added code to implement REVERSE_LATENCY using srl16

diff --git a/src/edu/berkeley/fleet/fpga/verilog/Verilog.java b/src/edu/berkeley/fleet/fpga/verilog/Verilog.java
index f0a25d4..3ac807f 100644 (file)
--- a/src/edu/berkeley/fleet/fpga/verilog/Verilog.java
+++ b/src/edu/berkeley/fleet/fpga/verilog/Verilog.java
@@ -42,9 +42,8 @@ import static edu.berkeley.fleet.two.FleetTwoFleet.*;
  */
 public class Verilog {
 
-    //public static final int FORWARD_LATENCY = 1;
-    public static final int FORWARD_LATENCY = 6;
-    public static final int REVERSE_LATENCY = 3;
+    public static final int FORWARD_LATENCY = 6;  // must be >=1
+    public static final int REVERSE_LATENCY = 4;  // must be >=3
 
     public static interface Value {
         public String  getVerilog();
@@ -508,8 +507,25 @@ public class Verilog {
             public String getResetCode() { return driven==null ? name+"_a<=1;" : ""; }
             public String getAssignments() {
                 StringBuffer sb = new StringBuffer();
-                if (external)
-                    sb.append("assign " +                 name +"_a_ = " + name + "_a;\n");
+                if (external) {
+                    int a = REVERSE_LATENCY - 3;
+                    if (a<0 || a>16) {
+                        throw new RuntimeException("cannot offer latency of " + REVERSE_LATENCY +"-3");
+                    } else if (a==0) {
+                        sb.append("assign " +                 name +"_a_ = " + name + "_a;\n");
+                    } else {
+                        a = a-1;  // CLK-to-Q gives us one cycle of latency anyways
+                        sb.append("SRL16E srl16_"+name+"_a\n");
+                        sb.append("              (.Q ("+name+"_a_),\n");
+                        sb.append("               .A0 ("+((a & (1<<0)) == 0 ? 0 : 1)+"),\n");
+                        sb.append("               .A1 ("+((a & (1<<1)) == 0 ? 0 : 1)+"),\n");
+                        sb.append("               .A2 ("+((a & (1<<2)) == 0 ? 0 : 1)+"),\n");
+                        sb.append("               .A3 ("+((a & (1<<3)) == 0 ? 0 : 1)+"),\n");
+                        sb.append("               .CE (1),\n");
+                        sb.append("               .CLK (clk),\n");
+                        sb.append("               .D ("+name+"_a));\n");
+                    }
+                }
                 return sb.toString();
             }
             public void connect(SinkPort driven) {