The vast majority of VHDL designs uses clocked logic, also known as synchronous logic or sequential logic. A clocked process is triggered only by a master clock signal, not when any of the other input signals change.
The basic building block of clocked logic is a component called the flip-flop. There are different variants of it, and in this tutorial we are going to focus on the positive-edge-triggered flip-flop with negative reset:
The flip-flop is a sample-and-hold circuit,
Creating modules is a great way to reuse code, but often you need the same module with smaller variations throughout your design. This is what generics and the generic map is for. It allows you to make certain parts of the module configurable at compile-time.
Constants are used when we want to avoid typing the same value over and over again. They can be used for defining bit-widths of signal vectors at compile-time,
A module is a self-contained unit of VHDL code. Modules communicate with the outside world through the entity. Port map is the part of the module instantiation where you declare which local signals the module’s inputs and outputs shall be connected to.
In previous tutorials in this series we have been writing all our code in the main VHDL file, but normally we wouldn’t do that. We create logic with the purpose of using it in an FPGA or ASIC design,
The Case-When statement will cause the program to take one out of multiple different paths, depending on the value of a signal, variable, or expression. It’s a more elegant alternative to an If-Then-Elsif-Else statement with multiple Elsif’s.
Other programming languages have similar constructs, using keywords such as a switch, case, or select. Among other things, Case-When statements are commonly used for implementing multiplexers in VHDL.
A concurrent statement in VHDL is a signal assignment within the architecture, but outside of a normal process construct. The concurrent statement is also referred to as a concurrent assignment or concurrent process.
When you create a concurrent statement, you are actually creating a process with certain, clearly defined characteristics. Concurrent statements are always equivalent to a process using a sensitivity list, where all the signals to the right of the signal assignment operator are on the sensitivity list.
The signed and unsigned types in VHDL are bit vectors, just like the std_logic_vector type. The difference is that while the std_logic_vector is great for implementing data buses, it’s useless for performing arithmetic operations.
If you try to add any number to a std_logic_vector type, ModelSim will produce the compilation error: No feasible entries for infix operator “+”. This is because the compiler doesn’t know how to interpret this collection of bits that the vector is.
Test your coding skills with this VHDL quiz after completing tutorials 6-11 from the Basic VHDL Tutorial series!
If you read the blog posts, watched the videos, and did the exercises you should be qualified to answer most of the questions in the quiz. The quiz is intentionally made a little bit challenging. If you don’t know the answer, try to use your reasoning skills to figure out the most likely answer.
The std_logic_vector type can be used for creating signal buses in VHDL. The std_logic is the most commonly used type in VHDL, and the std_logic_vector is the array version of it.
While the std_logic is great for modeling the value that can be carried by a single wire, it’s not very practical for implementing collections of wires going to or from components. The std_logic_vector is a composite type, which means that it’s a collection of subelements.
The most common type used in VHDL is the std_logic. Think of this type as a single bit, the digital information carried by a single physical wire. The std_logic gives us a more fine-grained control over the resources in our design than the integer type, which we have been using in the previous tutorials.
Normally, we want a wire in a digital interface to have either the value ‘1’ or ‘0’. These two values are the only values that a bit,