I was a little annoyed by the peculiarities of the AXI interface the first time I had to create logic to interface an AXI module. Instead of the regular busy/valid, full/valid, or empty/valid control signals, the AXI interface uses two control signals named “ready” and “valid”. My frustration soon changed to awe.

The AXI interface has built-in flow control without using additional control signals. The rules are easy enough to understand, but there are a few pitfalls one has to account for when implementing the AXI interface on an FPGA.

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I’m excited to announce that the VHDL and FPGA course that I have been working on for the last six months is starting to become complete. The course is in beta at the moment, and I am planning on launching it for the first time this autumn.

Who is the FPGA course for?

The FPGA course is intended for the developer who has knowledge of other programming languages, but is new to VHDL and FPGAs.

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Constrained random verification is a testbench strategy that relies on generating pseudo-random transactions for the device under test (DUT). The goal is to reach functional coverage of a number of predefined events through random interaction with the DUT.

Open Source VHDL Verification Methodology (OSVVM) is a free VHDL library which includes a number of convenient packages for creating constrained random testbenches. We are particularly interested in the RandomPkg and CoveragePck, which we will be using in this article.

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Circular buffers are popular constructs for creating queues in sequential programming languages, but they can also be implemented in hardware. In this article we will create a ring buffer in VHDL to implement a FIFO in block RAM.

There are many design decisions you will have to make when implementing a FIFO. What kind of interface do you need? Are you limited by resources? Should it be resilient to over-read and overwrite? Is latency acceptable?

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An interactive testbench is a simulator setup where input to the device under test (DUT) is provided by an operator while the testbench is running. Most often, this would mean you entering commands in the simulator console to provide the DUT with stimulus.

While you should always create a self-checking testbench, an interactive testbench can be a nice supplement. It’s easier to perform ad-hoc testing with an interactive testbench at hand, than it is to change the code of the self-checking testbench.

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A self-checking testbench is a VHDL program that verifies the correctness of the device under test (DUT) without relying on an operator to manually inspect the output. The self-checking testbench runs entirely on its own, and prints an “OK” or “Failed” message in the end.

Every VHDL module should have an associated self-checking testbench. It’s important to be able to verify that all modules have the intended behavior at any time. For example,

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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,

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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,

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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,

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