Learning Digital Systems Design in VHDL by Example in a Junior Course Darrin M. Hanna and Richard E. Haskell School of Engineering and Computer Science Oakland University dmhanna@oakland.edu haskell@oakland.edu Abstract Advancements in modern CAD tools enable students to design hardware, simulate their designs, synthesize them, and study common low-level issues such as timing, glitches, power consumption, and space requirements. Rather than emphasizing logic and gates to study issues in digital systems in a junior-level digital design course at Oakland University, the course presents the behavior of classical digital components, independent of target technology. Students learn VHDL and design tools by example through designing systems consisting of basic components, gradually increasing in complexity to larger digital systems covering most of the VHDL language. Using Xilinx ISE and Aldec Active-HDL, students describe, study, implement, and test basic gates, multiplexers, encoders and decoders, as well as more extensive designs including a VGA controller, memory-mapped video, a serial port transmit and receive, and special-purpose processors. A book called Learning By Example Using VHDL Advanced Digital Design has been written to cover this material. Instead of chapters this book contains over 40 worked examples from basic digital components to datapaths, control units, and a microcontroller. Useful tutorials and theoretical topics are included in appendices. This approach combines traditional topics in designing hardware with modern CAD tools to produce a unique learning experience for junior engineering students. Examples of the projects done in this course will be presented. Introduction At Oakland University a junior-level course, Digital Logic and Microprocessor Design, is taken by all computer engineering and electrical engineering students and some computer science majors. PowerPoint lectures and laboratory experiments including the latest in design and simulation software are used to present and practice digital design by example. Xilinx ISE and Aldec Active-HDL are used to design and test digital systems in VHDL including investigating power requirements using XPower, timing and glitches using pre- and post-implementation simulations, implementation using Floor Planner, and critical path including space/speed tradeoffs using synthesis information. Students also implement and test their designs on a Xilinx Spartan 3 FPGA using a prototyping board made by Digilent, Inc, which includes switches, buttons, LEDs, seven-segment displays, a VGA port, an RS-232 port, and a PS/2 port. During the course, students learn to use the software tools to design and test hardware by example while designing components starting with basic gates, multiplexers, encoders, decoders, to more extensive designs including a VGA controller, memory-mapped video, a serial port transmit and receive, and special-purpose processors that implement algorithms such as integer square root and Euclid s GCD. Both fundamental topics and tool tutorials are presented by example. After completing the 8 laboratory experiments, students work in groups of 3 or 4 to complete a design project during the last month of the term. These projects involve sophisticated special-purpose processors and components to produce impressive digital systems that use peripherals such as A/D and D/A converters, a USB port, sensors, and motors.
A book called Learning By Example Using VHDL Advanced Digital Design is being written to cover this material. Instead of chapters this book contains 49 worked examples ranging from basic digital components to datapaths, control units, and a microcontroller. Useful tutorials and theoretical topics are included in appendices. This approach combines traditional topics in designing hardware with modern CAD tools to produce a novel learning experience for junior engineering students. Section 1 describes the textbook Learning By Example Using VHDL. Section 2 describes the course materials and tools. Section 3 presents some examples of student projects. A conclusion is given in Section 4. 1. Learning By Example (LBE) Using VHDL Many digital design textbooks emphasize digital logic and logic reduction for implementing and studying digital systems using basic logic gates. Some include examples of implementing select digital components using a hardware description language such as VHDL or Verilog added to a later edition. On the other hand, there are textbooks for learning the VHDL language. These often take an approach traditional to learning a programming language emphasizing general syntax with some specific examples. The former typically do not include enough VHDL for students to effectively use the language and tools. The latter usually require significant knowledge of hardware as a prerequisite to learning VHDL. Further, it is rare to find a text that provides step-by-step directions for how to use a specific CAD tool for design entry, simulation, synthesis, implementation, power analysis, timing analyses, and other functions. This leaves much information for the instructor to coordinate and assemble for the students which can take considerable time and effort. We have begun publishing a series of books to address this need. The format of these books is different from that of standard texts. Instead of chapters these books contain a large number of worked examples with tutorials and theory topics relegated to appendices. The table of contents of the book Learning By Example Using VHDL Advanced Digital Design 1 is shown in Table 1 Table 1 Table of Contents of Learning By Example Using VHDL Advanced Digital Design Introduction Digital Logic and VHDL Example 1 2-Input Gates: Logic Equations Example 2 Multiple-Input Gates Example 3 4-Input Gates: for Loop Example 4 2-to-1 Multiplexer: if Statement Example 5 4-to-1 Multiplexer: Module Instantiation Example 6 4-to-1 Multiplexer: case Statement Example 7 Quad 2-to-1 Multiplexer Example 8 Generic Multiplexer: Parameters Example 9 Multiplexer as a Universal Element Example 10 Glitches Example 11 7-Segment Decoder: case Statement Example 12 Top-level VHDL Designs Example 13 7-Segment Display: Spartan-3 Board Example 14 4-Bit Binary-to-BCD Converter: case Statement Example 15 Bit Binary-to-BCD Converter: for Loops Example 16 Gray Code Converters Example 17 Adder Example 18 Shifters Example 19 Multiplier
