Efficient On-line Interconnect BIST in FPGAs with Provable Detectability for Multiple Faults Vishal Suthar and Shantanu Dutt Dept. of ECE University of Illinois at Chicago

Suthar & Dutt, University of Illinois at Chicago Outline On-Line BIST – Concepts Previous Work in Interconnect BIST New Interconnect BIST: I-BIST –Motivation –Types of faults –On-line issues –Global Testing –Detailed Testing –Simulation results Conclusions

Suthar & Dutt, University of Illinois at Chicago On-line Built-In Self-Testing in FPGAs CIRCUIT ROTE (ROving TEster) Two column left spare for ROTE; one for fault reconfiguration ROTE roves across the FPGA SPARE COLUMN CIRCUIT SPARE COLUMN TPG - Test Pattern Generator ORA - Output Response Analyser CUT - Cells Under Test BISTer: WUT - Wires Under Test WUT TC CO CT OC In each session diff. PLBs act as CUTs, TPG and ORA. TPG CUT ORA Pass / fail WUT

Suthar & Dutt, University of Illinois at Chicago Interconnect BIST-- Motivation Interconnects occupy ~ 90% of FPGA area Multiple faults can easily occur for current and emerging nano-meter FPGAs; no current work detects all multiple faults Many PLB BIST work, but much fewer on interconnect BIST

Suthar & Dutt, University of Illinois at Chicago Interconnect BIST– Past Work Only few on-line interconnect BIST methods # configurations high for previous on-line interconnect BIST: 10 for [Liu et al., CICC’01] # of test vectors high for all previoys interconnect BIST No prev. method has guarantd. 100% fault detectability for multiple faults Fault diagnosability (% of faults correctly located) low for past methods Our interconnect BIST method I-BIST will address and improve on all these issues many

Suthar & Dutt, University of Illinois at Chicago I-BIST – Objectives & Flow Objectives: 1.To maximize diagnosability, even in presence of multiple faults, by avoiding fault masking scenarios. 2.To reduce test time. Dominant component of test time = configuration time Hence minimize # configurations. Secondary objective: minimize # test vectors. Approach: 1. First isolate the possible fault locations to a small set of interconnects in very few configurations -> Suspect Set, 2. Then diagnose interconnects of suspect set for faults. -- Global testing -- Detailed testing

Suthar & Dutt, University of Illinois at Chicago I-BIST – Interconnect Faults Testing wire-segments and switches Faults considered: wire-segment -> stuck-at, stuck-open and bridged fault switches -> stuck-open and stuck-closed.

Suthar & Dutt, University of Illinois at Chicago Typical FPGA contains single-length, double-length and long wires. whereas, a ROTE can occupy only two to three columns. Hence, two types of ROTE are required: V-ROTE and H-ROTE Roving Tester Positions for I-BIST

Suthar & Dutt, University of Illinois at Chicago I-BIST – Tested Interconnects V-Set: Interconnects tested under V-ROTE Uncovered switches are covered in the next ROTE position or in the H-ROTE

Suthar & Dutt, University of Illinois at Chicago Multiple nets are formed Multiple ORAs each comparing adj. nets ORAs configured for 2-input XOR function Pass complementary vectors on each adj. net => 2 test vectors {0101..} & {1010…} I-BIST – Global Testing: Idea Test vector 1: Test vector 2:

Suthar & Dutt, University of Illinois at Chicago Consider ORA(n2, n3) Under fault-free conditions ORA (2-ip XOR) output = 1 1 Wire stuck-at n2 ORA output = 0 1 (n2 stuck-at-0) = 1 0 (n2 stuck-at-1) Multiple stuck-at fault on n2 is going to result in one type dominating at the ORA input Wires bridge fault ORA output = 0 0 Test vector 1: Test vector 2: I-BIST – Global Testing: Idea

Suthar & Dutt, University of Illinois at Chicago Under fault-free conditions ORA (2-ip XOR) output = 1 1 Exception: one of the adjacent wire is s-a-0 and other is s-a-1. output = 1 1 = fault-free output  one more test vector required {0000…} or {111…} Unexpected-comparison based test vectors Unexpected-comparison based test vectors Fault-free output = Faulty output = Test vector 1: Test vector 2: Test vector 0: I-BIST – Global Testing: Idea

Suthar & Dutt, University of Illinois at Chicago Stuck-open fault: Test vector 1: Test vector 2: Test vector 0: Assunption: Stuck-open fault essentially results in a s-a-0 fault in the affected part Only detected if present between TPG and ORA A second stage of global testing reqd. Switch stuck-open faults that are part of the nets are similarly detected All switches are included in some net across 5 configurations of net patterns I-BIST – Global Testing: Idea

