rtl: add sources for ethernet udp stack

rtl/ethernet-udp/src/eth/mac/arp_cache.v

@@ -0,0 +1,51 @@
+module arp_cache
+(
+ input                clk ,
+ input                rst_n ,
+ 
+ input                arp_found,
+ input  [31:0]        arp_rec_source_ip_addr,
+ input  [47:0]        arp_rec_source_mac_addr,
+ 
+ input  [31:0]        destination_ip_addr,
+ output reg [47:0]    destination_mac_addr,
+ 
+ output reg           mac_not_exist
+) ;
+       
+reg [79:0]  arp_cache ;
+
+//init arp cache
+always @(posedge clk or negedge rst_n)
+  begin
+    if (~rst_n)
+      arp_cache  <= 80'h00_00_00_00_ff_ff_ff_ff_ff_ff ;
+    else if (arp_found)
+      arp_cache  <= {arp_rec_source_ip_addr, arp_rec_source_mac_addr} ;
+    else
+      arp_cache  <= arp_cache ;
+  end
+  
+always @(posedge clk or negedge rst_n)
+  begin
+    if (~rst_n)
+      destination_mac_addr  <= 48'hff_ff_ff_ff_ff_ff ;
+    else if (destination_ip_addr == arp_cache[79:48])
+      destination_mac_addr  <= arp_cache[47:0] ;
+    else
+      destination_mac_addr  <= 48'hff_ff_ff_ff_ff_ff ;
+  end
+  
+always @(posedge clk or negedge rst_n)
+  begin
+    if (~rst_n)
+      mac_not_exist  <= 1'b0 ;
+    else if (destination_ip_addr != arp_cache[79:48])
+      mac_not_exist  <= 1'b1 ;
+    else if (destination_ip_addr == arp_cache[79:48] && arp_cache[47:0] == 48'hff_ff_ff_ff_ff_ff)
+      mac_not_exist  <= 1'b1 ;
+    else
+      mac_not_exist  <= 1'b0 ;
+  end
+  
+endmodule