Negative Hold Time Values in the Datasheet: Theory and Implications

Understand what negative Hold times in datasheets mean for your digital designs. This concise guide explains why these values exist, their benefits for timing closure and high-speed circuits, and clarifies why negative Setup times don't apply.

Introduction: The Basics of Setup and Hold Time

Please refer to these two pages for an in-depth explanation of Setup and Hold times and how they feed into the timing closure equation -

  1. Introduction to Static Timing Analysis
  2. Advanced Topics in Static Timing Analysis and Timing Closure

What is a Negative Hold Time?

A negative hold time means that the data input can change before the active clock edge, and the flip-flop will still correctly capture the old data.

It's counter-intuitive to the standard definition, but it effectively extends the "safe" window for data change before the clock edge.

Why Do Negative Hold Times Occur?

  • Internal Delays: This is the primary reason. It's due to the internal propagation delays within the flip-flop itself, specifically the difference in delays between the clock path and the data path to the internal latching element.
    • If the clock signal takes longer to reach the internal latching element than the data signal does, it creates a "margin" where the external data can change before the clock edge and still be correctly sampled by the delayed internal clock.
    • Think of it as the effective clock edge for the internal latching element being slightly delayed relative to the external clock pin.
  • Fabrication Process: It's a characteristic of the specific flip-flop design and the semiconductor process technology. Designers can intentionally design flip-flops with negative hold times.

Implications for Circuit Design

  • Benefits (Design Flexibility):
    • Eases Hold Constraints: A negative hold time provides more "slack" for the hold check. It means that the data path can be shorter (or faster) relative to the clock path without violating hold time.
    • Simplifies Routing/Layout: Designers have more flexibility in placing components and routing signals, especially for short data paths, as they don't need to intentionally add delay to meet a positive hold requirement.
    • Critical for High-Speed Designs (e.g., DDR Memory): In very high-speed interfaces like DDR SDRAM, where data rates are extremely high, negative hold times can be crucial to achieve the required timing margins. It allows data to transition almost immediately after the clock edge without causing a hold violation.
  • Potential Drawbacks/Considerations:
    • While it helps with hold time, it might sometimes come at the expense of a larger setup time requirement for the same flip-flop. (Though not always, and it's a design tradeoff).
    • Requires careful Static Timing Analysis (STA): Designers must accurately account for these negative values in their timing analysis tools to ensure correct operation across all operating conditions (PVT variations).
    • Doesn't eliminate the need for any hold time check: It simply shifts the "safe window." Data still needs to be stable for a certain total aperture, even if part of that is before the clock edge.

Conclusion

  • Negative hold times are not an error or a problem; they are a legitimate and often beneficial characteristic of modern digital circuits.
  • They provide valuable flexibility for designers, especially in high-speed applications, by relaxing hold constraints.
  • Understanding and properly accounting for them in timing analysis is key to robust digital design.

Why are there no Negative Setup Times in the Datasheet?


It's important to clarify that while negative hold times are a real and utilized phenomenon in digital design, negative setup times, on the other hand, do not practically exist.

A negative setup time would mean that data could change after the clock edge and still be captured correctly. It is intuitive to see what kind of chaos that would cause!!

Subscribe to fpgadesign.io

Don’t miss out on the latest issues. Sign up now to get access to the library of members-only issues.
jamie@example.com
Subscribe