From a5142f9abbb99992abe4201f3ad587159ffcb7ec Mon Sep 17 00:00:00 2001
From: Markus Koch <markus@notsyncing.net>
Date: Sun, 30 Jan 2022 18:03:09 +0100
Subject: [PATCH] des: Improve header synchronization

---
 trashernet/trashernet_phy.vhd | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/trashernet/trashernet_phy.vhd b/trashernet/trashernet_phy.vhd
index 694c261..de00ed1 100644
--- a/trashernet/trashernet_phy.vhd
+++ b/trashernet/trashernet_phy.vhd
@@ -128,7 +128,9 @@ begin
 
 			-- Bit recovery
 			type demanchestization_state_t is (SYNC, DATA, ERROR);
-			signal demanchestization_state : demanchestization_state_t;
+			signal demanchestization_state              : demanchestization_state_t;
+			constant DEMANCHESTIZATION_MIN_SYNC_CNT_MAX : integer := (4 * 8) - 1; -- 4 good sync bytes
+			signal demanchestization_min_sync_cnt       : integer range 0 to DEMANCHESTIZATION_MIN_SYNC_CNT_MAX;
 		begin
 			-- Detects spacing of transitions
 			transition_detector : process(clk, rst) is
@@ -200,21 +202,30 @@ begin
 			bit_recovery : process(clk, rst) is
 			begin
 				if rst then
-					demanchestization_state <= SYNC;
-					bit_stb                 <= '0';
-					phy_out.rx_active       <= '0';
-					phy_out.rx_error        <= '0';
+					demanchestization_state        <= SYNC;
+					bit_stb                        <= '0';
+					phy_out.rx_active              <= '0';
+					phy_out.rx_error               <= '0';
+					demanchestization_min_sync_cnt <= DEMANCHESTIZATION_MIN_SYNC_CNT_MAX;
 				elsif rising_edge(clk) then
 					bit_stb          <= '0';
 					phy_out.rx_error <= '0';
 
+					if (bit_ev = TOGGLE) and (demanchestization_min_sync_cnt /= 0) then
+						demanchestization_min_sync_cnt <= demanchestization_min_sync_cnt - 1;
+					elsif (bit_ev = KEEP) or (bit_ev = ERROR) then
+						demanchestization_min_sync_cnt <= DEMANCHESTIZATION_MIN_SYNC_CNT_MAX;
+					end if;
+
 					if (bit_ev /= NONE) then
 						case demanchestization_state is
 							when SYNC =>
 								if (bit_ev = KEEP) then
-									bit_value               <= '1';
-									demanchestization_state <= DATA;
-									phy_out.rx_active       <= '1';
+									if demanchestization_min_sync_cnt = 0 then
+										bit_value               <= '1';
+										demanchestization_state <= DATA;
+										phy_out.rx_active       <= '1';
+									end if;
 								end if;
 							when DATA => -- @suppress: Condition outside of case allows to exit this state
 								bit_value <= not bit_value when bit_ev = TOGGLE else bit_value;
@@ -225,7 +236,7 @@ begin
 					end if;
 
 					if (bit_ev = ERROR) then
-						phy_out.rx_error        <= '1';
+						phy_out.rx_error        <= '1' when (demanchestization_state = DATA) else '0';
 						demanchestization_state <= ERROR;
 					end if;