The linked list is a dynamic data structure. A linked list can be used when the total number of elements is not known in advance. It grows and shrinks in memory, relative to the number of items it contains.
Linked lists are most conveniently implemented using classes in an object-oriented programming language. VHDL has some object-oriented features which can be used for abstracting away the complexity of the implementation from the user.
In this article we are going to use access types,
VHDL includes few built-in types, but offers a number of additional types through extension packages. Two of the most widely used types are the std_logic and std_ulogic. The difference between them is that the former is resolved, while the latter isn’t.
Before we go on to investigate what it means that a type is resolved, let’s first look at the traits that the two types share in common.
Bit and boolean are part of the standard package,
Delta cycles are non time-consuming timesteps used by VHDL simulators for modelling events during execution of VHDL code. They are events that happen in zero simulation time after a preceding event.
VHDL is a parallel programming language, while computers and CPUs work in a sequential manner. When a normal programming language is run, the CPU executes one instruction after the other. While in VHDL, there can be multiple sequences of logic that react to each other in ways that are not compatible with the standard computer architecture.
Most of us stick to a certain way of writing a state machine. Perhaps you type out the construct that you are most familiar with without giving much thought to the alternatives. Depending on the method that you were taught when learning VHDL, you may prefer one method to another.
Over the years, I have seen many different state machine designs. To appease my curiosity, I set out to investigate the most common ways to design finite-state machines (FSMs) in VHDL.
Test your progress with this VHDL quiz after completing part 4 of the Basic VHDL Tutorial series!
It is possible to drive external signals from a procedure. As long as the signal is within the scope of the procedure, it can be accessed for reading or writing, even if it isn’t listed in the parameter list.
Procedures that are declared in the declarative region of the architecture, cannot drive any external signals. This is simply because there are no signals in its scope at compile time. A procedure declared within a process,
An impure function can read or write any signal within its scope, also those that are not on the parameter list. We say that the function has side effects.
What we mean by side effects is that it is not guaranteed that the function will return the same value every time it is called with the same parameters. If the function can read signals that are not on the parameter list,
Functions are subprograms in VHDL which can be used for implementing frequently used algorithms. A function takes zero or more input values, and it always returns a value. In addition to the return value, what sets a function apart from a procedure, is that it cannot contain Wait-statements. This means that functions always consume zero simulation time.
If you are familiar with functions or methods from other programming languages, VHDL functions should be easy to grasp.
A finite-state machine (FSM) is a mechanism whose output is dependent not only on the current state of the input, but also on past input and output values.
Whenever you need to create some sort of time-dependent algorithm in VHDL, or if you are faced with the problem of implementing a computer program in an FPGA, it can usually be solved by using an FSM.
State-machines in VHDL are clocked processes whose outputs are controlled by the value of a state signal.
A procedure is a type of subprogram in VHDL which can help us avoid repeating code. Sometimes the need arises to perform identical operations several places throughout the design. While creating a module might be overkill for minor operations, a procedure is often what you want.
Procedures can be declared within any declarative region. The scope of the procedure will be limited to wherever it’s declared, architecture, package, or process. Whenever you call the procedure,
