure.
The numbers I'm using to generate my numbers for timings is from Tiny VGA: tinyvga.com/vga-timing/640x480@60Hz Instead of counting lines for V_SYNC I am counting clock pulses, however, The numbers should be equivlent (ie. 2 800 count lines = 1600 pulses). I may change this behavior to line counting later.
Below is my code, and pictures of the timings from ModelSim.
module Top(RESET, H_SYNC, V_SYNC, RED);
input wire RESET;
output wire H_SYNC;
output wire V_SYNC;
output wire RED;
wire rgb_en;
/*** Test Bench Code ***/
//reg osc_clk, reset;
//initial begin
//#0 reset = 0;
//#0 osc_clk = 0;
//#2 reset = 1;
//end
//always #1 osc_clk = ~osc_clk;
OSCC OSCC_1 (.OSC(osc_clk)); /*< IP clock module for Lattice CPLD >*/
Controller CNTRL(.NRST(RESET), .CLK(osc_clk), .H_SYNC(H_SYNC), .V_SYNC(V_SYNC), .RGB_EN(rgb_en));
assign RED = (rgb_en ? 1:1'bz);
endmodule
module Controller(CLK, NRST, H_SYNC, V_SYNC, RGB_EN);
input wire CLK; /*< CLK input from Top module >*/
input wire NRST; /*< Reset input from Top module >*/
output reg H_SYNC; /*< Goes to VGA Horizontal Sync >*/
output reg V_SYNC; /*< Goes to VGA Verical Sync >*/
output reg RGB_EN ; /*< Enables RGB values durning display time on H_SYNC >*/
reg [9:0] h_counter; /*< Tracks amount of pulses from CLK >*/
reg [18:0] v_counter; /*< Tracks amount of pulses from H_SYNC >*/
`define H_SYNC_PULSE 10'd96 /*< Length of Sync Pulse >*/
`define H_BACK_PORCH_END 10'd144 /*< Pulse len + Porch Len >*/
`define H_FRONT_PORCH_STRT 10'd784 /*< Front Porch Len - Max >*/
`define H_COUNT_MAX 10'd799 /*< Max line pulses for resolution >*/
`define V_SYNC_PULSE 19'd1600 /*< 2 H_SYNC lines >*/
`define V_BACK_PORCH_END 19'd28000 /*< 33+2 H_SYNC lines >*/
`define V_FRONT_PORCH_STRT 19'd412000 /*< 525-10 H_SYNC lines >*/
`define V_COUNT_MAX 19'd419999 /*< 525 H_SYNC lines >*/
/*** Logic for H_SYNC ***/
always @(*) begin
if (h_counter < `H_SYNC_PULSE) begin
H_SYNC = 0;
RGB_EN = 0;
end
/* If H_Sync is in the display zone, enable RGB */
else if (h_counter > `H_BACK_PORCH_END && h_counter < `H_FRONT_PORCH_STRT) begin
H_SYNC = 1;
RGB_EN = 1;
end
/* During the Front Porch period, disable RGB */
else begin
H_SYNC = 1;
RGB_EN = 0;
end
end
/*** Logic for V_SYNC ***/
always @(*) begin
if (v_counter < `V_SYNC_PULSE) begin
V_SYNC = 0;
end
else begin
V_SYNC = 1;
end
end
/*** Counter logic ***/
always @(posedge CLK) begin
if (h_counter >= `H_COUNT_MAX || !NRST) begin
h_counter <= 11'b00;
end
else begin
h_counter <= h_counter + 1;
end
end
always @(posedge CLK) begin
if (v_counter >= `V_COUNT_MAX || !NRST) begin
v_counter <= 11'b00;
end
else begin
v_counter <= v_counter + 1;
end
end
endmodule