Suthar & Dutt, University of Illinois at Chicago Stuck-open fault (contd): TPG ORA n 2 n 3 n 4 n 2 w 1 w Stuck-open Fail TPG ORA n 2 n 3 n 4 n 2 w 1 w Stuck-open Fail Two stages of global testing phase needed – can be done simultaneously I-BIST – Global Testing: Idea (contd)

Suthar & Dutt, University of Illinois at Chicago Unlike other faults, switch stuck-closed fault cannot be detected by including the switch on a net I-BIST – Global Testing: Idea (contd) Switch stuck-closed fault:

Suthar & Dutt, University of Illinois at Chicago Switch stuck-closed fault (contd): Unlike other faults, switch stuck-closed fault cannot be detected by including the switch on a net Form nets in such a way that a switch stuck-closed fault bridges the two nets. We refer to such switches as Spanning switches I-BIST – Global Testing: Idea (contd)

Suthar & Dutt, University of Illinois at Chicago Theorem: Any number of faults in each net tested will be detected by the multiple ORA technique of global testing. Furthermore, a stuck-closed fault in any spanning switch between every pair of nets compared by an ORA will also be detected. Corollary: Fault masking cannot occur in global testing, even in presence of faults of the same type. I-BIST – Global Testing: Properties

Suthar & Dutt, University of Illinois at Chicago Dual-type multiple ORA technique multiple nets formed similarly configured nets – net-set e.g. net-sets n & l Global Testing

Suthar & Dutt, University of Illinois at Chicago Dual-type multiple ORA technique Two types of ORA comparison: 1. Intra-net-set comparison Adjacent nets of same net-set compared E.g., target: all faults other than switch stuck-closed fault 2.Inter-net-set comparison Nets i of the two net-sets compared E.g. target: switch stuck-closed faults in spanning switches Global Testing

Suthar & Dutt, University of Illinois at Chicago Theorem: Any # of fault(s) in the V-set (interconnects tested in V-ROTE) will be detected by the 5 configurations of the global testing phase. Global Testing – Five Configurations

Suthar & Dutt, University of Illinois at Chicago Diagnose faults on faulty nets of global testing phase. Simple approach: Flat intersection Suspect set = interconnects of failed nets found in global testing phase Each interconnect element of suspect set compared with fault-free components. Suspect interconnect Fault-free interconnect Drawback: Suspect set may be too large, even in presence of few fault. Detailed Testing

Suthar & Dutt, University of Illinois at Chicago Better approach: Divide & Conquer ( D & C) Failed nets divided into 2 sub- nets Failed configurations re- applied independently on both sets of sub-nets. Only the comparisons that failed in global testing phase are reqd. Switches between the two sub- nets tested using same configurations minimized to two rows A OO O Detailed testing – Divide & Conquer (D&C)

Suthar & Dutt, University of Illinois at Chicago Detailed Testing – D&C (contd): Global testingDetailed testing Switch stuck-closed A OO O O O

Suthar & Dutt, University of Illinois at Chicago Simulation Setup Simulated a 32 x 32 FPGA with single-length segments in C Simulated a 32 x 32 FPGA with single-length segments in C Fault injector injects faults in wire segments and switches randomly at the specified fault probability (density/100) Fault injector injects faults in wire segments and switches randomly at the specified fault probability (density/100)

Suthar & Dutt, University of Illinois at Chicago Fault latency ( # configurations) vs. Fault density Simulation Setup and Fault Latency Results Simulated a 32 x 32 FPGA with single-length segments in C Fault injector injects faults in wire segments and switches randomly at the specified fault probability (density/100) Fault injector injects faults in wire segments and switches randomly at the specified fault probability (density/100)

Suthar & Dutt, University of Illinois at Chicago Overall Fault Diagnosability Results Fault coverage (diagnosability) versus fault density Analytical Results & Comparison to Prev Work many

Suthar & Dutt, University of Illinois at Chicago Conclusions Presented a new interconnect BIST technique I-BIST for FPGAs—uses a hierarchical, adaptive approach, unexpected- comparison based test vectors Applicable to both on-line and off-line BIST I-BIST has 100% guaranteed fault detectability in the presence of multiple faults – a first I-BIST has ~ 100% fault diagnosability (empirically) I-BIST has the fewest configurations—5—per WUT-set in global testing I-BIST has the fewest # of test vectors—3—per WUT-set testing phase I-BIST uses an adaptive D&C phase in detailed testing to home in to the offending faults quickly New work: Combined PLB and interconnect BIST w/o fault- free assumptions about any components—to appear at VTS’06