Static Timing Analysis: Cell Delay Vs. Cell Drive Strength

Have you ever wondered how a predator succeeds or its 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.

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:

1. Higher drive strength → Lower cell delay: A stronger cell can charge/discharge the output node faster, reducing delay.
2. 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:

1. Increased area: Higher drive strength cells have larger transistors, which take up more silicon area.
2. 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:

1. For high-speed paths, stronger drive strength cells are used to reduce delay.
2. For power-efficient design, weaker drive strength cells are used where speed is not critical.
3. 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.