Digital systems, such as microprocessors, digital signal processors, and field-programmable gate arrays (FPGAs), are used in a wide range of applications, including consumer electronics, automotive systems, medical devices, and aerospace. These systems are designed to perform complex functions, and their failure can have significant consequences, including financial losses, damage to reputation, and even loss of life.
A primary barrier to high-quality testing is the internal isolation of complex circuitry.
| Fault Model | Description | Coverage Target | | :--- | :--- | :--- | | | Node permanently tied to 0 or 1. | >99% (industry standard) | | Transition Delay | Signal fails to propagate within clock period (slow-to-rise/fall). | >95% for timing-critical paths | | Path Delay | Cumulative delay along a specific path exceeds limit. | Critical for high-speed designs | | Bridging (Wired-AND/OR) | Two nets shorted together. | Requires IDDQ or specialized ATPG | | Open (Stuck-open) | Transistor gate disconnected (sequential behavior). | Hard; needs two-pattern tests | | Fault Model | Description | Coverage Target
[ DL = 1 - (1 - Y)^1 - FC ] where (Y) = yield. For (Y=90%) and (FC=99%), (DL \approx 1000) ppm.
: The ease with which internal nodes and flip-flops can be set to a specific value through primary inputs. | Critical for high-speed designs | | Bridging
Perform full timing analysis on scan paths to eliminate hold-time violations introduced by physical routing constraints across the die.
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Testable design, or DFT, involves incorporating special circuitry into the IC design to facilitate testing. Without DFT, testing complex sequential circuits is nearly impossible. 1. Scan Design (Scan-Based Testing)
The relationship between design and testing is symbiotic. Without testable design principles, high-quality testing is impossible due to a lack of access. Without high-quality testing, testable design is meaningless because defects go undetected.
Digital systems testing poses several challenges, including: