A robust pulsed flip-flop and its use in enhanced scan design

Abstract

Delay faults are frequently encountered in nanometer technologies. Therefore, it is critical to detect these faults during factory test. Testing for a delay fault requires the application of a pair of test vectors in an at-speed manner. To maximize the delay fault detection capability, it is desired that the vectors in this pair are independent. Independent vector pairs cannot always be applied to a circuit implemented with standard scan design approaches. However, this can be achieved by using enhanced scan flip-flops, which store two bits of data. This paper has two contributions. First, we develop a pulsed flip-flop (PFF) design. Second, we present an enhanced scan flipflop design, based on our PFF circuit. We have compared the performance of our pulse based flip-flop with recently published pulse based flip-flop designs, as well as a traditional master-slave D flip-flop. Our PFF shows significant improvements in power and timing compared to the other designs. Our pulse based enhanced scan flip-flop (PESFF) has 13% lower power dissipation and 26% better timing than a conventional D flipflop based enhanced scan flip-flop (DESFF). The layout area of our PESFF is 5.2% smaller than the DESFF. Monte Carlo simulations demonstrate that our design is more robust to process variations than the DESFF.