ATPG Pattern Generation — Page Text (English)
Introduction
ATPG tools generate deterministic test patterns that target specific structural fault models.
These patterns leverage scan architecture to control internal states, observe logic responses, and validate the integrity of the design.
Why ATPG Patterns Matter
ATPG patterns are essential for:
- Achieving high structural fault coverage
- Validating logic correctness and detecting manufacturing defects
- Supporting safety‑critical verification flows
- Enabling systematic fault injection campaigns
- Ensuring predictable and repeatable test behavior
ATPG is the bridge between DFT architecture and measurable test quality.
Core Concepts
Pattern Generation
ATPG algorithms compute deterministic input sequences that activate faults and propagate their effects to observable points.
Patterns are generated for fault models such as stuck‑at, transition, bridging, and path delay.
Pattern Compaction
To reduce test time and tester memory, ATPG tools compact patterns by merging compatible vectors and eliminating redundant ones.
Compaction preserves coverage while minimizing pattern count.
Deterministic vs. Pseudo‑Random
- Deterministic patterns: precisely target specific faults; required for high coverage and safety validation.
- Pseudo‑random patterns: used in LBIST or as a pre‑conditioning phase; fast but less targeted.
Modern flows often combine both approaches.
Pattern Application Through Scan
ATPG patterns are applied through scan chains using the shift–capture–shift sequence:
- Shift‑in the test stimulus
- Capture the logic response
- Shift‑out the observed data
This mechanism provides full controllability and observability of internal nodes.
Safety Connection
ATPG plays a key role in functional safety:
- Provides deterministic fault activation for safety validation
- Supports FMEDA assumptions with measurable diagnostic coverage
- Enables systematic fault injection campaigns
- Complements LBIST for startup and periodic diagnostics
ATPG is a foundational tool for safety‑critical verification.
Examples
Figure 1: Example ATPG pattern
An ATPG pattern activates a specific fault, propagates its effect through the combinational logic, and captures the resulting response in the scan flip‑flops. The pattern is applied by shifting in the stimulus, performing a capture cycle, and shifting out the observed response. This activate–propagate–observe sequence is the foundation of deterministic structural testing.
Figure 2: Example fault activation and propagation
A fault is activated when a specific value from SFF_A sensitizes the fault site inside the combinational logic. The resulting faulty effect propagates through the logic and reaches SFF_B, where it can be captured and later observed through scan. This activate–propagate–observe mechanism is the foundation of deterministic ATPG.
Figure 3: Example compaction effect
ATPG tools reduce the number of test patterns by merging compatible vectors. Don’t‑care bits (X) allow multiple patterns to be combined into a single deterministic pattern without losing fault coverage. In this example, P1, P2, and P3 are compatible and merge into a single compacted pattern, while P4 remains independent. Compaction eliminates redundant patterns and minimizes test time and tester memory.
