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--- dt-code-comb [2024/03/18 19:03] – created beckmanf
+++ dt-code-comb [2024/03/26 09:39] (current) – type in process beckmanf
@@ Line 151: / Line 151: @@
 </code>
+===== Process for Combinational Circuit =====
+Processes for combinational circuits can also use variables. The assignment operator for a variable assignment is ":=". Variables change the value immediately, but the variable only has a scope inside the process. Inside a process the sequential statements like "if..then..else if..else..end if" or "case" or "for"  can be used.
+The following example code shows the use of a process with a for loop and and if statement to compute the majority function. m is '1' when more than half of the bits of "a" are '1'. Otherwise m is '0'.
+<code vhdl>
+architecture rtl of majority_circuit is
+  signal a : std_ulogic_vector(7 downto 0);
+  signal m : std_ulogic;
+begin
+  maj_p : process(a)
+    variable cnt : integer range 0 to 8;
+  begin process
+    m <= '0';
+    cnt := 0;
+    for i in 0 to 7 loop
+      if a(i) = '1' then
+        cnt := cnt + 1;
+      end if;
+    end loop;
+    if cnt > 4 then
+      m <= '1';
+    end if;
+  end process;
+end architecture rtl;
+</code>
 ===== Multiple Drivers not allowed =====
@@ Line 164: / Line 190: @@
 end architecture rtl;
 </code>
+===== Combinational Circuits - Always assign a value =====
+Combinational circuits must always have an assignment of a value when a statement is evaluated. Otherwise the synthesis software will infer an unwanted memory element. That is usually a latch. You must make sure that vhdl code that is supposed to describe combinational logic always assigns a value.
+The following code will assign the value of "b" to "y" only when "a" has the value '1'. What should happen when a is '0'? The synthesis will infer a latch to remember the last value when "a" is going from '1' to '0'. This is really bad and will result in timing problems.
+<code vhdl>
+architecture rtl of crap_code is
+  signal a,b,y : std_ulogic;
+begin
+  y <= b when a = '1';
+end architecture rtl;
+</code>