Add deadlock detection example using SymbiYosys

docs/examples/deadlock_detection/fsm.v

@@ -0,0 +1,47 @@
+module fsm (
+    input clk,
+    input rst,
+    input req,
+    output reg grant
+);
+
+parameter IDLE  = 2'b00;
+parameter WAIT  = 2'b01;
+parameter GRANT = 2'b10;
+
+reg [1:0] state;
+
+always @(posedge clk) begin
+    if (rst) begin
+        state <= IDLE;
+        grant <= 0;
+    end else begin
+        case (state)
+            IDLE: begin
+                if (req)
+                    state <= WAIT;
+            end
+
+            WAIT: begin
+                // Deadlock: no exit from WAIT state
+                state <= WAIT;
+            end
+
+            GRANT: begin
+                grant <= 1;
+                state <= IDLE;
+            end
+        endcase
+    end
+end
+
+`ifdef FORMAL
+
+// Check if GRANT state is reachable
+always @(posedge clk) begin
+    cover(grant);
+end
+
+`endif
+
+endmodule