Digital Systems Testing And Testable Design Solution May 2026

As chip sizes grow, the volume of test data becomes enormous. A 100-million-gate design may require gigabytes of test vectors. Test compression reduces this by:

| Term | Definition | |------|-------------| | Fault | Physical defect (e.g., stuck-at-0, stuck-at-1) | | Error | Incorrect output caused by a fault | | Test vector | Set of input values applied to detect a fault | | Fault coverage | % of detected faults / total possible faults | | Test set | Collection of test vectors | | Testability | Ease of setting/observing internal states | digital systems testing and testable design solution

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The difficulty of testing any digital system can be distilled into two metrics: controllability (how easily a specific internal node can be set to a desired logic state) and observability (how easily the state of that node can be propagated to a primary output). In a complex sequential circuit, internal state registers act as both barriers and black holes. To test a deep logic path, a tester must sequence the chip through a long chain of clock cycles, a process that is time-consuming and error-prone. As chip sizes grow, the volume of test data becomes enormous

The fundamental dilemma is that normal functional operation and testing mode have contradictory requirements. Functionality seeks to minimise pins, hide internal states, and optimise speed. Testing seeks maximum access, full visibility, and deterministic control. The difficulty of testing any digital system can