GHDL and VHDL Simulation

GHDL is the most common open-source VHDL simulator for students. It follows the VHDL build model: analyze files, elaborate a top-level design, then run the simulation.

Learning Objectives

You will learn to:

• analyze VHDL source files with GHDL;
• elaborate and run a testbench;
• generate VCD waveforms;
• use assertions to make tests self-checking.

Basic GHDL Flow

ghdl -a counter4.vhd
ghdl -a counter4_tb.vhd
ghdl -e counter4_tb
ghdl -r counter4_tb

The usual meanings are:

Some GHDL backends allow -r to elaborate automatically, but beginners should learn the three-step model.

Dumping a Waveform

ghdl -r counter4_tb --vcd=counter4.vcd
gtkwave counter4.vcd

Waveforms help you see timing relationships, but assertions should still decide pass/fail.

Self-Checking Testbench Habit

assert count = "0001"
    report "counter did not increment"
    severity error;

If the design is wrong, the simulator should complain without relying on your eyes.

Larger Projects

For multiple files, analyze packages first:

ghdl -a rtl/common_pkg.vhd
ghdl -a rtl/uart_tx.vhd
ghdl -a tb/uart_tx_tb.vhd
ghdl -e uart_tx_tb
ghdl -r uart_tx_tb --vcd=uart_tx.vcd

VHDL compile order matters because a file can depend on packages, entities, or architectures analyzed earlier.

Exercise

Run the VHDL counter exercise using GHDL:

1. intentionally break the increment line;
2. confirm the assertion fails;
3. fix the design;
4. regenerate the VCD;
5. inspect reset, enable, and count in GTKWave.

Common Mistakes

• Analyzing files in the wrong order.
• Running the design entity instead of the testbench entity.
• Forgetting to stop the testbench.
• Treating a waveform as proof without assertions.
• Mixing generated files with source files.

Summary

GHDL gives VHDL students a transparent simulation workflow: analyze, elaborate, run, and inspect waveforms. Use assertions so your testbench becomes a checking tool, not just a waveform generator.

Next: Exercise: Find and Fix a Timing Failure.

Further Reading

• GHDL quick start
• GHDL invoking guide
• GTKWave manual

Mind Map