为什么出现这个错误

sdisen

expected choices in case statement      --- case ad is那句出错

begin
  if(cp'event and cp='1') then
    ad:=ad+1;
     case ad is
      when "0000" =>Atime<=A_g(3 downto 0);
      when "0001" =>Atime<=A_y(3 downto 0);
    ....