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dt-code [2010/12/20 13:42]
beckmanf created 		dt-code [2025/04/10 16:09] (current)
beckmanf bilder in dokuwiki
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-=== VHDL Code Beispiele ​===+===== VHDL Entity and Architecture =====
  
-== Entity ​and Architecture ==+VHDL splits interface (entity) ​and implementation (architecture). The interface of a circuit is defined in the entity. The implementation is defined in the architecture.
  
 <code vhdl> <code vhdl>
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   a_i : in std_ulogic;   a_i : in std_ulogic;
   b_i : in std_ulogic; ​   b_i : in std_ulogic; ​
-  ​y_i : out std_ulogic);​ +  ​y_o : out std_ulogic);​ 
-end first;+end and_gate;
  
 architecture rtl of and_gate is architecture rtl of and_gate is
 begin begin
   y_o <= a_i and b_i;    y_o <= a_i and b_i; 
-end rtl;  +end architecture ​rtl; 
-  ​+
 </​code>​ </​code>​
 +Listing 1: Entity and Architecture for and_gate
  
-== Strukturelle Beschreibung von Schaltungen ​==+The code in listing 1 describes the full code for a circuit called "​and_gate"​. The entity with the name "​and_gate"​ describes two input ports "​a_i"​ and "​b_i"​ and an output port "​y_o"​. All ports here have the type "​std_ulogic"​ which defines that those ports basically can have a logic 1 or 0 value. (Some other values for simulation purposes are left for now). An entity can have an arbitrary number of inputs and outputs. Each input and output can have a different type. 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​and_gate.svg?​width=300 |AND Gate Block}} 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​and_gate.svg?​linkonly |Figure 1}}: Block and_gate with a_i and b_i as inputs and y_o as output 
 + 
 +Figure 1 shows the block "​and_gate"​ with the inputs and outputs. This represents the entity. The entity code does not imply any functionality. The name "​and_gate"​ is an identifier, i.e. it does not imply that this block actually works as an and gate. 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​and_gate_arch.svg?​width=300 |}} 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​and_gate_arch.svg?​linkonly |Figure 2}}: Architecture of "​and_gate"​ which just contains an AND gate. 
 + 
 +The architecture with the name "​rtl"​ describes the implementation. Here it is a signal assignment with a boolean expression using the and function. This makes this circuit behave like an AND gate. Figure 2 shows the schematic with the AND gate. As the AND function can be described in one line of code, this block "​and_gate"​ does not make too much sense, but it explains the idea of entity and architecture. 
 + 
 +===== Instantiation and hierarchical design ===== 
 + 
 +Circuits are very often composed of subcircuits. And the subcircuits can themself be composed of subcircuits. This is called hierarchical design. The following example shows a block "​mux"​.  
 + 
 +{{ :​public:​praktikum_digitaltechnik:​mux.svg?​width=200 | Multipexer block}} 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​mux.svg?​linkonly |Figure 3}}: "​mux"​ block with a_i, b_i and s_i inputs and y_o output 
 + 
 +Figure 3 shows the block interface of a circuit "​mux"​ which has three inputs a_i, b_i and s_i and an output y_o.  
 + 
 +This "​mux" ​ is composed of two instances of "​and_gate",​ an "​or_gate"​ and an "​inv_gate"​. 
 +Assume that we have defined not only the "​and_gate"​ but also an "​or_gate"​ and an "​inv_gate"​. The mux circuit shall be composed of the xxx_gate subcircuits as shown in figure 4. 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​mux-vhdl.svg?​width=800 | Mux Schematic}} 
 + 
 +{{ :​public:​praktikum_digitaltechnik:​mux-vhdl.svg?​linkonly |Figure 4}}: Schematic of "​mux"​ 
 + 
 +Figure 4 shows a schematic of a multiplexer using and_gate, or_gate and the inv_gate. The corresponding VHDL code is shown in listing 2.
  
 <code vhdl> <code vhdl>
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 end mux; end mux;
  
-architecture structure of mux is +architecture structure of mux is    ​
- +
-  component and_gate  +
-    port( +
-      a_i : in std_ulogic;​ +
-      b_i : in std_ulogic;​ +
-      y_o : out std_ulogic);​ +
-       +
-  component or_gate +
-    port( +
-      a_i : in std_ulogic;​ +
-      b_i : in std_ulogic;​ +
-      y_o : out std_ulogic);​ +
-   +
-  component inv_gate +
-    port( +
-      a_i : in std_ulogic;​ +
-      y_o : out std_ulogic);​ +
-      ​+
   signal s1 : std_ulogic;   signal s1 : std_ulogic;
   signal s2 : std_ulogic;   signal s2 : std_ulogic;
   signal s3 : std_ulogic; ​       signal s3 : std_ulogic; ​    
-    ​ 
 begin begin
  
-  inv_gate_i0 : inv_gate+  inv_gate_i0 : work.inv_gate
   port map(   port map(
     a_i => s_i,     a_i => s_i,
     y_o => s1);     y_o => s1);
     ​     ​
-  and_gate_i0 : and_gate+  and_gate_i0 : work.and_gate
   port map(   port map(
     a_i => s1,     a_i => s1,
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     y_o => s2);     y_o => s2);
     ​     ​
-  and_gate_i1 : and_gate+  and_gate_i1 : work.and_gate
   port map(   port map(
     a_i => s_i,     a_i => s_i,
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     y_o => s3);     y_o => s3);
   ​   ​
-  or_gate_i0 : or_gate+  or_gate_i0 : work.or_gate
   port map(   port map(
     a_i => s2,     a_i => s2,
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     y_o => y_o);     y_o => y_o);
  
-end mux; +end architecture structure; 
  
 </​code>​ </​code>​
 +
 +Listing 2: Structural VHDL code that instantiates and_gate, or_gate and inv_gate
 +
 +The instantiation code introduces the name of the instance. In this example the names of the "​and_gate"​ instances are "​and_gate_i0"​ and "​and_gate_i1"​. The instantiation code instantiates "​work.and_gate"​ which means that there is a component "​and_gate"​ assumed to be in library "​work"​. The "​and_gate"​ component is created in the work library when the corresponding vhdl code that defines the "​and_gate"​ is compiled.
 +
 +===== Signals =====
 +
 +Note the signals s1, s2 and s3 in listing 2 which work as internal nets in "​mux"​. But it is also possible to assign the value of an input port to a signal. The value of a signal can also be assign to an output port.
 +
 +Signals are defined between "​architecture"​ and the following "​begin"​.
 +
 +===== Multiplexer Code - the real thing =====
 +
 +The previous code example in listing 2 and listing 1 show the hierarchical design of a multiplexer. Nobody would ever do it like that - it only serves as an example for hierarchical design which is used when the subcircuits have some complexity. The multiplexer function as described in listing 2 can be written in one line of code in VHDL. Listing 3 shows the full code of "​mux"​ with entity and architecture. The function inside the architecture is only one line of code.
 +
 +<code vhdl>
 +library ieee;
 +use ieee.std_logic_1164.all;​
 +
 +entity mux is
 +port (
 +  a_i : in std_ulogic;
 +  b_i : in std_ulogic;
 +  s_i : in std_ulogic;
 +  y_o : out std_ulogic);​
 +end mux;
 +
 +architecture rtl of mux is    ​
 +begin
 +y_o <= a_i when s_i = '​0'​ else b_i;
 +end architecture;​
 +</​code>​
 +
 +Listing 3: Multiplexer Code in one line
 +
 +
 +
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