Скачать или смотреть Creating a VGA Controller with an Accurate Fractional Counter in Verilog

Learn how to design a VGA controller in Verilog that effectively approximates a fraction for enabling signals with minimal error using a fractional counter.
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Crafting a VGA Controller with Minimal Error Using a Fractional Counter

In the realm of digital design, particularly in creating a VGA controller, one common challenge arises: how to enable a signal at specific intervals when exact timing cannot be achieved with integer division. This guide explores a heartfelt question from a developer faced with this dilemma while working with a 100 MHz clock and needing to enable the VPIXEL signal 480 times over a duration of approximately 16670 milliseconds. Let’s delve into the solution that minimizes error over time without introducing a new clock signal.

The Challenge at Hand

The user wishes to activate the VPIXEL signal:

480 times within 16670 milliseconds.

Given the 100 MHz clock, this equates to enabling VPIXEL every approximately 34729.166 clock cycles (or 34729.166 counts).

However, achieving this exact count with integer division leads to timing inaccuracies—with pulses occurring earlier than expected.

To maintain signal accuracy, a new approach must be implemented—one that allows tracking of time without direct integer division errors.

Solution: Implementing a Fractional Counter

One of the ancient approaches to solve this problem can be traced back to the design of the Gregorian calendar, which sought a similar solution for timing accuracy over extended periods. The user can create a fractional counter that tracks the numerator of the desired fraction to achieve the required accuracy.

Key Concepts

Integer Division Errors: Relying solely on integer values leads to cumulative timing errors, which can degrade performance over time.

Tracking the Fraction: By keeping track of the numerator and adjusting it adaptively, we can achieve the desired output without rounding errors.

Reset Mechanism: The design must include a reset state to start the count accurately.

Code Implementation

Below is an example of how to implement a fractional counter in Verilog. This solution is illustrative and may require further adjustment based on specific project requirements:

[[See Video to Reveal this Text or Code Snippet]]

Simplifications and Considerations

Reducing Bit Width: The current_num can be optimized by dividing both N and D by 160 to lower bit count while maintaining resolution.

Analysis of Constants: The choice of 16670 is worth investigating—it could serve as an approximation of 50000/3, advocating for using fractions for both N and D if that is indeed the case.

Conclusion

Creating a VGA controller in Verilog requires careful consideration of timing and counting methods, particularly when facing integer division limitations. By implementing a fractional counter, you can improve timing accuracy and minimize long-term errors in your design. This thoughtful approach not only enhances functionality but also reflects a depth of understanding in digital design principles.

Remember, while coding can often seem straightforward, the nuances of digital timing can introduce unexpected complexities. Embrace these challenges, and turn them into learning experiences that make you a better designer!