1 files changed, 81 insertions, 0 deletions
diff --git a/control_lib/ss_rcvr.v b/control_lib/ss_rcvr.v
new file mode 100644
index 000000000..8e650d21b
--- /dev/null
+++ b/control_lib/ss_rcvr.v
@@ -0,0 +1,81 @@
+
+
+// Source-synchronous receiver
+// Assumes both clocks are at the same rate
+// Relative clock phase is
+//    unknown
+//    variable
+//    bounded
+// The output will come several cycles later than the input
+
+// This should synthesize efficiently in Xilinx distributed ram cells,
+//   which is why we use a buffer depth of 16
+
+// FIXME Async reset on rxclk side?
+
+module ss_rcvr
+  #(parameter WIDTH=16)
+    (input rxclk,
+     input sysclk,
+     input rst,
+     
+     input [WIDTH-1:0] data_in,
+     output [WIDTH-1:0] data_out,
+     output reg clock_present);
+   
+   wire [3:0] rd_addr, wr_addr;
+
+   // Distributed RAM
+   reg [WIDTH-1:0] buffer [0:15];
+   always @(posedge rxclk)
+     buffer[wr_addr] <= data_in;
+   
+   assign 	   data_out = buffer[rd_addr];
+   
+   // Write address generation
+   reg [3:0] 	   wr_counter;
+   always @(posedge rxclk or posedge rst)
+     if (rst)
+       wr_counter <= 0;
+     else
+       wr_counter <= wr_counter + 1;
+   
+   assign 	   wr_addr = {wr_counter[3], ^wr_counter[3:2], ^wr_counter[2:1], ^wr_counter[1:0]};
+   
+   // Read Address generation
+   wire [3:0] 	   wr_ctr_sys, diff, abs_diff;
+   reg [3:0] 	   wr_addr_sys_d1, wr_addr_sys_d2;
+   reg [3:0] 	   rd_counter;
+   
+   assign 	   rd_addr = {rd_counter[3], ^rd_counter[3:2], ^rd_counter[2:1], ^rd_counter[1:0]};
+   
+   always @(posedge sysclk)
+     wr_addr_sys_d1 <= wr_addr;
+   
+   always @(posedge sysclk)
+     wr_addr_sys_d2 <= wr_addr_sys_d1;
+   
+   assign 	   wr_ctr_sys = {wr_addr_sys_d2[3],^wr_addr_sys_d2[3:2],^wr_addr_sys_d2[3:1],^wr_addr_sys_d2[3:0]};
+   
+   assign 	   diff = wr_ctr_sys - rd_counter;
+   assign 	   abs_diff = diff[3] ? (~diff+1) : diff;
+   
+   always @(posedge sysclk)
+     if(rst)
+       begin
+	  clock_present <= 0;
+	  rd_counter <= 0;
+       end
+     else 
+       if(~clock_present)
+	 if(abs_diff > 5)
+	   clock_present <= 1;
+	 else
+	   ;
+       else
+	 if(abs_diff<3)
+	   clock_present <= 0;
+	 else
+	   rd_counter <= rd_counter + 1;
+
+endmodule // ss_rcvr