I wrote a vhdl code, that would display 4 digits on cpld 7-segment displays. I used a state machine to select the display, and with ... select instruction to select a set of bits given to the current display.

entity disp is
    Port ( clk_in : in  STD_LOGIC;
           an1 : out  STD_LOGIC;
           an2 : out  STD_LOGIC;
           an3 : out  STD_LOGIC;
           an4 : out  STD_LOGIC);
           g : out  STD_LOGIC;
           f : out  STD_LOGIC;
           e : out  STD_LOGIC;
           d : out  STD_LOGIC;
           c : out  STD_LOGIC;
           b : out  STD_LOGIC;
           a : out  STD_LOGIC);
end disp;

architecture Behavioral of disp is
    type states is (s0, s1, s2, s3);
    signal present_state, next_state: states;
    signal outp: std_logic_vector (0 to 3);
    signal counter: integer range 0 to 199999:=0;
    signal tym: std_logic:='0';
    signal clk_out: std_logic;
    signal WY : std_logic_vector (6 downto 0);
    signal WE : std_logic_vector (3 downto 0);


--state machine
process (present_state)
    case present_state is
        when s0 =>
            outp <= "0111";
            next_state <= s1;                
        when s1 =>
            outp <= "1011";
            next_state <= s2;    
        when s2 =>
            outp <= "1101";
            next_state <= s3;
        when s3 =>
            outp <= "1110";
            next_state <= s0;
    end case;
end process;

--frequency divider
process (clk_in)
    if (rising_edge(clk_in)) then
            if (counter = 199999) then
                    tym <= not tym;
                    counter <= 0;
                    counter <= counter+1;
            end if;
    end if;
end process;

clk_out <= tym;

--state changing
process (clk_out)
    if (rising_edge(clk_out)) then
        present_state <= next_state;
    end if;
end process;


--set of bits for 7-segment display
WE <= an4 & an3 & an2 & an1;
with WE select
WY <= "1000000" when "0111", --display 0
           "1111001" when "1011", --display 1
           "0100100" when "1101", --display 2
           "0000000" when "1110", --display  8
           "1111111" when others; --display nothing

g <= WY(6);
f <= WY(5);
e <= WY(4);
d <= WY(3);
c <= WY(2);
b <= WY(1);
a <= WY(0);

end Behavioral;

I do something wrong with the with ... select instruction input, because Xilinx shows an error for the bolded line saying "Cannot read from 'out' object an4 ; use 'buffer' or 'inout'". How to solve this problem?

  • \$\begingroup\$ The closing paren an4 : out STD_LOGIC); doesn't belong. Instead of declaring an1-an4 mode buffer you could WE <= outp(3) & outp(2) & outp(1) & outp(0); -- WE <= an4 & an3 & an2 & an1;. \$\endgroup\$
    – user8352
    May 22, 2016 at 2:46

2 Answers 2


Cannot read from 'out' object an4 ; use 'buffer' or 'inout'

This error indicates that you're using an old version of the VHDL standard. The simplest fix is to pass the option that causes the analyzer to use the VHDL-2002 or VHDL-2008 rules.

an1 : out  STD_LOGIC;
an2 : out  STD_LOGIC;
an3 : out  STD_LOGIC;
an4 : out  STD_LOGIC);

As the error message suggests change these from "out" to "buffer".


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