Implement VGA controller

1 files changed, 166 insertions, 16 deletions

diff --git a/rtl/vga.sv b/rtl/vga.sv
index 4ff0ad1..d720578 100644
--- a/rtl/vga.sv
+++ b/rtl/vga.sv
@@ -2,15 +2,52 @@
 
 `define VGA_PIXCLK_HZ 25_175_000
 
+/* Todas las constantes mágicas están en el DE1-SoC CD-ROM bajo
+ * Datasheet/SDRAM/IS42R16320D.pdf
+ * 
+ * Este módulo es el CRTC de VGA. Consideraciones:
+ *
+ * - Se necesita una resolución clásica de 640x480 @ ~60Hz.
+ *
+ * - La VRAM en este caso es una unidad de memoria dinámica real, externa a la
+ *   fábrica de la FPGA, que tiene todas las propiedades esperables de SDRAM
+ *   real, como cálculos de temporización e impedancias, latencia de más de un
+ *   ciclo, strobe, etc.
+ *
+ * - Este módulo debe generar una señal para el DAC que no puede detenerse,
+ *   sino que debe emitir exactamente un píxel por ciclo de pixclk, excepto
+ *   fuera de las regiones activas horizontal y vertical.
+ *
+ * - La VRAM tiene 4 bancos de 10 columnas y 13 filas con celdas de 16 bits.
+ *
+ * - El CPU puede escribir a VRAM al mismo tiempo que el CRTC lee, ambos son
+ *   maestros de un mismo esclavo.
+ *
+ * Por lo tanto:
+ *
+ * - El formato de framebuffer es row-major r5g6b5.
+ *
+ * - Existen dos buffers de scanline, uno para filas pares y otro para
+ *   impares.
+ *
+ * - Mientras el CRTC muestra una scanline en pantalla, el maestro Avalon lee
+ *   la siguiente de VRAM usando pipelining para lograr esto último lo más
+ *   rápido que la VRAM sea capaz. Según el *_hw.tcl del IP para VRAM, este
+ *   soporta hasta siete transacciones en pipeline. Ocurrirán glitches si VRAM
+ *   no es capaz de producir una scanline antes de que el CRTC la necesite.
+ */
+
 module vga
 (
 	input  logic       clk,
-	input  logic       rst_n,
+	                   rst_n,
 
+	// 26 bits direccionan 64MiB
 	output logic[25:0] avl_address,
 	output logic       avl_read,
-	input  logic[31:0] avl_readdata,
+	input  logic[15:0] avl_readdata,
 	input  logic       avl_waitrequest,
+	                   avl_readdatavalid,
 
 	output logic       vga_clk,
 	                   vga_hsync,
@@ -22,24 +59,137 @@ module vga
 	                   vga_b
 );
 
-	localparam H_ACTIVE = `COORD_BITS'd640;
-	localparam H_FPORCH = `COORD_BITS'd16;
-	localparam H_SYNC   = `COORD_BITS'd96;
-	localparam H_BPORCH = `COORD_BITS'd48;
-	localparam V_ACTIVE = `COORD_BITS'd480;
-	localparam V_FPORCH = `COORD_BITS'd11;
-	localparam V_SYNC   = `COORD_BITS'd2;
-	localparam V_BPORCH = `COORD_BITS'd31;
+	localparam H_ACTIVE_NO = 640;
+	localparam H_ACTIVE    = `COORD_BITS'd640;
+	localparam H_FPORCH    = `COORD_BITS'd16;
+	localparam H_SYNC      = `COORD_BITS'd96;
+	localparam H_BPORCH    = `COORD_BITS'd48;
+	localparam V_ACTIVE    = `COORD_BITS'd480;
+	localparam V_FPORCH    = `COORD_BITS'd11;
+	localparam V_SYNC      = `COORD_BITS'd2;
+	localparam V_BPORCH    = `COORD_BITS'd31;
+
+	localparam H_FPORCH_AT = H_BPORCH + H_ACTIVE;
+	localparam H_SYNC_AT   = H_FPORCH_AT + H_FPORCH;
+	localparam H_TOTAL     = H_SYNC_AT + H_SYNC;
+	localparam V_FPORCH_AT = V_BPORCH + V_ACTIVE;
+	localparam V_SYNC_AT   = V_FPORCH_AT + V_FPORCH;
+	localparam V_TOTAL     = V_SYNC_AT + V_SYNC;
+
+	typedef struct packed
+	{
+		logic[4:0] r;
+		logic[5:0] g;
+		logic[4:0] b;
+	} pix;
 
