added sync retention to dp_packet_merge

Closes L2SDP-713

libraries/base/dp/src/vhdl/dp_packet_merge.vhd

@@ -201,6 +201,7 @@ ARCHITECTURE rtl OF dp_packet_merge IS
     pkt_cnt     : NATURAL RANGE 0 TO g_nof_pkt+1;
     align_cnt   : NATURAL RANGE 0 TO g_nof_pkt+1;
     busy        : STD_LOGIC;
+    sync        : STD_LOGIC;
     src_out     : t_dp_sosi;
   END RECORD;
 
@@ -324,9 +325,17 @@ BEGIN
     IF snk_in.sop = '1' THEN
       IF r.pkt_cnt = 0 THEN
         v.src_out.sop     := '1';
-        v.src_out.sync    := snk_in.sync;     -- only preserve sync when sync interval and nof_pkt interval are aligned, else sync gets lost but could still be recovered from BSN
+        v.src_out.sync    := r.sync;          -- use sync from previous merged packet if it occurred on pkt_cnt /= 0.
         v.src_out.bsn     := snk_in.bsn;      -- use BSN     of first packet for merged packet
         v.src_out.channel := snk_in.channel;  -- use channel of first packet for merged packet
+        v.sync            := '0';             -- reset captured sync.
+        IF snk_in.sync = '1' THEN
+          v.src_out.sync  := '1';     -- set out sync to '1' if this first block contains the sync.
+        END IF;
+      ELSE
+        IF snk_in.sync = '1' THEN
+          v.sync := '1'; -- Capture sync if it occurs on a pkt_cnt /= 0 so we can use it in the next merged packet
+        END IF;
       END IF;
     END IF;
     
@@ -359,6 +368,7 @@ BEGIN
       v.pkt_cnt     := 0;
       v.align_cnt   := 0;
       v.busy        := '0';
+      v.sync        := '0';
       v.src_out     := c_dp_sosi_rst;
     END IF;