What is the difference between OCV, AOCV, and POCV? Why POCV?

Sources of Variation: The primary sources of variation that necessitate derates are:

·         PVT (Process, Voltage, Temperature) Variations: These are inter-chip variations.

o    Process: Variations in manufacturing (e.g., lithography wavelength, defects) can alter transistor parameters like oxide thickness, dopant levels, and physical dimensions (W/L), which in turn affect threshold voltage (Vt​) and current (I), and thus cell delay. Dies at the center of a wafer are more accurate than those at the periphery.

o    Voltage: Supply voltage fluctuations (due to IR drop, supply noise, battery charge levels) directly impact current and cell delay. Lower technology nodes with lower supply voltages see a higher percentage of variation.

o    Temperature: Ambient temperature and on-chip hot spots (due to high switching activity) affect cell delays. For deep sub-micron technologies, temperature inversion (where delay decreases with increasing temperature at lower voltages) can occur.

·         RC Variations: Interconnect delay is increasingly dominant. Variations in CMP, photolithography, and etch processes affect wire dimensions, leading to RC variations.

·         OCV (On-Chip Variation) / Local Variations: These are intra-chip variations, meaning identical cells at different locations on the same chip and in the same corner can exhibit different delays. Factors include IR drop, metallization mismatches, CMP effects, Vth​/mobility mismatches, local temperature variations, and toggling activity mismatches (aging). OCV is categorized into:

o    Random Variations: Non-deterministic, no spatial correlation, hard to predict (e.g., gate oxide thickness, implant doses). Their effect tends to cancel out over longer path depths.

o    Systematic Variations: Deterministic, can be modelled, exhibit spatial correlation (e.g., gate length/width, interconnect width due to proximity/density effects). Proportional to cell location.

·         Simple OCV Derates (Global Derates): Applying a single, fixed derate percentage (e.g., +/-6%) to all cells/nets for late/early paths using set_timing_derate.

o    Problem: Overly pessimistic because it applies the worst-case variation (derived from the most sensitive cell) to all cells, leading to over-optimization, increased power consumption, and longer STA cycles.

·         AOCV (Advanced OCV): Reduces pessimism by applying derates based on cell type, path depth (random variations tend to cancel), and distance (systematic variations increase with distance). AOCV is more accurate than OCV, especially for nodes like 65nm and below.

o    Flaws: AOCV often uses the worst-case timing arc for a cell's derate and doesn't account for variations due to input slew, output load, or operating conditions of side inputs, potentially being too optimistic or pessimistic.

·         POCV (Parametric OCV): A more realistic, statistical approach that overcomes AOCV limitations by making OCV derate factors dependent on input slew and output load.

o    It assumes cell delay follows a normal distribution (Gaussian curve) around a nominal delay (mean, μ) with a standard deviation (σ). Typically, analysis is done at 3σ (covering ~99.7% of variations).

o    Random variations are modeled using this statistical approach (μ, σ) instead of min-max values, reducing pessimism as extreme delay values are less probable.

o    Systematic variations are still handled using distance-based derates, similar to AOCV.