-	localparam H_SYNC_AT = H_BPORCH + H_ACTIVE + H_FPORCH;
-	localparam H_TOTAL   = H_SYNC_AT + H_SYNC;
-	localparam V_SYNC_AT = V_BPORCH + V_ACTIVE + V_FPORCH;
-	localparam V_TOTAL   = V_SYNC_AT + V_SYNC;
+	enum int unsigned
+	{
+		A,
+		B
+	} reading, next_reading, fill_start_reading;
 
-	logic[7:0] r, g, b;
+	pix current, read_a, read_b;
+	pix scanline_a[H_ACTIVE_NO];
+	pix scanline_b[H_ACTIVE_NO];
+
+	logic next_active;
+	logic[24:0] addr;
+	logic[`COORD_BITS - 1:0] x, y, next_x, next_y, next_hsync,
+	                         pending_read, pending_data, read_idx, write_idx;
 
 	assign vga_clk = clk;
-	assign vga_blank_n = 1;
 	assign vga_sync_n = 0;
+	assign vga_blank_n = 1;
+	assign avl_address = {addr, 1'b0};
+
+	assign vga_r = {current.r, current.r[4], current.r[4], current.r[4]};
+	assign vga_g = {current.g, current.g[5], current.g[5]};
+	assign vga_b = {current.b, current.b[4], current.b[4], current.b[4]};
+
+	assign read_idx = next_x - H_BPORCH;
+	assign next_reading = next_y[0] ^ (next_x < H_FPORCH_AT) ? A : B;
+
+	assign next_active
+		=  next_x >= H_BPORCH && next_x < H_FPORCH_AT
+		&& next_y >= V_BPORCH && next_y < V_FPORCH_AT;
+
+	always_comb begin
+		unique case(reading)
+			A: current = read_a;
+			B: current = read_b;
+		endcase
+
+		if(x != H_TOTAL - 1) begin
+			next_x = x + 1;
+			next_y = y;
+		end else begin
+			next_x = 0;
+			next_y = y != V_TOTAL - 1 ? y + 1 : 0;
+		end
+	end
+
+	always @(posedge clk or negedge rst_n)
+		if(!rst_n) begin
+			x <= H_TOTAL - 1;
+			y <= V_TOTAL - 1;
+			reading <= A;
+			write_idx <= 0;
+			pending_read <= 0;
+			pending_data <= 0;
+			fill_start_reading <= A;
+
+			read_a <= 0;
+			read_b <= 0;
+
+			addr <= 0;
+			avl_read <= 0;
+
+			vga_hsync <= 0;
+			vga_vsync <= 0;
+		end else begin
+			if(next_active)
+				unique case(next_reading)
+					A: read_a <= scanline_a[read_idx];
+					B: read_b <= scanline_b[read_idx];
+				endcase
+
+			if(avl_readdatavalid) begin
+				unique case(fill_start_reading)
+					A: scanline_b[write_idx] <= avl_readdata;
+					B: scanline_a[write_idx] <= avl_readdata;
+				endcase
+
+				write_idx <= write_idx + 1;
+				pending_data <= pending_data - 1;
+			end
+
+			if(!avl_read || !avl_waitrequest) begin
+				avl_read <= 0;
+
+				if(pending_read != 0) begin
+					addr <= addr + 1;
+					avl_read <= 1;
+					pending_read <= pending_read - 1;
+				end
+			end
+
+			if(pending_read == 0 && pending_data == 0 && next_reading != reading) begin
+				if(y >= V_BPORCH - 2 && y < V_FPORCH_AT - 2) begin
+					write_idx <= 0;
+					pending_read <= H_ACTIVE;
+					pending_data <= H_ACTIVE;
+					fill_start_reading <= next_reading;
+				end else
+					addr <= {$bits(addr){1'b1}};
+			end
+
+			x <= next_x;
+			y <= next_y;
+			reading <= next_reading;
+
+			vga_hsync <= next_x < H_SYNC_AT;
+			vga_vsync <= next_y < V_SYNC_AT;
+		end
 
 endmodule