A generic low power scan chain wrapper for designs using scan compression

Abstract

Shrinking power budgets in low power system-on-chips (SoCs) have elevated test power consumption as a major consideration for chip design and test engineering teams. Many traditional automatic test pattern generation (ATPG) and design-for-test (DFT) techniques for test power reduction are either effective for circuits not using test data compression hardware or have implications on the physical design cycle. This paper describes a technique for reducing peak current during scan based testing that can work in the presence of compression, and impose no restrictions on physical design, e.g. related to chip clocking. We propose low-design effort modifications to the test compression logic (wrapper-like changes) that enable us to (a) bypass scan chains or groups of them and (b) shift in constant values into the bypassed flip-flops for lowering the instantaneous current drawn. The modifications are easily localized to a scan chain wrapper that can be used with any scan compression solution. An SoC using lowpower scan chain wrappers provides sufficient configurability (scan chains bypassed or scan chains included) to explore different power reductions with test cost trade-offs. We describe a methodology that allows us to manage the inherent configurability available in our solution. For empirical validation, we have implemented low-power scan chain wrappers for a subset of scan chains in a recently taped-out 65nm low-power SoC. We present experimental data from ATPG and initial silicon power measurements for this chip to demonstrate the benefits and limitations of the proposal.