DFT bypass?

Understanding scan-insertion bypass logic for asynchronous resets

This is an easy conceptual question about design-for-test (DFT) methodology, specifically the scan-insertion phase. It tests whether you understand why test infrastructure must account for asynchronous control signals that would otherwise interfere with deterministic scan operation.

When scan chains are inserted into a design, the goal is to shift in known test patterns and observe shifted-out responses in a fully deterministic manner. Asynchronous reset pins present a problem: they can force flip-flops into a known state independently of the scan clock, disrupting the controlled flow of data through the chain. A multiplexer on the reset path, gated by a test_mode signal, lets you selectively disable or redirect these asynchronous controls during test, ensuring the scan chain behaves predictably.

• Scan chain architecture and test-mode control signals
• Asynchronous vs. synchronous reset behavior
• Interactions between functional logic and test infrastructure