A major revolution in digital design has taken place over the past decade and more.  Field programmable gate arrays (FPGAs) can now contain over a million equivalent logic gates and tens of thousands of flip-flops.  This means that it is not possible to use traditional methods of logic design involving the drawing of logic diagrams when the digital circuit may contain thousands of gates.  The reality is that today digital systems are designed by writing software in the form of hardware description languages (HDLs).  The most common HDLs used today are VHDL and Verilog.  Both are in widespread use.  When using these hardware description languages the designer typically describes the behavior of the logic circuit rather than writing traditional Boolean logic equations.  Computer-aided design tools are used to both simulate the Verilog or VHDL design and to synthesize the design to actual hardware.

This book assumes no previous knowledge of digital design.  You start at the beginning, learning about basic gates, logic equations, Boolean algebra, and Karnaugh maps.  In over 70 VHDL examples, you learn how to design combinational and sequential digital circuits, and even a complete microprocessor core, simulating them using the Aldec Active-HDL simulator.  A free student edition of the Aldec Active-HDL simulator is available from Aldec, Inc. (www.aldec.com).   

Once you simulate your digital design, you may want to download it to one of the many low-cost FPGA boards that are available on the market.  This book could be used with any type of FPGA board.  You would simply embed the examples we present in this book in a top-level design in which the inputs and outputs are tailored to your particular hardware.  We will show some examples in this book on how to do this for a particular FPGA board made by Digilent, Inc., which contains a Xilinx FPGA, but you can easily adapt these examples to your particular FPGA board.

A new pedagogical method called “flipping” has been shown to be very effective in enhancing the classroom experience.  Instead of coming to class unprepared and listening to an hour lecture and then going home to do homework, the order is “flipped” and you first watch short video clips online about a particular topic, and then the class time can be used to do “homework” problems, discussion, and answering questions.  We have prepared 112 short video clips that cover all of the material in this book and they are available free on YouTube.  To find these, go to www.youtube.com/user/LBEbooks/playlists and click on Digital Design VHDL.  If you view these videos before going to class or reading the chapter, it will help you a lot!

Many colleagues and students have influenced the development of this book.  Their stimulating discussions, probing questions, and critical comments are greatly appreciated. 

Richard E. Haskell
Darrin M. Hanna