18-03-2014, 11:18 AM
VHDL
VHDL.ppt (Size: 401.5 KB / Downloads: 21)
Basic Language Elements
Identifiers
Composed of sequence of 1 or more characters, upper case, lower case, digit or ( _) underscore character.
The first character in an identifier must be a letter and the last character may not be an underscore.
Upper case and Lower case are considered identical when used as identifier. Eg COUNT = Count = CouNT
Process Statement
A process statement contains sequential statements that describe the functionality of a portion of an entity in sequential terms.
Recall concurrent statements where all the statements were executed parallel, but inside a process statements, statements are executed to introduce sequential control.
Process statement itself is a concurrent statement.
Operations like case, if else can be used with
process statement.
Process Statement Syntax
A process has a sensitivity list.
The execution of the statements inside of the process happens when the value of any signal in the sensitivity list changes.
So, the signals that should be listed in the sensitivity list are those that can cause outputs to change inside of the process.
WAIT Condition
When a wait statement is encountered, the process in which appears that statement suspends. When the condition specified in the wait statement is met, the process resumes and its statements are executed until another wait statement is encountered.
The VHDL language allows several wait statements in a process. When used to model combinational logic for synthesis, a process may contain only one wait statement. If a process contains a wait statement, it cannot contain a sensitivity list.