Table 1 (cont d) Table of Contents of Learning By Example Using VHDL Advanced Digital Design Example 20 Divider Example 21 Arithmetic Logic Unit (ALU): case Statement Example 22 Comparators Example 23 Encoders Example 24 Priority Encoders Example 25 3-to-8 Decoder Example 26 Edge-Triggered D Flip-Flops Example 27 D Flip-Flops in VHDL Example 28 Divide-by-2 Counter Example 29 JK and T Flip-Flops Example 30 Registers Example 31 Shift Registers Example 32 Ring Counter Example 33 Counters Example 34 Arbitrary Waveform Example 35 Pulse-Width Modulation (PWM) Example 36 Finite State Machines Example 37 Datapaths Example 38 Control Units Example 39 VGA Controller Example 40 Memory: ROM Example 41 Memory: On-chip RAM, Distributed and Block Memory Example 42 Memory: External RAM Example 43 Asynchronous Serial Communications: Transmit Example 44 Asynchronous Serial Communications: Receive Example 45 PS/2 Controller Keyboard Example 46 PS/2 Controller Mouse Example 47 Test Benches using a Liquid Crystal Display Controller Example 48 Power Consumption Example 49 Combinational and Sequential Multiplication: Speed/Space Tradeoff Appendix A Aldec Active-HDL Tutorial Part 1 Appendix B Aldec Active-HDL Tutorial Part 2 Appendix C Number Systems Appendix D Basic Logic Gates and De Morgan s Theorem Appendix E Implementing Digital Circuits Appendix F Basic Digital Design Appendix G Boolean Algebra Appendix H Karnaugh Maps Appendix I Adders Appendix J Latches and Flip-Flops Appendix K State Machines Appendix L VHDL Quick Reference Guide The text begins with an extremely basic example of implementing basic gates. For students who wish to learn or review digital logic basics and prerequisites, Appendix C through K contain tutorials for various topics. Appendix A contains a step-by-step tutorial for completing the first example using Aldec Active-HDL with Xilinx Web Pack installed for synthesis and implementation. This tutorial illustrates completing the exercise using explicit screen shots for each step, easy for anyone to follow to learn how to use the tools for design entry, functional simulation, synthesis, implementation, and downloading the design to the FPGA. Appendix L contains a VHDL quick reference guide.
The examples become more complex as the student completes earlier examples. It is not necessary to complete all of the examples; examples do not require skills from all previous examples. In some cases, a single hardware component is used in multiple examples to illustrate different ways to do things in VHDL. For example, examples 4 through 9 all use a multiplexer to illustrate different VHDL constructs and digital topics. Example 4 uses an if-statement, Example 5 teaches instantiating modules, Example 6 uses a case statement, Example 7 demonstrates a quad 2-to-1 multiplexer as opposed to the 4-to-1 multiplexers used in the other examples, Example 8 introduces a generic multiplexer using parameters, and finally Example 9 presents the multiplexer as a universal element. Later examples emphasize more sophisticated digital designs including special-purpose processors, serial communication, a VGA controller, a memory controller, and more. In addition to more sophisticated designs are the more advanced digital issues including measuring power consumption and battery life, post-implementation timing and critical paths, and creating and using test benches. This mixture of digital logic, digital design, and VHDL is valuable for both advancing knowledge in digital logic and design as well as providing practice in using modern design tools that are used in industry for designing chips. This book contains teacher s resources including solutions to the examples and PowerPoint slides for lectures. Other Learning By Example books include one for learning basic digital design using Verilog 2 and one for learning to program microcontrollers using C 3,4. 2. Course Materials and Tools A 4-credit junior-level course in digital design is offered at Oakland University using this textbook. Students have taken, as a prerequisite, an introductory course in electrical and computer engineering where approximately five weeks are devoted to numbers systems and digital logic. In the junior-level digital design course students attend two lectures each week plus a 3-hour laboratory session where a lab instructor is available to help students and check their assignments. PowerPoint slides from the LBE book are used in the lectures. Each assigned lab is comprised of one or more examples. Typically, there are eight labs. Aldec s Active-HDL is used for simulation and the design flow. Xilinx ISE is installed for synthesis and implementation, accessed from the main design flow menu in Active-HDL. Each student is required to purchase a development board so that they can explore the tools and digital design on their own. Xilinx Web Pack is offered free for students to download. Using a virtual private connection, students can connect to the School s license server to obtain full access to Active- HDL. This makes it convenient for students to setup the digital design environment at home or on a laptop, often increasing the time that they spend with the tools and labs as opposed to only offering a laboratory environment on campus. At Oakland, we have used Digilent s FPGA development boards for this course. In particular we are currently using Digilent s Spartan 3 Development Board ranging from a Spartan 3 200 to 1000 (approximately 200,000 to 1,000,000 equivalent gate capacity, respectively). This development board is JTAG programmable and contains: 8 slide switches, 4 pushbuttons, 8 LEDs, and 4-digit seven-segment display, 1MByte fast asynchronous RAM on board, Serial port, VGA port, and PS/2 mouse/keyboard port, and three 40-pin expansion connectors and three high-current voltage regulators (3.3V, 2.5V, and 1.2V).
