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Enhanced Scan Methodology to Increase Delay Fault coverage and Reduce Scan Test Time

IP.com Disclosure Number: IPCOM000179328D
Publication Date: 2009-Feb-12
Document File: 5 page(s) / 123K

Publishing Venue

The IP.com Prior Art Database

Abstract

Here we describe a method to partition scan chains into multiple scan groups, each controlled by independent scan enable signal, to achieve higher delay fault coverage using broadside scan test. The proposed technique can be easily adopted by existing design methodologies and can be used for implementation in industrial designs. Experimental results obtained from the implementation of this technique on industrial designs are also presented.

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Enhanced Scan Methodology to Increase Delay Fault coverage and Reduce Scan Test Time

Abstract:

Here we describe a method to partition scan chains into multiple scan groups, each controlled by independent scan enable signal, to achieve higher delay fault coverage using broadside scan test. The proposed technique can be easily adopted by existing design methodologies and can be used for implementation in industrial designs. Experimental results obtained from the implementation of this technique on industrial designs are also presented.

Introduction:

To increate quality of test patterns and reduce defect-per-million (DPM) at-speed tests are becoming a necessity in today’s DSM designs. There are two widely accepted methods of performing at-speed tests: Launch-off-shift (LoS) and Launch-off-capture (LoC or broadside).

a)      Launch-off-Capture (LoC)

In this method, the second pattern of the two pattern test is obtained from the circuit response of the first pattern [1], i.e. the launch patterns are obtained from the circuit response of the last shifted pattern. As the design can take only a limited number of states, numbers of launch patterns are limited.

b)      Launch-off-Shift (LoS)

In this method, the second pattern of the two-pattern tests required for testing delay faults is obtained by a one-bit shift of the first pattern [1], i.e. the launch patterns are generated from the shift path. This enables generation of more launch patterns compared to broadside test, and hence helps achieve higher coverage.

Why LoC?

Though LoS can achieve higher delay fault coverage, it requires an at-speed scan enable signal for proper launch and capture operations. This requires significant design and routing effort. Due to the resulting design and area overhead broadside approach is preferred over LoS in industrial designs.

Prior Arts

The proposed method is based on the work done in [1], [2] & [3], which achieve higher delay fault coverage by controlling a group of flip flops by independent scan enable signals. Work done in [3] suggests using multiple scan enable signals to control alternate, group or a subset of scan elements in a scan chain. Although the approaches proposed in [1], [2] & [3] would provide greater controllability to launch vector but they may not be suitable for use in industrial design flows due to overheads resulting from routing and they also required repeated prior analysis of design in order to identify groups of flips flops that are to be controlled by independent scan enable signals.

Proposed Methodology:

The proposed method as described uses multiple scan enables signals to control a subset of scan chains. Each such subset of scan chains is defined as a separate scan group. Alternate scan chains can be grouped together to form a scan group, each controlled by an independent scan enable (SEN) signal.

Scan chai...