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Hello, this is my first post. I'm very new to digital logic and VHDL. I started by making some VHDL for just a clock, which I was able to compile with VHDL just fine, and simulate, and view the output in gtkwave. What I was hoping to do next is just an 8bit ripple adder. However, I'm stuck on an error. The following VHDL: library ieee; use ieee.std_logic_1164.all; entity and_gate is port ( a, b: in std_logic; a_and_b: out std_logic ); end; architecture behavior of and_gate is begin process (a, b) begin a_and_b <= a and b; end process; end; entity xor_gate is port ( a: in std_logic; b: in std_logic; a_xor_b: out std_logic ); end; Fails with the errors: $ ghdl a 8_bit_adder.vhdl 8_bit_adder.vhdl:23:15: no declaration for "std_logic" 8_bit_adder.vhdl:24:15: no declaration for "std_logic" 8_bit_adder.vhdl:25:22: no declaration for "std_logic" ghdl: compilation error The error goes away if I delete the "xor_gate" entity. I'm surprised at the error as I don't know why I wouldn't be able to use the same type (std_logic) several times. Please help! :)
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Edmund wrote: > library ieee; > use ieee.std_logic_1164.all; Thats missing before the second entity. Each VHDL module starts with the used libraries...
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Unfortunately, I've run into another error that I don't know how to interpret. The following VHDL:
library ieee; use ieee.std_logic_1164.all; entity and_gate is port ( a: in std_logic; b: in std_logic; a_and_b: out std_logic ); end; architecture behavior of and_gate is begin process (a, b) begin a_and_b <= a and b; end process; end; library ieee; use ieee.std_logic_1164.all; entity xor_gate is port ( a: in std_logic; b: in std_logic; a_xor_b: out std_logic ); end; architecture behavior of xor_gate is begin process (a, b) begin a_xor_b <= a xor b; end process; end; library ieee; use ieee.std_logic_1164.all; entity half_adder is port ( a: in std_logic; b: in std_logic; sum: out std_logic; carry: out std_logic ); end; architecture structure of half_addr is component and_gate is port ( a: in std_logic; b: in std_logic; a_and_b: out std_logic ); end component; component xor_gate is port ( a: in std_logic; b: in std_logic; a_xor_b: out std_logic ); end component; begin g1: xor_gate port map ( a => a, b => b, a_xor_b => sum ); g2: and_gate port map ( a => a, b => b, a_and_b => carry ); end; 
Yields: $ ghdl a 8_bit_adder.vhdl 8_bit_adder.vhdl:53:14: entity 'half_addr' was not analysed ghdl: compilation error It is probably obvious that this is my first structural VHDL architecture, I'm having trouble finding a good resource on VHDL syntax/semantics. Thank you in advance.
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Edited by Moderator
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Oh uh.. nevermind. "half_addr" versus "half_adder".
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Edmund wrote: > Oh uh.. nevermind Ok, but in the next post with VHDL code try that (its to be found a few lines above the edit box):
Formatting options [vhdl]VHDL code[/vhdl] 
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Edited by Moderator
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Hi, I was trying to combine my 4:1 mux and register together. By using the method above, where in the code should i put my processes of both the components? in the port map?
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Danial Sadiq wrote: > By using the method above There you can see two entities and_gate and xor_gate which are used as components in a top level entity half_adder. > in the port map? In a port map you have to put the ports of the entity. And that entity afterwards is instantiated as a component in a top level entity. You can see, that the two processes in the two entities and_gate and xor_gate do not show up in any port map. > I was trying to combine my 4:1 mux and register together. Which mux and which registers? I can't see no code with that... Post your code (wrap it as described with the [vhdl] tags), then we can discuss your particular problem.
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Please use in one VHDLfile only one entry. Your code will be better readable. Halfadd and fulladd are only simple demonstrators. The VHDLlanguage has for this the operator + for signed and unsigned signal.