Lexical Conventions

Verilog HDL uses lexical conventions similar to those in the C programming language. It is a case-sensitive language with all keywords in lowercase.

Comments

Comments in Verilog enhance code readability and documentation. There are two types of comments:

Single-Line Comments:
Start with // and continue to the end of the line.

Example:

 // This is a single-line comment

Multiple-Line Comments:
Start with /* and end with */. Single-line comments can be embedded within multiple-line comments. However, multiple-line comments cannot be nested.

Example:

 This is a multiple-line comment.
  It can span multiple lines.
  // You can use single-line comments within multiple-line comments.
*/

Attempting to nest multiple-line comments will result in an error:

 /* 
  This will cause an error
  /* Nested comment */
*/

Whitespace

Whitespace characters (spaces \b, tabs \t, and newlines \n) are ignored by Verilog except when they separate tokens. Whitespace is not ignored within strings.

Operators

Operators in Verilog are categorized as follows:

  • Unary Operators: Operate on a single operand.
    Example: a = ~b; // ~ is a unary operator

  • Binary Operators: Operate between two operands.
    Example: a = b && c; // && is a binary operator

  • Ternary Operators: Use two operators to operate on three operands.
    Example: a = b ? c : d; // ?: is a ternary operator

Data Types

In Verilog, data types represent different logic states:

  • 1'b0: Logic 0 (false or ground)
  • 1'b1: Logic 1 (true or power)
  • 1'bX: Unknown logic state
  • 1'bZ: High impedance (floating)

Number Specification

Verilog supports two types of number specifications: sized and unsized.

Sized Numbers:
Represented as <size>'<base format> <number>.

  • <size> specifies the number of bits (decimal).
  • <base format> specifies the number's base: decimal ('d or 'D), hexadecimal ('h or 'H), binary ('b or 'B), and octal ('o or 'O).
  • <number> is the actual number.

Examples:

 4'b1111    // 4-bit binary number (15 in decimal)
12'habc    // 12-bit hexadecimal number (2748 in decimal)
16'd255    // 16-bit decimal number
12'h13x    // 12-bit hexadecimal number with 4 least significant bits unknown

Unsized Numbers:
By default, these are decimal and 32 bits in size unless otherwise specified.

Examples:

 23456      // 32-bit decimal number
'hc3       // 32-bit hexadecimal number
'o21       // 32-bit octal number
'bz        // 32-bit high impedance number

Negative Numbers:
Indicate negative values by prefixing the size with a minus sign.

Examples:

 -6'd3      // 8-bit negative number stored as 2's complement of 3
3'd-1      // Illegal specification

Underscore Characters:
Used to improve readability by separating digits in numbers.

Example:

 8'b1000_1010 // Binary number with underscores

Question Mark (?):
Represents an unknown state or high impedance, alternative to Z.

Example:

 2'b?? // Equivalent to 2'bzz