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resources:combinational_logic_styles [2019/06/28 10:46] jgoeders |
resources:combinational_logic_styles [2020/02/07 16:22] (current) nelson |
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| Line 18: | Line 18: | ||
| <code SystemVerilog> | <code SystemVerilog> | ||
| - | logic [3:0] in; | + | logic [2:0] in; |
| logic out; | logic out; | ||
| Line 25: | Line 25: | ||
| </code> | </code> | ||
| - | Assuming the above SystemVerilog code, there are many different ways to implement the combinational logic: | + | Assuming the above SystemVerilog code, there are many different ways to implement the same combinational logic. Here are some different examples: |
| + | |||
| + | ---- | ||
| === Structural SV === | === Structural SV === | ||
| + | Sum of Products: | ||
| <code SystemVerilog> | <code SystemVerilog> | ||
| - | logic not0, not1, not2; | + | logic [2:0] in_not; |
| - | not(not0, in[0]); | + | not(in_not[0], in[0]); |
| - | not(not1, in[1]); | + | not(in_not[1], in[1]); |
| - | not(not2, in[2]); | + | not(in_not[2], in[2]); |
| - | and(term1, not2, not1, in[0]); | + | and(term1, in_not[2], in_not[1], in[0]); |
| - | and(term1, not2, in[1], not0); | + | and(term2, in_not[2], in[1], in_not[0]); |
| - | and(term1, in[2], not1, not0); | + | and(term3, in[2], in_not[1], in_not[0]); |
| - | and(term1, in[2], in[1], in[0]); | + | and(term4, in[2], in[1], in[0]); |
| - | or(out, term1, term2, term3, term4) | + | or(out, term1, term2, term3, term4); |
| </code> | </code> | ||
| - | OR | + | Minimized (single XOR gate): |
| <code SystemVerilog> | <code SystemVerilog> | ||
| Line 49: | Line 52: | ||
| </code> | </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: | ||
| + | 1; | ||
| + | </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) | ||
| + | 3'b001: out = 1'b1; | ||
| + | 3'b010: out = 1'b1; | ||
| + | 3'b100: | ||
| + | out = 1'b1; | ||
| + | 3'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. | ||
