TABLE OF CONTENTS As mentioned in chapter 5, VHDL code is inherently concurrent. PROCESSES, FUNCTIONS, and PROCEDURES are the only sections of code that are executed sequentially. However, as a whole, any of these blocks is still concurrent with any other statements placed outside it.
One important aspect of sequential code is that it is not limited to sequential logic. Indeed, with it we can build sequential circuits as well as combinational circuits. Sequential code is also called behavioral code.
The statements discussed in this section are all sequential, that is, allowed only inside PROCESSES, FUNCTIONS, or PROCEDURES. They are: IF, WAIT, CASE, and LOOP.
VARIABLES are also restricted to be used in sequential code only (that is, inside a PROCESS, FUNCTION, or PROCEDURE). Thus, contrary to a SIGNAL, a VARIABLE can never be global, so its value can not be passed out directly.
We will concentrate on PROCESSES here. FUNCTIONS and PROCEDURES are very similar, but are intended for system-level design, being therefore seen in Part II of this book.
A PROCESS is a sequential section of VHDL code. It is characterized by the presence of IF, WAIT, CASE, or LOOP, and by a sensitivity list (except when WAIT is used). A PROCESS must be installed in the main code, and is executed every time a signal in the sensitivity list changes (or the condition related to WAIT is fulfilled). Its syntax is shown below.
[label:] PROCESS (sensitivity list) [VARIABLE name...
