说明该程序逻辑功能。 Library ieee; Use ieee.std_logic_11.all; Use ieee.std_logic_unsigned.all; Entity up_down is Port(clk,rst,en,up: in std_logic; Sum: out std_logic_vector(2 downto 0); Cout: out std_logic); End; Architecture a of up_down is Signal count: std_logic_vector(2 downto 0); Begin Process(clk,rst) Begin If rst=’0’ then Count<=(others=>’0’); Elsif rising_edge(clk) then If en=’1’ then Case up is When ‘1’ => count<=count+1; When others =>count<=count-1; End case; End if; End if; End process; Sum<=count; Cout <=’1’ when en=’1’ and ((up=’1’ and count=7) or (up=’0’ and count=0)) else ‘0’; 进位信号 End;