Flip-flop design and technique for scan chain diagnosis

Abstract

A modification to conventional scan chain design is disclosed which can identify whether any connection in the scan chain is shorted to the supply voltage or ground (i.e., shorted to a logical 1 or logical 0) and the precise location of the short. Circuitry in the flip-flops (or other sequential elements) forming the scan chain allows the scan output of each flip-flop to be set or reset by switching a scan enable signal between logic states. If there is a fault in the scan chain where a node is stuck at a logical 1, then resetting the scan outputs of the flip-flops to 0 and clocking the flip-flops will result in a logical 1 being output from the last flip-flop after a number of clock pulses. The number of clock pulses indicates the position of the flip-flop in the scan chain which is associated with the fault. A similar technique detects a stuck-at-0 fault by setting the flip-flops to 1. In a preferred embodiment, each adjacent pair of flipflops in the chain consists of a flip-flop whose scan output can be reset to a 0 and a flip-flop whose scan output can be set to a 1.