SetVCD

Programmatically inspect hardware VCD signals using a high-level functional interface.

Higher-order programming constructs and set operations are a natural fit for inspecting VCD signals, and this Python library allows you to easily specify, in text, what simulation timesteps matter to functional correctness.

Motivating Example

Say you are debugging a streaming input interface (Accelerator.io_input) and you would like to extract only the values when the streaming transaction is valid. Typically when viewing a wavefile in a viewer, the points we care about look like this:

We can write this desired condition, parameterised by simulation timestep t as a formal statement:

(clk(t - 1) == 0 and clk(t) == 1) and
reset(t) == 0 and
input_ready(t) == 1 and
input_valid(t) == 1

This is a very natural fit for higher-order functions and set operations.

This library provides you two important methods:

• Get set of timesteps with condition:

   SetVCD.get(signal_name: String,
              condition: (ValueType, ValueType, ValueType) -> Bool,
              value_type: Raw or String or FP)
              -> SignalExpression
  

  • SignalExpression represents subset of timesteps of the wavefile that meet the provided condition.
  • Two SignalExpression objects (a and b) be combined with set operations to create a higher SignalExpression
    • c = a & b: c is the intersection of set a and b.
    • c = a | b: c is the union of set a and b.
    • c = a ^ b: c is the difference of set a and b.

• A SignalExpression object obtained from get and combined with operations can then be evaluated on the wavefile via:

  SetVCD.get_value(signal_name: str,
                   expr: SignalExpression,
                   value_type: Raw or String or FP)
                   -> List(Value)
  SetVCD.get_value_with_t(signal_name: str,
                   expr: SignalExpression,
                   value_type: Raw or String or FP)
                   -> List((Timestep, Value))
  

  • This yields the set of values you would be interested in.

For a practical example, please see our pre-filled notebook: example.ipynb

Overview

SetVCD is a Python package for analyzing Verilog VCD files and extracting time points where specific signal conditions are met. It provides a simple, type-safe interface for working with simulation waveforms using set-based operations.

Installation

The package is available in PyPI:

pip install setVCD

Future Enhancements

Planned for future versions:

• Higher-order operations for signal conditions
• Performance optimization for large VCD files
• Streaming interface for very large files
• MCP (Model Context Protocol) integration