Enumerations

An enumerated type declares a set of integral named constants. Enumerated data types provide the capability to abstractly declare strongly typed variables without either a data type or data value(s) and later add the required data type and value(s) for designs that require more definition.

Enumerated data types also can be easily referenced or displayed using the enumerated names as opposed to the enumerated values. In the absence of a data type declaration, the default data type shall be int. Any other data type used with enumerated types shall require an explicit data type declaration.

An enumerated type defines a set of named values. In the following example, light1 and light2 are defined to be variables of the anonymous (unnamed) enumerated int type that includes the three members: red, yellow and green.

enum {red, yellow, green} light1, light2; // anonymous int type

An enumerated name with x or z assignments assigned to an enum with no explicit data type or an explicit 2-state declaration shall be a syntax error.

// Syntax error: IDLE=2’b00, XX=2’bx , S1=2’b01, S2=2’b10
enum {IDLE, XX=’x, S1=2’b01, S2=2’b10} state, next;

An enum declaration of a 4-state type, such as integer, that includes one or more names with x or z assignments shall be permitted.

// Correct: IDLE=0, XX=’x, S1=1, S2=2
enum integer {IDLE, XX=’x, S1=’b01, S2=’b10} state, next;

An unassigned enumerated name that follows an enum name with x or z assignments shall be a syntax error.

// Syntax error: IDLE=2’b00, XX=2’bx, S1=??, S2=??
enum integer {IDLE, XX=’x, S1, S2} state, next;

The values can be cast to integer types, and increment from an initial value of 0. This can be overridden.

enum {a=3, b=7, c} alphabet;

If an automatically incremented value is assigned elsewhere in the same enumeration, this shall be a syntax error.

// Syntax error: c and d are both assigned 8
enum {a=0, b=7, c, d=8} alphabet;

If the first name is not assigned a value, it is given the initial value of 0.

// a=0, b=7, c=8

enum {a, b=7, c} alphabet;
Any enumeration encoding value that is outside the representable range of the enum shall be an error. If any of the enum members are defined with a different sized constant, this shall be a syntax error.

// Correct declaration - bronze and gold are unsized
enum bit [3:0] {bronze='h3, silver, gold='h5} medal4;

// Correct declaration - bronze and gold sizes are redundant
enum bit [3:0] {bronze=4'h3, silver, gold=4'h5} medal4;

// Error in the bronze and gold member declarations
enum bit [3:0] {bronze=5'h13, silver, gold=3'h5} medal4;

// Error in c declaration, requires at least 2 bits
enum bit [0:0] {a,b,c} alphabet;