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Verilog Define Statements

Verilog provides a text macro substitution facility. This may be useful for defining constants, opcodes, or with conditional compilation. To define a text macro, use the define keyword.

 `define myCode 13 

When you want to use the macro, simple use the macroname with a accent key.

a = `myCode;

The following compile directives are available for use

`include   Includes another file, avoid using pathnames
`define    Define a text macro
`ifdef     Starts conditional compilation, dependant on if a macro has been defined
`else      Alternate condiational compilation
`endif     Ends conditional compilation
`ifndef    Starts conditional compilation, like `ifdef
`elseif    Alternative conditional compilation

The use of `define doesn't require that the macro be given a value. This allows for the following code style (exceprt from a defines.v file).

// Number of bits used for devider register. If used in system with
// low frequency of system clock this can be reduced.
// Use SPI_DIVIDER_LEN for fine tuning theexact number.
//
`define SPI_DIVIDER_LEN_8
//`define SPI_DIVIDER_LEN_16
//`define SPI_DIVIDER_LEN_24
//`define SPI_DIVIDER_LEN_32

`ifdef SPI_DIVIDER_LEN_8
  `define SPI_DIVIDER_LEN       5    // Can be set from 1 to 8
`endif                                                          
`ifdef SPI_DIVIDER_LEN_16                                       
  `define SPI_DIVIDER_LEN       16   // Can be set from 9 to 16
`endif                                                          
`ifdef SPI_DIVIDER_LEN_24                                       
  `define SPI_DIVIDER_LEN       24   // Can be set from 17 to 24
`endif                                                          
`ifdef SPI_DIVIDER_LEN_32                                       
  `define SPI_DIVIDER_LEN       32   // Can be set from 25 to 32 
`endif

Likewise, conditional compilation of code is possible

module add23( 
  a,
  b,
  `ifdef ADD3 
  c,
  `endif
sum);

output [3:0] sum;
input [3:0] a, b;
`ifdef ADD3
input [3:0] c;
`endif

assign sum = 
`ifdef ADD3
  c+
`endif
  a+b;

endmodule

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