Have you ever wondered how a predator succeeds (or) a prey escapes in the jungle? It’s the breathtaking speed and agility of the predator (say, a leopard) 🐆 as it chases prey (say, a deer)🦌.
The VLSI circuit operation is very similar. If the driving cell is strong, it takes less delay and changes the output quicker than a weaker driver, which produces a sluggish response and takes longer to produce the same effect at the output pin Q.
Before going into the details, let us know what these terms (cell delay and cell drive strength) mean and why should we take care?
Drive strength and cell delay are two key concepts in digital circuit design, particularly in standard cell libraries used for ASIC and FPGA design. They are closely related and impact timing, power, and area considerations in a design.
Cell Delay
Cell delay refers to the propagation delay of a logic cell, which is the time taken for a change at the input to reflect at the output. It is a function of the input transition time and the effective load at the output pin.
Drive Strength
Drive strength refers to the ability of a logic cell (such as an inverter, buffer, NAND, NOR, etc.) to drive a load. It is usually represented as multiples of a base drive (e.g., X1, X2, X4, X8, etc.), where higher drive strength (X32, X16, etc.) means the cell can provide more current and drive larger loads (longer interconnects, multiple fan-outs). Lower drive strength (X1, X2, etc.) means the cell is weaker and is suitable for driving small loads. As shown in the example below, BUFX2 has lower drive strength, and its delay is higher (high transition time ). BUFX16 has higher drive strength, and its delay is lesser (low transition time).
Relationship Between Drive Strength and Cell Delay
There is an inverse relationship between drive strength and cell delay:
a) Higher drive strength → Lower cell delay: A stronger cell can charge/discharge the output node faster, reducing delay.
b) Lower drive strength → Higher cell delay: A weaker cell takes longer to switch the output due to limited drive current.
However, increasing drive strength comes with trade-offs:
a) Increased area: Higher drive strength cells have larger transistors, which take up more silicon area.
b) Higher power consumption: Larger transistors have higher leakage and dynamic power consumption.
In the below example, you can notice that a higher drive strength cell(BUFX20) has less transition time (0.0349ns), results in less cell delay, and a lower drive strength cell (BUFX2) has a higher transition time (0.1063ns) that results in high cell delay. You can also understand how the area will be affected by different drive strength cells. 
Choosing the Right Drive Strength
In timing optimization, drive strength selection is a key technique:
a) For high-speed paths, stronger drive strength cells are used to reduce delay.
b) For power-efficient design, weaker drive strength cells are used where speed is not critical.
c) Buffer insertion and sizing: Designers optimize timing by inserting buffers of appropriate drive strengths.
Practical Example
Consider a NAND gate with different drive strengths:
- A NAND2_X1 might have a cell delay of 50 ps when driving a small load.
- A NAND2_X8 might have a cell delay of 15 ps for the same load but will consume more power.
Thus, designers must balance drive strength and delay to meet timing constraints while minimizing power and area.
Would you like further details on a specific aspect, such as how EDA tools optimize drive strength?
This Cadence RTL-to- GDSII Flow training course contains the Timing Signoff module that demonstrates the impact of cell delay for different drive strengths for you. Please reach out to Cadence Training for further information.
The course includes slides with audio and downloadable laboratory exercises designed to emphasize the topics covered in the lecture. There is also a Digital Badge available for the training.
You can also access the lab demos created for each course module through a channel video RTL-to-GDSII lab demo videos.
If you want to learn in detail about the Timing Concepts, STA, and Timing Signoff, you can take the following two courses.
- Basic Static Timing Analysis— In this course, you can learn all the basics of STA.
- Tempus Signoff Timing Analysis and Closure with Stylus Common UI— In this course, you can learn the detailed flow of timing sign-off, with multiple techniques to analyze and resolve all the timing violations (including manual ECOs flow).
Also, the following short training bytes help you understand the concepts quickly, and a few demonstrate how to debug and resolve errors; for more, look here: Training Bytes (Videos)
Training Byte References
- Demo: How to Run Timing Analysis Using the Innovus Implementation System? (Video)
- Demo: How to Run an Independent Timing Analysis In Tempus? (Video)
- Demo: How to Rerun the Innovus to fix all timing violations in Tempus? (Video)
- Demo: How to Highlight the Timing path in Innovus Implementation System
- How to Run Placement Optimization in the Innovus Implementation System?
- How to Run the Synthesis Without DFT?
- How to Run the Synthesis Flow With DFT?
- Creating Power Rings, Power Stripes, and Power Rails in the Innovus Implementation System
- How to Run Power Analysis and Analyze the Results in Innovus Implementation System?
- What are On-Chip Variations?
- What is RTL Coding In VLSI Design?
- What is Digital Verification?
- What Is Synthesis in VLSI Design?
- What Is Logic Equivalence Checking in VLSI Design?
- What Is DFT in VLSI Design?
- What is Digital Implementation?
- What is Power Planning?
- What are DRC and LVS in Physical Verification?
Related Blogs
- Here Is the Recording of the RTL-to-GDSII Flow FrontEnd Webinar!
- Training Insights – Why Is RTL Translated into Gate-Level Netlist?
- Training Bytes: They May Be Shorter, But the Impact Is Stronger!
For more information on Cadence's digital design and signoff products and services, visit www.cadence.com.
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Happy learning! Thank You.