From e1f3a0e577f5aa9d52f3053969171c675430878f Mon Sep 17 00:00:00 2001
From: Wojciech Kosior <kwojtus@protonmail.com>
Date: Tue, 1 Sep 2020 17:03:42 +0200
Subject: fix line counting in test model (avoid having picture shifted 1 line
 down on virtual display)

---
 models/vga_display.v | 88 +++++++++++++++++++++++++++-------------------------
 1 file changed, 45 insertions(+), 43 deletions(-)

(limited to 'models')

diff --git a/models/vga_display.v b/models/vga_display.v
index bb683e2..6e30248 100644
--- a/models/vga_display.v
+++ b/models/vga_display.v
@@ -134,11 +134,11 @@ module VGA_640_480_60Hz(
       end // else: !if(missing_h_sync_pulse)
 
 
-      /* Vertical stuff is almost entirely the same as horizontal */
-      if (new_line) begin
+      /* Vertical stuff is similar to horizontal */
+      if (h_state == ACTIVE_VIDEO) begin
 	 if (unexpected_v_sync_pulse) begin
 	    v_state <= SYNC_PULSE;
-	    v_counter <= 1;
+	    v_counter <= 0;
 
 	    if (v_state != UNKNOWN_STATE) begin
 	       `MSG(({"VGA: error: unexpected vertical sync pulse signal ",
@@ -146,52 +146,54 @@ module VGA_640_480_60Hz(
 	    end
 	 end else if (missing_v_sync_pulse) begin
 	    v_state <= BACK_PORCH;
-	    v_counter <= 1;
+	    v_counter <= 0;
 
 	    `MSG(({"VGA: error: missing expected vertical sync pulse signal ",
 		   "(line %0d of sync pulse)"}, v_counter));
-	 end else begin
-	    case (v_state)
-	      FRONT_PORCH : begin
-		 if (v_counter + 1 != V_FRONT_PORCH_LENGTH) begin
-		    v_counter <= v_counter + 1;
-		 end else begin
-		    v_state <= SYNC_PULSE;
-		    v_counter <= 0;
-		 end
+	 end
+      end // if (h_state == ACTIVE_VIDEO)
+
+      if (new_line) begin
+	 case (v_state)
+	   FRONT_PORCH : begin
+	      if (v_counter + 1 != V_FRONT_PORCH_LENGTH) begin
+		 v_counter <= v_counter + 1;
+	      end else begin
+		 v_state <= SYNC_PULSE;
+		 v_counter <= 0;
 	      end
-	      SYNC_PULSE : begin
-		 if (v_counter + 1 != V_SYNC_PULSE_LENGTH) begin
-		    v_counter <= v_counter + 1;
-		 end else begin
-		    v_state <= BACK_PORCH;
-		    v_counter <= 0;
-		 end
+	   end
+	   SYNC_PULSE : begin
+	      if (v_counter + 1 != V_SYNC_PULSE_LENGTH) begin
+		 v_counter <= v_counter + 1;
+	      end else begin
+		 v_state <= BACK_PORCH;
+		 v_counter <= 0;
 	      end
-	      BACK_PORCH : begin
-		 if (v_counter + 1 != V_BACK_PORCH_LENGTH) begin
-		    v_counter <= v_counter + 1;
-		 end else begin
-		    v_state <= ACTIVE_VIDEO;
-		    v_counter <= 0;
-		 end
+	   end
+	   BACK_PORCH : begin
+	      if (v_counter + 1 != V_BACK_PORCH_LENGTH) begin
+		 v_counter <= v_counter + 1;
+	      end else begin
+		 v_state <= ACTIVE_VIDEO;
+		 v_counter <= 0;
 	      end
-	      ACTIVE_VIDEO : begin
-		 if (v_counter + 1 != V_ACTIVE_VIDEO_LENGTH) begin
-		    v_counter <= v_counter + 1;
-		 end else begin
-		    v_state <= FRONT_PORCH;
-		    v_counter <= 0;
-
-		    if (!error) begin
-		       $writememb("VGAdump.mem", picture);
-		       `DBG(("VGA: written VGAdump.mem"));
-		       image_writes <= image_writes + 1;
-		    end
-		 end // else: !if(v_counter + 1 != V_ACTIVE_VIDEO_LENGTH)
-	      end // case: ACTIVE_VIDEO
-	    endcase // case (v_state)
-	 end // else: !if(missing_v_sync_pulse)
+	   end
+	   ACTIVE_VIDEO : begin
+	      if (v_counter + 1 != V_ACTIVE_VIDEO_LENGTH) begin
+		 v_counter <= v_counter + 1;
+	      end else begin
+		 v_state <= FRONT_PORCH;
+		 v_counter <= 0;
+
+		 if (!error) begin
+		    $writememb("VGAdump.mem", picture);
+		    `DBG(("VGA: written VGAdump.mem"));
+		    image_writes <= image_writes + 1;
+		 end
+	      end // else: !if(v_counter + 1 != V_ACTIVE_VIDEO_LENGTH)
+	   end // case: ACTIVE_VIDEO
+	 endcase // case (v_state)
       end // if (new_line)
 
       if (unexpected_h_sync_pulse || unexpected_h_sync_pulse ||
-- 
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