How to use the command 'ghdl' (with examples)

GHDL is an open-source simulator for the VHDL language, a hardware description language used to model electronic systems. GHDL allows for the analysis, simulation, and synthesis of VHDL code, providing a suite of features and tools that help both novice and experienced developers verify and refine their designs. Below, we explore various use cases of the GHDL command, illustrating how it can be employed effectively.

Use case 1: Analyze a VHDL source file and produce an object file

Code:

ghdl -a filename.vhdl

Motivation: The initial step in working with VHDL code is ensuring that the design is syntactically correct and can be understood by the simulator. Analyzing the source file with GHDL translates the written code into an object file, a crucial step for anyone preparing to simulate or further elaborate their VHDL designs.

Explanation:

• ghdl: Invokes the GHDL simulator.
• -a: The -a command-line argument means “analyze” and instructs GHDL to parse the VHDL file and check it for errors while producing an intermediate object file.
• filename.vhdl: This is the name of the VHDL source file to be analyzed.

Example Output:

analyzing entity <entity_name>
<filename>.o

The output confirms successful analysis and the generation of an object file with no syntax errors.

Use case 2: Elaborate a design

Code:

ghdl -e design

Motivation: After analyzing VHDL files and ensuring they are error-free, the next step is elaboration. This process involves finalizing and interconnecting various design units. Elaborating a design is essential as it prepares it for simulation by resolving all placeholder references within your VHDL code.

Explanation:

• ghdl: This calls the GHDL tool.
• -e: The -e flag stands for “elaborate” and directs GHDL to work on the specified design unit.
• design: This specifies the name of the design configuration (which may be an entity, architecture, or configuration unit).

Example Output:

elaborated design <design_name>

This indicates that the design has been successfully elaborated and is ready for execution.

Use case 3: Run an elaborated design

Code:

ghdl -r design

Motivation: Running the design after elaboration is a crucial part of simulating VHDL code. This step enables developers to verify the behavior of their design under the provided constraints and check for any run-time issues or unexpected behavior.

Explanation:

• ghdl: Starts the GHDL execution program.
• -r: The -r option stands for “run”, a command invoking execution of the elaborated design.
• design: The specific design unit you wish to simulate.

Example Output:

simulation finished successfully

This output shows a successful simulation execution, confirming that the behavior is as expected.

Use case 4: Run an elaborated design and dump output to a waveform file

Code:

ghdl -r design --wave=output.ghw

Motivation: During the development and verification phase, observing the signal transitions and behavior over time in a visual medium can aid significantly in debugging and understanding complex interactions. This is where dumping the output into a waveform file becomes invaluable, allowing engineers to visualize their design’s operations using a waveform viewer.

Explanation:

• ghdl: Initiates the GHDL program.
• -r: The run command to execute the design.
• design: The design unit to be simulated.
• --wave=output.ghw: This triggers GHDL to generate a waveform output file (.ghw) that captures the simulation’s execution details over time for later examination.

Example Output:

simulation finished after 100 ns
waveform data dumped to output.ghw

The output confirms the creation of a waveform file, which can be analyzed with software like GTKWave to scrutinize design behavior.

Use case 5: Check the syntax of a VHDL source file

Code:

ghdl -s filename.vhdl

Motivation: Syntax checking a VHDL file independently allows for rapid identification and correction of code errors before engaging in deeper analysis or elaboration processes. This step can be particularly beneficial for generating bug-free code more efficiently.

Explanation:

• ghdl: Calls the GHDL tool.
• -s: The -s option stands for “syntax check”, which tells GHDL to verify the syntax of the VHDL source file.
• filename.vhdl: The name of the VHDL file whose syntax needs checking.

Example Output:

syntax check passed

A simple confirmation indicating that the file’s syntax is correct, making it ready for further deployment or simulation.

Use case 6: Display help

Code:

ghdl --help

Motivation: For users less familiar with command-line tools or those needing a quick refresher on command options and usage, the help command provides a quick way to access comprehensive instructions directly from the console.

Explanation:

• ghdl: Executes the GHDL binary.
• --help: This parameter directs GHDL to display various command-line options and additional usage information.

Example Output:

Usage: ghdl [command] [options] <files...>
Commands:
  -a, --analyze      Analyze the VHDL source files
  -e, --elaborate    Elaborate the design
  -r, --run          Run the elaborated design
...

The output summarizes available commands and options, serving as a handy guide for users exploring GHDL functionalities.

Conclusion:

GHDL is a powerful tool for VHDL designers, enabling them to analyze, elaborate, simulate, and inspect their designs efficiently. Through diverse commands, GHDL offers a streamlined workflow for verifying and validating VHDL code. By understanding and utilizing the various use cases of GHDL, developers can enhance their ability to manage complex hardware designs accurately and effectively.