After completing the eight labs listed in Table 2, the remaining three to four weeks are spent on a team design project. For the design project, students are put into groups of three or four based on a list of preferred group members. Each group decides what their project will do and after receiving instructor approval and discussion, development commences. To implement their design, students can use a combination of digital components that they have developed throughout the course from the LBE examples and a state machine and datapath specific to their project. Each group gives a fifteen minute presentation and submits a written report and project poster as their final deliverables for the course. They are also required to include a group picture in the presentation, poster, and report, and a video of their working demonstration in their presentation. This makes it easy to identify students who have completed projects years later and show their demonstrations without actually setting them up and downloading them. Students receive individual grades based on mandatory peer evaluations. Table 2 Eight lab assignments leading to the project 1. Multiplexers -- Simulation and Synthesis Using Aldec Active-HDL 7.1 2. Priority Encoders and Seven-Segment Decoder 3. Comparators and Using the FPGA Editor 4. Binary to BCD Converter and the Seven-Segment Displays 5. Datapaths and Controllers: Integer Square Root 6. VGA Controller 7. UART and RAM 8. Combinational and Sequential Multiplication Space vs. Speed Trade-off 3. Examples of Student Projects We have found that most student projects resulting from this course are of impressive quality and complexity. Table 3 shows a list of selected student projects. Table 3 Some examples of student projects A. Interactive Sudoku game B. Wheel of Fortune C. Blackjack D. Digital clock using a touch screen monitor E. Mastermind F. Memory G. Pong H. A typing tutor I. FPGA Mario J. Catch em Sudoku, also known as Number Place, is a logic-based placement puzzle. The objective is to fill the grid so that every column, every row and every 3 3 box contains the digits 1 to 9. The puzzle setter provides a partially completed grid so that there is only one solution. This was implemented on an FPGA. The game board is displayed on a VGA monitor. Using the VGA
controller developed in Example 39, assigned as one of their lab assignments, some students developed their own font. The cursor is moved using the arrow keys. The player presses a number key when the cursor is located over the desired cell. When the last cell is full, if all the answers are correct, the numbers the user input turn green and the clock stops. The object is to beat the clock, or have the best time. Figure 1 shows screenshots of this Sudoku implementation. (Player loses) (Player wins) Figure 1: Interactive Sudoku on an FPGA Using the keyboard, players enter the phrase and set the wheel spinning on Wheel of Fortune. The wheel spins on the VGA screen and stop at a random dollar amount. Using the keyboard again, players select a letter and the game reveals where that letter appears in the phrase and awards points accordingly or reports that the letter does not appear in the phrase. Figure 2 shows a block diagram of the data path for keydata keyboard keyflg VGA_adr gameover Vga_adr ScoreCtrl Scoreboard intro store and guess turncontrol turn nxtpos Wheelposition spin winner Num2asc Wheelclock poscnt row Fnt_data wheelclk vgasel 255 gendata Figure 2: A block diagram of the components in Wheel of Fortune phrasedata
the Wheel of Fortune component. This component is integrated with a VGA controller and control unit designed by the group as shown in Figure 3. clk clr gendata gendata clk clr hs phasedata phasedata vs kc kd wheelclk a turn wheelclk aturn red green vga_adr plno vga_adr plno blue keydata keyflag gover next_turn spin nextpos row_out pos_cnt vgasel row_out pos_cnt vgasel gover screen keydata keyflg gover next_turn spin nextpos screen clk clr Figure 3: Wheel of fortune integrated with keyboard input, a VGA controller, and a control unit A shortened version of Blackjack was implemented using the basic rule that the hand that is highest, but less than or equal to 21 wins. This version of blackjack includes 2 players and a computer player. This consists of a graphics engine, card randomizer, and fonts for the suits and numbers/letters on the cards. Players select whether to stand or hit using buttons on the development board. Extreme Memory supports custom card faces and patterns for card backs using serial communication to transmit these items to the game. Using the keyboard arrow keys, players can move a selection box from frame to frame and select a card. The next step is to remember where the match is and select that card next in this classic game of memory. Figure 4 shows screenshots of extreme memory. Figures 5 and 6 show a block diagram of the data path and a state diagram of the high-level state machine. Custom card faces and backs Splash Screen Ready to Play Winning Marquee Figure 4: Screenshots of Extreme Memory
