Output blank image when LCD is off

The screen stays blank until 1 frame after the LCD is turned on.
Fixes glitches during transitions in some games.

Gameboy.sv

@@ -542,6 +542,7 @@ wire lcd_clkena;
 wire [14:0] lcd_data;
 wire [1:0] lcd_mode;
 wire lcd_on;
+wire lcd_vsync;
 
 assign AUDIO_S = 0;
 
@@ -590,6 +591,7 @@ gb gb (
 	.lcd_data    ( lcd_data   ),
 	.lcd_mode    ( lcd_mode   ),
 	.lcd_on      ( lcd_on     ),
+	.lcd_vsync   ( lcd_vsync  ),
 	.speed       ( speed      ),
 	
 	// serial port
@@ -620,10 +622,13 @@ lcd lcd
 (
 	// serial interface
 	.clk_sys( clk_sys    ),
-	.pix_wr ( sgb_lcd_clkena & ce_cpu ),
+	.ce     ( ce_cpu         ),
+
+	.lcd_clkena ( sgb_lcd_clkena ),
 	.data   ( sgb_lcd_data   ),
 	.mode   ( sgb_lcd_mode   ),  // used to detect begin of new lines and frames
 	.on     ( sgb_lcd_on     ),
+	.lcd_vs ( sgb_lcd_vsync  ),
 
 	.isGBC  ( isGBC      ),
 
@@ -658,7 +663,7 @@ wire [1:0] joy_p54;
 wire [3:0] joy_do_sgb;
 wire [14:0] sgb_lcd_data;
 wire [15:0] sgb_border_pix;
-wire sgb_lcd_clkena, sgb_lcd_on;
+wire sgb_lcd_clkena, sgb_lcd_on, sgb_lcd_vsync;
 wire [1:0] sgb_lcd_mode;
 wire sgb_pal_en;
 wire [1:0] sgb_en = {~status[24] ^ status[23], status[23]};
@@ -685,6 +690,7 @@ sgb sgb (
 	.lcd_clkena  ( lcd_clkena  ),
 	.lcd_data    ( lcd_data    ),
 	.lcd_mode    ( lcd_mode    ),
+	.lcd_vsync   ( lcd_vsync   ),
 
 	.h_cnt       ( h_cnt      ),
 	.v_cnt       ( v_cnt      ),
@@ -694,7 +700,8 @@ sgb sgb (
 	.sgb_lcd_data    ( sgb_lcd_data    ),
 	.sgb_lcd_on      ( sgb_lcd_on      ),
 	.sgb_lcd_clkena  ( sgb_lcd_clkena  ),
-	.sgb_lcd_mode    ( sgb_lcd_mode    )
+	.sgb_lcd_mode    ( sgb_lcd_mode    ),
+	.sgb_lcd_vsync   ( sgb_lcd_vsync   )
 );
 
 reg hs_o, vs_o;

rtl/gb.v

@@ -52,6 +52,7 @@ module gb (
 	output [14:0] lcd_data,
 	output [1:0] lcd_mode,
 	output lcd_on,
+	output lcd_vsync,
 
 	output [1:0] joy_p54,
 	input  [3:0] joy_din,
@@ -484,7 +485,8 @@ video video (
 	.lcd_clkena  ( lcd_clkena    ),
 	.lcd_data    ( lcd_data      ),
 	.mode        ( lcd_mode      ),
-	
+	.lcd_vsync   ( lcd_vsync     ),
+
 	.vram_rd     ( video_rd      ),
 	.vram_addr   ( video_addr    ),
 	.vram_data   ( vram_do       ),

rtl/lcd.v

@@ -6,7 +6,10 @@
 module lcd
 (
 	input        clk_sys,
-	input        pix_wr,
+	input        ce,
+	input        lcd_clkena,
+	input        lcd_vs,
+
 	input [14:0] data,
 
 	input  [1:0] mode,
@@ -45,22 +48,54 @@ module lcd
 reg [14:0] vbuffer_inptr;
 reg vbuffer_in_bank;
 reg lcd_off;
+reg blank_de, blank_output;
+reg [14:0] blank_data;
+
+wire pix_wr = ce & (lcd_clkena | blank_de);
 always @(posedge clk_sys) begin
-	reg old_lcd_off;
+	reg old_lcd_off, old_on, old_lcd_vs;
+	reg [8:0] blank_hcnt,blank_vcnt;
 
 	lcd_off <= !on || (mode == 2'd01);
+	blank_de <= (!on && blank_output && blank_hcnt < 160 && blank_vcnt < 144);
 
