Compacting output responses containing unknowns using an embedded processor

In system-on-chip (SOC) designs, embedded processors are frequently present as part of the functional design and can be used to help test the chip or system by providing a software-based test. Previous work has looked at compacting output responses in software by performing signature analysis using either arithmetic operations or by implementing a multi-input signature register (MISR) in software. However, these approaches cannot be used when the output response contains unknown (X) values. While it is possible to precisely mask all X´s present in the output response in software, a straightforward approach would require a very large amount of mask data to specify which bits to mask. This paper proposes an efficient method for compacting output responses with X´s in software using the concept of canceling X´s from signatures as proposed in [Touba 07], Whereas the efficiency of the hardware implementation in [Touba 07] is constrained by needing to minimize the hardware overhead, in software these constraints are not present. Thus, a novel and more efficient implementation is proposed here. Moreover, the efficiency is further improved by incorporating a low cost partial X-masking step in software as well. Results indicate that output responses with significant X densities can be very efficiently compacted using the proposed software-based scheme.