Tri state buffer

vhdl n. OP (Company: none) #3417911
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library ieee;

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use ieee.std_logic_1164.all;

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entity tristate_buffer is 

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port (enable: in std_logic;

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d_in: in std_logic_vector ( 7 downto 0 ) ;

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d_out: out std_logic_vector (7 downto 0 )) ;

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end tristate_buffer;

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architecture behavioral of tristate_buffer is

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begin

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process (enable,d_in)

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begin

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if (enable = ‘1’) then

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d_out <= d_in;

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else

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d_out <= (‘zzzzzzzz’);

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end if;

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end process;

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end behavioral;


what is the error in this program?
i get the following errors when i compile it in modelsim

tri_state_buffer.vhd(14): near "?": syntax error
tri_state_buffer.vhd(17): near "?": syntax error
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vhdl n. OP (Company: none) #3418177
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Thanks

1
library ieee;

2
use ieee.std_logic_1164.all;

3


4


5
entity tristate_buffer is 

6
port (enable: in std_logic;

7
d_in: in std_logic_vector ( 7 downto 0 ) ;

8
d_out: out std_logic_vector (7 downto 0 )) ;

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end tristate_buffer;

10


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architecture behavioral of tristate_buffer is

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begin

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process (enable,d_in)

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begin

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if (enable = '1') then

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d_out <= d_in;

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else

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d_out <= ("zzzzzzzz");

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end if;

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end process;

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end behavioral;


now the program is like this.
but new errors are coming like

tri_state_buffer.vhd(19): String literal has a character 'z' not in the 
enumeration type ieee.std_logic_1164.std_logic.

tri_state_buffer.vhd(22): VHDL Compiler exiting

pls help
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