-	if (pix_wr & ~lcd_off) vbuffer_inptr <= vbuffer_inptr + 1'd1;
+	if (pix_wr) vbuffer_inptr <= vbuffer_inptr + 1'd1;
 
 	old_lcd_off <= lcd_off;
-	if(~old_lcd_off & lcd_off) begin  //lcd disabled or vsync restart pointer
+	if(old_lcd_off ^ lcd_off) begin
 		vbuffer_inptr <= 0;
-		vbuffer_in_bank <= ~vbuffer_in_bank;
+		if (lcd_off) vbuffer_in_bank <= ~vbuffer_in_bank; //LCD disabled or VBlank
 	end
+
+	old_on <= on;
+	if (old_on & ~on & ~blank_output) begin // LCD disabled, start blank output
+		blank_output <= 1'b1;
+		{blank_hcnt,blank_vcnt} <= 0;
+	end
+
+	// Regenerate LCD timings for filling with blank color when LCD is off
+	if (ce & ~on & blank_output) begin
+		blank_data <= data;
+		blank_hcnt <= blank_hcnt + 1'b1;
+		if (blank_hcnt == 9'd455) begin
+			blank_hcnt <= 0;
+			blank_vcnt <= blank_vcnt + 1'b1;
+			if (blank_vcnt == 9'd153) begin
+				blank_vcnt <= 0;
+				vbuffer_inptr <= 0;
+				vbuffer_in_bank <= ~vbuffer_in_bank;
+			end
+		end
+	end
+
+	// Output 1 blank frame until VSync after LCD is enabled
+	old_lcd_vs <= lcd_vs;
+	if (~old_lcd_vs & lcd_vs & blank_output)
+		blank_output <= 0;
 end
 
 reg [14:0] vbuffer[65536];
-always @(posedge clk_sys) if(pix_wr) vbuffer[{vbuffer_in_bank, vbuffer_inptr}] <= data;
+always @(posedge clk_sys) if(pix_wr) vbuffer[{vbuffer_in_bank, vbuffer_inptr}] <= (on & blank_output) ? blank_data : data;
 
 
 // Mode 00:  h-blank

rtl/sgb.v

@@ -13,6 +13,7 @@ module sgb (
 	input [14:0] lcd_data,
 	input [1:0]  lcd_mode,
 	input        lcd_on,
+	input        lcd_vsync,
 
 	input [8:0]  h_cnt,
 	input [8:0]  v_cnt,
@@ -31,7 +32,8 @@ module sgb (
 	output reg [14:0] sgb_lcd_data,
 	output reg        sgb_lcd_clkena,
 	output reg [1:0]  sgb_lcd_mode,
-	output reg        sgb_lcd_on
+	output reg        sgb_lcd_on,
+	output reg        sgb_lcd_vsync
 );
 
 localparam CMD_PAL01    = 5'h00;
@@ -833,7 +835,7 @@ end
 
 reg [14:0] lcd_data_r;
 reg [1:0] pal_no;
-reg lcd_clkena_r, lcd_on_r;
+reg lcd_clkena_r, lcd_on_r, lcd_vsync_r;
 reg [1:0] lcd_mode_r;
 reg [1:0] mask_en_r;
 wire [1:0] lcd_data_2 = lcd_data_r[1:0];
@@ -848,6 +850,7 @@ always @(posedge clk_sys) begin
 		lcd_clkena_r <= lcd_clkena;
 		lcd_mode_r <= lcd_mode;
 		lcd_on_r <= lcd_on;
+		lcd_vsync_r <= lcd_vsync;
 
 		if (~sgb_en | ((~output_sgb_pal | tint) & !mask_en_r) ) begin
 			sgb_lcd_data <= lcd_data_r;
@@ -863,6 +866,7 @@ always @(posedge clk_sys) begin
 		sgb_lcd_mode <= lcd_mode_r;
 		sgb_lcd_on <= lcd_on_r;
 		sgb_pal_en <= sgb_en & ( (output_sgb_pal & ~tint) || |mask_en_r);
+		sgb_lcd_vsync <= lcd_vsync_r;
 	end
 
 end

rtl/video.v

@@ -39,6 +39,7 @@ module video (
 	output lcd_on,
 	output lcd_clkena,
 	output [14:0] lcd_data,
+	output lcd_vsync,
 
 	output irq,
 	output vblank_irq,
@@ -404,12 +405,14 @@ reg [7:0] v_cnt;            // max 153
 // This results in v_cnt 0 lasting for almost 2 lines.
 wire line153 = (v_cnt == 8'd153);
 reg vcnt_reset;
+reg vsync;
 always @(posedge clk) begin
 	if (!lcdc_on) begin
 		v_cnt <= 8'd0;
 		vcnt_reset <= 1'b0;
+		vsync <= 1'b0;
 	end else if (ce) begin
-		if (~vcnt_reset && h_cnt == 9'd455) begin
+		if (~vcnt_reset && h455) begin
 			v_cnt <= v_cnt + 1'b1;
 		end
 
@@ -419,9 +422,15 @@ always @(posedge clk) begin
 				v_cnt <= 8'd0;
 			end
 		end
+
+		// VSync goes high on line 0 but it takes a full frame after the LCD is enabled
+		// because the first line where end_of_line is high after LCD is enabled is line 1.
+		if (end_of_line) vsync <= !v_cnt;
 	end
 end
 
+assign lcd_vsync = vsync;
+
 // line inside the background currently being drawn
 wire [7:0] bg_line = v_cnt + scy;
 wire [7:0] bg_col  = pcnt + scx;
@@ -750,6 +759,9 @@ always @(posedge clk) begin
 		if (lcd_clk) begin
 			lcd_data_out <= (sprite_pixel_visible) ? sprite_pix : pix_rgb_data;
 		end
+
+		// Output blank pixels if lcd is off.
+		if (~lcd_on) lcd_data_out <= isGBC ? 15'h7FFF : 15'd0;
 	end
 end