Advanced DFT: Real-Time Debugging, Challenges & Scan Compression Techniques

🔹 Real-Time Debugging Scenarios in DFT

1️⃣ Scan Chain Debugging (Shift Errors & Scan Integrity Issues)

Problem: Incorrect data shifting in the scan chain leads to test failures.
Possible Causes:

• Misconnected or broken scan chains.
• Hold timing violations in scan paths.
• Incorrect scan enable (SE) control.

Debugging Approach:

• Run scan shift test with simple patterns (101010... or 11110000...).
• Verify scan input-to-output connectivity.
• Check hold timing during shift operation.

Command (Check Scan Chain Integrity in Synopsys DFT Compiler)

report_scan_chains
report_dft -scan_connectivity

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2️⃣ ATPG Failures & Low Fault Coverage

Problem: ATPG reports low fault coverage (< 98%).
Possible Causes:

• Uncontrolled/unobservable logic.
• X-values in scan chains due to floating signals.
• Incorrect scan chain connection.

Debugging Approach:

• Identify uncontrollable/unobservable points.
• Enable test points (testable registers) for scan observability.
• Use X-masking to avoid unknown states.

Command (Generate Fault Report in Synopsys TetraMAX)

report_faults -summary

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3️⃣ JTAG TAP Controller Fails in Board Testing

Problem: Boundary scan chain does not respond in IEEE 1149.1 (JTAG) testing.
Possible Causes:

• TAP (Test Access Port) controller misconfigured.
• Incorrect TDI-TDO connections.
• Power issues in JTAG pins.

Debugging Approach:

• Run JTAG IDCODE test (Instruction 0001).
• Check TAP state transitions using an oscilloscope.
• Verify JTAG control signals TMS, TCK, TDI, TDO.

Command (Verify JTAG TAP in Synopsys)

report_jtag -summary

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🔹 Challenges in DFT & How to Overcome Them

DFT Challenge	Cause	Solution
Scan chain routing congestion	Too many scan paths	Use scan chain reordering & grouping
High shift power consumption	Simultaneous switching of many FFs	Use power-aware scan stitching
X-values in scan patterns	Floating values, uninitialized memories	Use X-masking & ATPG constraints
Low test coverage	Uncontrollable logic, unscanned FFs	Insert test points & better pattern generation
Scan compression inefficiency	Poor compaction ratio	Optimize compaction logic & balancing

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🔹 Advanced Scan Compression Techniques

1️⃣ X-Masking (Handling Unknown Values in Scan Chains)

Problem: X-values reduce ATPG efficiency & coverage.
Solution: X-mask registers block Xs during scan capture.

Command (X-Masking in Synopsys DFT Compiler)

set_dft_configuration -x_handling xmask
insert_dft

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2️⃣ Adaptive Scan Compression (Dynamic Test Data Reduction)

Problem: Traditional scan compression reduces test time but not dynamically.
Solution: Adaptive compression selects only needed patterns.

Command (Enable Scan Compression in Synopsys)

set_scan_compression -enable
insert_dft

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3️⃣ Test Points Insertion (Improve Observability & Controllability)

Problem: ATPG detects unobservable faults.
Solution: Add test points (TPs) to improve controllability.

Command (Enable Test Point Insertion in Synopsys DFT)

set_dft_configuration -test_point_insertion enable

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🔹 Summary: How to Optimize DFT for Real-World Use?

 Debug scan failures using scan shift tests.
 Improve ATPG fault coverage with test points & X-masking.
 Fix JTAG issues by verifying TAP controller states.
 Reduce test cost & power with adaptive scan compression.
 Use DFT tools like TetraMAX, Tessent, and Modus for testing.