在VHDL中,一些信号属性(例如’TRANSACTION)是隐含信号.其他人(例如’EVENT)则不然.为什么是这样？ 最佳答案 返回的VHDL对象,其类型和值不受语言限制.用户定义的属性仅限于常量值.

6.7 Attribute declarations
An attribute is a value, function, type, range, signal, or constant that may be associated with one or more named entities in a description. There are two categories of attributes: predefined attributes and user-defined attributes. Predefined attributes provide information about named entities in a description. Clause 16 contains the definition of all predefined attributes. Predefined attributes that are signals shall not be updated.

User-defined attributes are constants of arbitrary type. Such attributes are defined by an attribute declaration.

§ 6.7 on p. 92, IEEE Standard VHDL Language Reference Manual, IEEE Standard 1076-2008

因此,内置属性可以映射到几乎所有内容.在’事务的情况下,返回对象是类型为bit的信号.

在VHDL语言中,属性或tick语法是一个很好的紧凑的东西.它用于多种用途.