From a3f07390e31ee8b8e3c5be5928a0f21b558e535a Mon Sep 17 00:00:00 2001
From: krisd <krisdover@hotmail.com>
Date: Sun, 20 Aug 2023 14:41:07 +1200
Subject: [PATCH] fixes #43: enforce hdmi 12px minimum control period during
 video and data-island preambles; fixed off-by-one errors on VG, VP, and VSync

---
 src/hdmi.sv | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/src/hdmi.sv b/src/hdmi.sv
index 373c33c..2c91550 100644
--- a/src/hdmi.sv
+++ b/src/hdmi.sv
@@ -192,9 +192,9 @@ always_comb begin
     hsync <= invert ^ (cx >= screen_width + hsync_pulse_start && cx < screen_width + hsync_pulse_start + hsync_pulse_size);
     // vsync pulses should begin and end at the start of hsync, so special
     // handling is required for the lines on which vsync starts and ends
-    if (cy == screen_height + vsync_pulse_start)
+    if (cy == screen_height + vsync_pulse_start - 1)
         vsync <= invert ^ (cx >= screen_width + hsync_pulse_start);
-    else if (cy == screen_height + vsync_pulse_start + vsync_pulse_size)
+    else if (cy == screen_height + vsync_pulse_start + vsync_pulse_size - 1)
         vsync <= invert ^ (cx < screen_width + hsync_pulse_start);
     else
         vsync <= invert ^ (cy >= screen_height + vsync_pulse_start && cy < screen_height + vsync_pulse_start + vsync_pulse_size);
@@ -254,8 +254,8 @@ generate
             end
             else
             begin
-                video_guard <= cx >= frame_width - 2 && cx < frame_width && (cy == frame_height - 1 || cy < screen_height);
-                video_preamble <= cx >= frame_width - 10 && cx < frame_width - 2 && (cy == frame_height - 1 || cy < screen_height);
+                video_guard <= cx >= frame_width - 2 && cx < frame_width && (cy == frame_height - 1 || cy < screen_height - 1 /* no VG at end of last line */);
+                video_preamble <= cx >= frame_width - 10 && cx < frame_width - 2 && (cy == frame_height - 1 || cy < screen_height - 1 /* no VP at end of last line */);
             end
         end
 
@@ -264,7 +264,7 @@ generate
         logic [4:0] num_packets_alongside;
         always_comb
         begin
-            max_num_packets_alongside = ((frame_width - screen_width) /* VD period */ - 2 /* V guard */ - 8 /* V preamble */ - 12 /* 12px control period */ - 2 /* DI guard */ - 2 /* DI start guard */ - 8 /* DI premable */) / 32;
+            max_num_packets_alongside = (frame_width - screen_width  /* VD period */ - 2 /* V guard */ - 8 /* V preamble */ - 4 /* Min V control period */ - 2 /* DI trailing guard */ - 2 /* DI leading guard */ - 8 /* DI premable */ - 4 /* Min DI control period */) / 32;
             if (max_num_packets_alongside > 18)
                 num_packets_alongside = 5'd18;
             else
@@ -272,9 +272,9 @@ generate
         end
 
         logic data_island_period_instantaneous;
-        assign data_island_period_instantaneous = num_packets_alongside > 0 && cx >= screen_width + 10 && cx < screen_width + 10 + num_packets_alongside * 32;
+        assign data_island_period_instantaneous = num_packets_alongside > 0 && cx >= screen_width + 14 && cx < screen_width + 14 + num_packets_alongside * 32;
         logic packet_enable;
-        assign packet_enable = data_island_period_instantaneous && 5'(cx + screen_width + 22) == 5'd0;
+        assign packet_enable = data_island_period_instantaneous && 5'(cx + screen_width + 18) == 5'd0;
 
         logic data_island_guard = 0;
         logic data_island_preamble = 0;
@@ -289,8 +289,11 @@ generate
             end
             else
             begin
-                data_island_guard <= num_packets_alongside > 0 && ((cx >= screen_width + 8 && cx < screen_width + 10) || (cx >= screen_width + 10 + num_packets_alongside * 32 && cx < screen_width + 10 + num_packets_alongside * 32 + 2));
-                data_island_preamble <= num_packets_alongside > 0 && cx >= screen_width && cx < screen_width + 8;
+                data_island_guard <= num_packets_alongside > 0 && (
+                    (cx >= screen_width + 12 && cx < screen_width + 14) /* leading guard */ || 
+                    (cx >= screen_width + 14 + num_packets_alongside * 32 && cx < screen_width + 14 + num_packets_alongside * 32 + 2) /* trailing guard */
+                );
+                data_island_preamble <= num_packets_alongside > 0 && cx >= screen_width + 4 && cx < screen_width + 12;
                 data_island_period <= data_island_period_instantaneous;
             end
         end