VHDL digital circuit design: A step-by-step guide

Digital circuit design is a critical part of electronics. In fact, most electronic components are digital electronic circuits. Even simple integrated circuits (ICs) contain thousands or hundreds of thousands of transistors. Microprocessors and microcontroller ICs typically have millions or billions of transistors.

Most ICs are produced in fabs, but digital circuit designs can be made using FPGA boards. It’s an important experience for any electronics learner.

The first step in designing digital circuits is learning the hardware description language (HDL). VHDL, Verilog, and SystemVerilog are the popular HDLs for digital circuit design. VHDL is beginner-friendly. It’s verbose but offers a strong type system with a structured syntax and good readability.

Verilog is concise and more efficient. However, it has less strict type checking, and certain constructs may remain ambiguous. SystemVerilog combines efficiency with advanced verification features, making it ideal for complex designs.

Verilog or VHDL are the only two viable options when new to digital circuit design, and the latter has some advantages. As a verbose hardware description language, VHDL builds a strong foundation of concepts in digital circuit design. Many existing hardware designs are written in VHDL, and many industries that focus on safety-critical systems prefer it over Verilog or SystemVerilog. Plus, any electronics student will learn VHDL as part of a university curriculum.

While learning VHDL, it’s necessary to design combinational logic like adders, subtractors, multipliers, comparators, encoders, decoders, multiplexers, and demultiplexers. With VHDL, it’s possible to describe hardware behavior, model hardware structure, simulate circuits, and synthesize them. You can build sequential logic like flip-flops, registers, counters, and shift registers. You can progress to designing ALUs, memory units, finite state machines, serial interfaces (UART, SPI, and I2C), parallel interfaces, and bus protocols.

At an advanced VHDL level, you can also:

Design core components of a microprocessor or microcontroller or implement digital signal processing circuits like FFTs, filters, and DSP algorithms.
Build communication systems like modems, encoders, and decoders.
Create image and video processing circuits, like implementing image and video compression, filtering, or enhancement algorithms.
Customize hardware accelerators for real-world applications using FPGA boards for image processing, video processing, cryptography, or machine learning.
Synthesize ASICs or control logic for embedded systems like automotive, consumer electronics, or industrial automation.

VHDL is a fundamental tool for designing and simulating digital circuits, building a strong foundation before learning Verilog or SystemVerilog. By writing VHDL codes, users will develop strong problem-solving skills and the essential logical thinking for digital circuit designing.

Overall, VHDL is relatively easy to learn. Here is a step-by-step guide:

Get familiar with VHDL: Learn about VHDL’s advantages and disadvantages. Also, learn about the program structure of this hardware description language and the design flow.
Begin your VHDL journey: By writing simple programs. These could be a full adder, 2-to-4 decoder, binary to gray code converter, 4-bit parity generator, 4X1 multiplexer, and more. This step is essential as it lays a strong foundation for tackling more complex designs.
Simulate and verify designs: Learn to compile, simulate, and verify VHDL programs.
Design digital gates: After you have learned to design, compile, and verify codes in VHDL, start designing logic gates like AND, OR, NOT, NAND, NOR, XOR, and XNOR.
Additional digital gates: Learn to design NAND, XOR, NOR, and XNOR gates with AND, OR, and NOT gates.
De Morgan’s Theorem: Design and verify De Morgan’s theorem in VHDL.
Universal gate: Design and verify a 7 NAND universal gate in VHDL.
NOR universal gate: Design and verify a NOR universal gate.
Digital circuits with boolean equations: Learn to translate boolean equations into real digital circuits in VHDL.
Half & full adder: Design and verify half and full-adder circuits in VHDL.
Half & full subtractor: Design and verify half and full subtractor in VHDL.
Parity generator and checker: Design and verify an 8-bit parity generator and checker in VHDL.
Decoder and encoder: Design and verify 3×8 decoder and 8×3 encoder in VHDL.
Multiplexer and demultiplexer: Design and verify an 8X1 multiplexer and 1×8 demultiplexer in VHDL.
SR Flipflop: Design and verify a clocked SR latch.
D Flipflop: Design and verify a D flipflop.
JK Flipflop: Design and verify a JK flipflop in VHDL.
T Flipflop: Design and verify a T flipflop in VHDL.
Counter: Design and verify a 4-bit binary counter.
Binary-to-gray and gray-to-binary converter: Design and verify 4-bit binary-to-gray and gray-to-binary converter in VHDL.
An 8-bit adder: Design and verify an 8-bit full adder in VHDL.
Comparator: Design and verify 1-bit and 8-t comparator.

The ideal way to learn digital circuit design is by writing HDL code for various circuits. Practice writing the code, compiling, verifying, and synthesizing if you have an FPGA board. Start with basic digital circuits like logic gates, boolean equations, combinational, and sequential logic circuits. Then, it gradually progresses to more complex digital circuits.

The step-by-step guide listed above will help build a foundation in digital circuit design using VHDL. You can then progress to more complex circuits like ALU, memory units, serial interfaces, parallel interfaces, bus protocols, and components of microprocessors and microcontrollers, ASICs, and DSPs. There’s virtually no limit.