Month: September 2017

Testbench != Simulation

There is a difference between testbench files and simulation files:
Testbench files are independent of the simulator and comparable tools. They include testbench configuration files, test case descriptions or stimuli and golden reference output files.

Simulation files are setup and command files for a simulator or similar vendor tool. Related configuration files include waveform settings and simulation scripts.

A project’s folder structure should also draw this distinction to keep simulator/vendor independent files separate. Log files and results should therefor be put in an output folder alongside the testbench files.

Spartan3E Starter Board Gotcha

On the Spartan 3E Starter Board the signals A and B of the rotary shaft encoder are actually inverted!
This is not mentioned in the documentation, the figure found there is strongly misleading . At least the pullup resistors are mentioned, which can bring you on the right track.
Oh and remember to put a reset on your FSMs :o)

References: Spartan 3E Starter Board documentation

VHDL package use

Try to avoid

use work.mypack.all;

This can cause name conflicts, e.g. if two packages define a type of the same name. It is far less likely to have conflicting type and package names. Even if so, its a hell lot easier to fix, e.g. by renaming one of the packages.
This problem may be found when integrating 3rd party code as a black box (i.e. not knowing or caring whats inside).
Therefor choose package names as descriptive as possible, while keeping them reasonably short.

I am aware that name conflict are unlikely in VHDL. Very few widely used packages exist and the whole library concept is omitted by most developers. I bet 99% of digital design engineers never ever used a different library than work for their packages. But that’s no reason to ignore best practices.

What Is a Hardware Description Language Anyway?

What is a HDL?

A hardware description language (HDL) is a computer language used to describe the structure and behavior of digital (and sometimes analog) electronic circuits. A HDL may look quite similar to a traditional programming language and indeed will behave similar as well in some aspects, however in other aspects there are important differences that one must be aware of when writing HDL code.

This article is meant to give an extremely brief overview of HDLs, their origins, what’s so special about them and what state of the art HDLs of today can offer.

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