Functional Tests for Scan Chain Latches

New digital designs often include scan chains; high quality economical test is the reason. While many techniques exist for testing combinational circuitry, little attention has been paid to testing the sequential elements (latches and flip-flops). This paper presents techniques for testing latches included in either shift registers or scan chains. We show that a test that applies all transitions to a latch-based shift register is an exhaustive functional test of all of the latches in the register. A more complex test is required for a scan chain. We present a procedure for deriving an exhaustive functional test of the latches in a scan chain. The shift register and scan chain tests presented do not depend on the latch implementation; they detect all detectable combinational defects (those that do not introduce additional states into a latch). We assume that only one latch is defective.