Supply Voltage Noise Aware ATPG for Transition Delay Faults Nisar Ahmed and M. Tehranipoor University of Connecticut Vinay Jayaram Texas Instruments, TX

2/7 Overview Objective Prior Work Statistical IR-drop Analysis Power Model Switching Cycle Average Power (SCAP) SCAP Calculator Pattern Generation Framework Experimental Results Conclusions

3/7 Objective High switching activity during test compared to functional operation Increased sensitivity of VDSM designs to supply noise IR-drop during at-speed test: A BIG CONCERN Present ATPG tools are supply noise unaware Targets as many faults as possible per pattern Random filling of X’s during ATPG Increases switching activity of test patterns How to generate IR-drop tolerant patterns without significantly increasing number of patterns?

4/7 IR-drop Effects? Delay Failure An excessive IR-drop can increase the delay of targeted paths Logic Failure An excessive IR-drop can significantly reduce the voltage reaching a device -- may function unpredictably Question: What is the impact of IR-drop on long and short paths Answer: Slack of a path and number of switching define how tolerant a path is to IR-drop

5/7 Prior Work IR-drop issue during at-speed test [Saxena, ITC03] Static Verification of Test Vectors for IR-drop Failure [Kokrady, ICCAD03] Power Supply Noise Analysis in Test Compaction [Wang, ITC05] Preferred-Fill [Remersaro, IEEE D&T 07] Low-capture TDF pattern generation [Wen, VTS05, VTS06] Faster-than-at-speed test considering IR-drop [Ahmed, ICCAD03]

6/7 Contributions Novel Power model to measure the average power during at-speed test Called Switching cycle average power (SCAP) Consider both the length of paths affected by each pattern and the number of transitions Pattern generation procedure: Set SCAP threshold Perform TDF pattern generation Identify high IR-drop patterns using SCAP Replace them with IR-drop patterns

7/7 Case Study ITC’99 benchmark (b19) 200K gates, 6642 scan cells 8 scan chains 4 VDD (VSS) pads 0.18 nm technology Frequency = 142MHz Power rings Width = 20um Stripes Width = 10um Power/Ground Distribution Network

8/7 Statistical IR-drop analysis Vector-less approach to estimate IR-drop analysis Assumptions: Uniform activity over the entire design region Switching time frame = clock period 20% Net toggle activity 2.8% voltage drop in VDD network 4.5% voltage drop in VSS network Underestimates average functional IR-drop and power Non-uniform switching activity Most activity occurs during early cycle period IR-drop inversely proportional to switching time-frame window

9/7 Procedure: Generate transition fault test patterns Measure time-span of all switching activity during launch- to-capture window Average switching time frame = Half cycle period Statistical IR-drop analysis (cont.) Avg. Switching Power [mW] Avg. IR-drop [V] VDDVSS Full cycle period Half cycle period What is an average switching time-frame window to estimate average functional IR-drop and power ? Functional power threshold to identify high IR-drop test patterns

10/7 Power Model (SCAP) Average IR-drop directly relates to average power Cycle average power (CAP) Power measured over entire cycle period Switching cycle average power model (SCAP) Measured over switching time frame window (STW) ATE clock Scan Enable FF clock STW (P2) STW (P1) Clock Network Switching CAP = ∑(C i * VDD 2 ) T SCAP = ∑(C i * VDD 2 ) STW T

11/7 Power Model (SCAP) (cont.) Pattern P1 and P2 Almost same switching activity Different switching time frame window (STW) P2 has smaller STW SCAP (P2) > SCAP (P1) IR-drop effects on VDD and VSS during pattern P1 and P2 application within 7ns capture window.

12/7 Power Model (SCAP) (cont.) SCAP (P2) > SCAP (P1) P1 P2 Switching time frame window (STW) is an important parameter ITC’99 benchmark (b19)

13/7 SCAP Model Validation SCAP is a good power model to represent average IR-drop P1 P2 Another example: Cadence SOC Design (Turbo Eagle)

14/7 PLI routine to measure power during launch-to- capture window Avoids huge VCD file generation for large designs SCAP = ∑(C i * VDD) 2 STW SCAP Calculator VCS Pattern Power Profile Design (.v) Test Patterns SDF Parasitics (SPEF) Physical Design (DEF) STAR-RXCT Instance Capacitance extractor PLI SCAP Calculator SDF – Standard delay format (Timing Information) VCS – Gate level simulator (Synopsys) PLI – Programmable language Interface STAR-RXCT – Extraction tool (Synopsys)

15/7 Pattern Generation Framework SCAP Calculator Pattern Set FS Commercial ATPG tool ATPG Short-listed Patterns Yes Exit No Fault List With X-fill options If SCAP>Thr ? Statistical IR-drop Analysis Thr Pattern Generation Procedure: Step 1: Run ATPG for all faults Step 2: Exclude high IR-drop Patterns (short-listed patterns) Step 3: Fault Simulate for Short-listed patterns  fault list Step 4: Run ATPG for this fault list

16/7 Experimental Results Conventional transition fault pattern set Random fill option for don’t-care bits 2360 patterns generated VSS network VDD network 860 patterns with SCAP value above threshold SCAP threshold = 20% toggle activity over avg. switching time frame window

17/7 Results (cont.) New patterns generated with fill-0 and fill-1 options for faults exclusively detected by short-listed patterns Fill-0 generates 957 low-power patterns instead of 860 patterns with high SCAP value 97 extra patterns but significantly reduces the SCAP value Fill-0 option

18/7 Results (cont.) Fill-1 option Fill Adjacent

19/7 Results (cont.) Comparison of conventional ATPG and the new pattern generation procedure Slight increase in pattern volume 4% increase in number of patterns

20/7 Conclusion New pattern generation procedure for IR-drop tolerant pattern generation Switching cycle average power (SCAP) model for identifying high IR-drop patterns Considers both switching capacitance and time frame of activity PLI based SCAP calculator Efficient way to measure SCAP during launch-to- capture window