Clarified func_apertif_unb1_correlator_inter_channel_delay_min() and added...

Clarified func_apertif_unb1_correlator_inter_channel_delay_min() and added comment on why 63*300 is too small, but could still work on HW.

applications/apertif/designs/apertif_unb1_correlator/src/vhdl/apertif_unb1_correlator_pkg.vhd

@@ -90,7 +90,7 @@ PACKAGE apertif_unb1_correlator_pkg IS
   CONSTANT c_input_db      : t_apertif_unb1_correlator_config := ( TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE,  TRUE,  TRUE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE, 48,     32, 63*300, 8, 3,  88);
   CONSTANT c_mesh_db       : t_apertif_unb1_correlator_config := ( TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE, FALSE,  TRUE,  TRUE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE, 48,     32, 63*300, 8, 3,  88);
   CONSTANT c_bg_output     : t_apertif_unb1_correlator_config := (FALSE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE,  TRUE, FALSE, FALSE,  TRUE, FALSE,  TRUE,  TRUE,  TRUE,  TRUE,  TRUE, FALSE, FALSE, FALSE,  TRUE, 48, 800000, 63*300, 8, 3,  88); -- 12500 * 64 = 800000
-  CONSTANT c_bg_output_sim : t_apertif_unb1_correlator_config := (FALSE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE,  TRUE, FALSE, FALSE,  TRUE, FALSE,  TRUE,  TRUE,  TRUE,  TRUE,  TRUE, FALSE, FALSE, FALSE,  TRUE, 48,  10*64,    300, 2, 2,  2);
+  CONSTANT c_bg_output_sim : t_apertif_unb1_correlator_config := (FALSE, FALSE,  TRUE,  TRUE, FALSE, FALSE, FALSE, FALSE,  TRUE, FALSE, FALSE,  TRUE, FALSE,  TRUE,  TRUE,  TRUE,  TRUE,  TRUE, FALSE, FALSE, FALSE,  TRUE, 48,  10*64,    279, 2, 2,  2);
   
   -- Function to select the revision configuration. 
   FUNCTION func_sel_revision_rec(g_design_name : STRING) RETURN t_apertif_unb1_correlator_config;
@@ -121,6 +121,7 @@ PACKAGE apertif_unb1_correlator_pkg IS
 
   -----------------------------------------------------------------------------
   -- Check inter channel delay
+  -- . See tb_node_apertif_unb1_correlator_processing_output.vhd for more info
   -----------------------------------------------------------------------------
 
   FUNCTION func_apertif_unb1_correlator_inter_channel_delay_max(block_period          : NATURAL;
@@ -247,7 +248,9 @@ PACKAGE BODY apertif_unb1_correlator_pkg IS
   --             127*300    0.1        = 12.8G/128 < 0.144 Gbps, so too small for the required capacity
   -- 
   -- From the available link capacity 0.15625 Gbps it follows that the minimal n = 12.8G / 0.15625 G = 81.92
-  -- and c_inter_channel_delay_min = 24576.
+  -- and c_inter_channel_delay_min = 24576. Maybe c_inter_channel_delay = 63*300 = 18900 < 24576 still
+  -- works on HW in Apertif because the 1GbE /10GbE switch network to the Data Writer has sufficient capacity
+  -- to buffer the packets. However better choose > 24576 instead.
   -- Choose a required capacity of 0.144 Gbps to fit both 8 bit and 6 bit beamlet mode, however due to packet
   -- overhead the required capacity needs to be about 4.2 % more than for the payload data alone. This packet
   -- overhead can be accounted for by increasing the required link capacity by 1.042, so then
@@ -270,8 +273,10 @@ PACKAGE BODY apertif_unb1_correlator_pkg IS
   
   FUNCTION func_apertif_unb1_correlator_inter_channel_delay_min(pinfo            : t_apertif_unb1_correlator_packet_info;
                                                                 nof_visibilities : NATURAL) RETURN INTEGER IS
-    -- Derive minimum inter_channel_delay from avaliable link data rate (independent of nof_beamlets, because the
-    -- burst rate is the same)
+    -- Derive minimum inter_channel_delay from avaliable link data rate. The burst rate is independent of the packet size,
+    -- but the minimum inter_channel_delay does dependent on the packet size, because the inter_channel_delay is applied
+    -- between packets (so not between samples). The inter_channel_delay is independent of nof_beamlets, because a
+    -- packet contains all visibilities for one channel in one beamlet.
     CONSTANT c_inter_channel_delay_min  : INTEGER := pinfo.eth_packet_nof_dp_clk - nof_visibilities;
   BEGIN
     RETURN c_inter_channel_delay_min;