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
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
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
🔹 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 |
🔹 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
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
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
🔹 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.