Figure 5: Block diagram of the data path for Extreme Memory Any time the escape key is pressed, all states return to start-state and clear. start keyout=x"0d" keychecks setupgo key Out=X"00" mov eright key Out=X"19" key Out=X"00" mov elef t keyout=x"1a" key Out=X"00" key Out=X"17" moveup key Out=X"00" setdn='0' randomset setdn='1' display flip keycheckf key Out=X"18" key Out=X"00" movedown mcount = "1000" check "not a match" unf lip win "match f ound" keycheckd key Out=X"00" Figure 6: State diagram for the high-level state machine controlling Extreme Memory
The FPGA Mario game consists of three levels of graphics: the background, the player, and the good/bad object to catch/avoid. Using a standard NES controller made by Nintendo Entertainment Systems, players can move the Mario player from left to right to catch or avoid the falling object. As the game progresses, the objects fall more quickly. Points are awarded for catching good objects and deducted for catching bad objects. Figure 7 shows a screenshot of the FPGA Mario Game, the NES controller, and the images. The FPGA Mario group also created a software FPGA Image Utility using Microsoft Visual Basic.NET to open an image and generate a COE file containing data encoding the image for an IP core memory. This utility is shown in Figure 9. Figure 8 shows a screenshot of the Ping Pong game using a potentiometer and A-to-D converter for playing against the computer. The computer anticipates ball movement and the ball speeds up as the game progresses. Figure 8 also shows the ball/angle scheme where students mapped the number of units to the right/left/up/down the ball should move to achieve a particular angle. NES Controller Mario Sprites (16 x 32) Good and Bad Sprites (16 x 16) Screenshot Background Tiles Graphics Components Figure 7: The FPGA Mario Game
Potentiometer Paddle Screenshot Ball Angle Scheme Figure 8: Ping Pong Game
Figure 9: The FPGA Image Utility created by the FPGA Mario team The game of Catch em consisted of a football goal post displayed on a VGA monitor that moves left and right using the arrow keys on a PS/2 keyboard. The object of the game is to move the goal post to catch the football that falls from the top of the screen. The football falls faster as the game progresses and points are awarded for catching the football. Figure 10 shows this game. Figure 10: Catch em Screenshot
4. Conclusions Using the LBE approach, students can refer to appendices for basic digital logic material in a junior-level course of digital design. Students each purchase a development board and setup the design tools at home or on their laptop. Lectures teach topics in digital logic, digital design, and VHDL according to examples selected by the instructor. The students are assigned eight labs to complete which are comprised of one or more examples. After completing these labs, a group of students completes a design project. We have found that students are well-prepared for designing digital systems having designed and implemented a sophisticated project of their own in only three weeks. This approach has produced better, more sophisticated projects than other, more traditional approaches to teaching digital logic with VHDL. 5. References 1. Richard E. Haskell and Darrin M. Hanna, Learning By Example Using VHDL Advanced Digital Design, LBE Books, Rochester, MI, 2007. 2. Richard E. Haskell, Learning By Example Using Verilog Basic Digital Design, LBE Books, Rochester, MI, 2007. 3. Richard E. Haskell, Learning By Example Using C Programming the HCS12 Microcontroller Using CodeWarrior, LBE Books, Rochester, MI, 2007. 4. Richard E. Haskell, Darrin M. Hanna, and Michael Latcha, Learning Microcontroller Programming in C by Example in a Sophomore Core Course, ASEE North Central Section Conference, Mar. 30-31, 2007, Charleston, WV.