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logic [N-1] cntr; // count to 2^N - 1 
always_ff @ (posedge clk) begin
   if (rst) begin
      cntr <= ($bits(cntr))’(‘h0);
   end
   else begin
      if (cntr == elapsed_time) begin
         cntr <= ($bits(cntr))’(‘h0);
      end
      else begin
         cntr <= cntr + ($bits(cntr))’(‘h1);
      end
   end
end