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--- resources:combinational_logic_styles [2019/06/28 10:34]
jgoeders created
+++ resources:combinational_logic_styles [2020/02/07 16:22] (current)
nelson
@@ Line 1: / Line 1: @@
 ====== Combinational Logic Styles ======
+There are many different styles of SystemVerilog code that you can use to generate combinational circuits.
+Consider the following logic function that outputs a 1 when an odd number of the three input bits are 1.  (This is a 3-input XOR).
+^ in[2] ^ in[1] ^ in[0] ^ out ^
+| 0 | 0 | 0 | 0 |
+| 0 | 0 | 1 | 1 |
+| 0 | 1 | 0 | 1 |
+| 0 | 1 | 1 | 0 |
+| 1 | 0 | 0 | 1 |
+| 1 | 0 | 1 | 0 |
+| 1 | 1 | 0 | 0 |
+| 1 | 1 | 1 | 1 |
+<code SystemVerilog>
+logic [2:0] in;
+logic       out;
+<Combinational Logic Here>
+</code>
+Assuming the above SystemVerilog code, there are many different ways to implement the same combinational logic.   Here are some different examples:
+----
+=== Structural SV ===
+Sum of Products:
+<code SystemVerilog>
+logic [2:0] in_not;
+not(in_not[0], in[0]);
+not(in_not[1], in[1]);
+not(in_not[2], in[2]);
+and(term1, in_not[2], in_not[1], in[0]);
+and(term2, in_not[2], in[1], in_not[0]);
+and(term3, in[2], in_not[1], in_not[0]);
+and(term4, in[2], in[1], in[0]);
+or(out, term1, term2, term3, term4);
+</code>
+Minimized (single XOR gate):
+<code SystemVerilog>
+xor(out, in[2], in[1], in[0]);
+</code>
+----
+=== Dataflow SV ===
+Using assign statement, and the ternary operator (also known as the ?: operator):
+<code SystemVerilog>
+assign out =
+  (in==3’b000)?0:
+  (in==3’b001)?1:
+  (in==3’b010)?1:
+  (in==3’b011)?0:
+  (in==3’b100)?1:
+  (in==3’b101)?0:
+  (in==3’b110)?0:
+;
+</code>
+Using assign statement, with sum of products, and dataflow operators :
+<code SystemVerilog>
+assign out = (~in[2] & ~in[1] & in[0]) | (~in[2] & in[1] & ~in[0]) |
+             (in[2] & ~in[1] & ~in[0]) | (in[2] & in[2] & in[0]);
+</code>
+Using assign statement, vectored comparison operators, binary literals:
+<code SystemVerilog>
+assign out = (in == 3'b001) || (in == 3'b010) || (in == 3'b100) || (in == 3'b111);
+</code>
+Using assign statement, vectored comparison operators, decimal literals:
+<code SystemVerilog>
+assign out = (in == 3'd1) || (in == 3'd2) || (in == 3'd4) || (in == 3'd7);
+</code>
+----
+=== Behavioral SV ===
+Using ''always_comb'' block with if statement.
+<code SystemVerilog>
+always_comb begin
+    out = 1'b0;
+    if ((in == 3'b001) || (in == 3'b010) || (in == 3'b100) || (in == 3'b111)) begin
+        out = 1'b1;
+    end else begin
+        // This else isn't necessary because of the default value at the top, but is included here
+        // to show you the syntax.
+        out = 1'b0;
+    end
+end
+</code>
+Using ''always_comb'' block with case statement.
+<code SystemVerilog>
+always_comb begin
+    out = 1'b0;
+    case(in)
+'b001: out = 1'b1;
+'b010: out = 1'b1;
+'b100:
+            out = 1'b1;
+'b111: begin
+            out = 1'b1;
+        end
+        default: begin
+            // This default isn't necessary because of the default value at the top, but is included here
+            // to show you the syntax.
+            out = 1'b0;
+        end
+    endcase
+end
+</code>
+The example above mixes different formatting of the case statements to show you the variations available.  //Note:// Similar to ''if'' and ''always'' blocks, you will need to include a ''begin'' and ''end'' if your case contains more than one statement.

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