Spring 07, Jan 25 ELEC 7770: Advanced VLSI Design (Agrawal) 1 ELEC 7770 Advanced VLSI Design Spring 2007 VLSI System DFT Vishwani D. Agrawal James J. Danaher Professor ECE Department, Auburn University Auburn, AL

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)2 SOC Design: A DFT Problem  Given the changing scenario in VLSI:  Mixed-signal circuits  System-on-a-chip  Multi-chip modules  Intellectual property (IP) cores  A system must be designed for testability.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)3 Conventional Test: In-Circuit Test (ICT)  A bed-of-nails fixture provides direct access to each chip on the board.  Advantages: Thorough test for devices; good interconnect test.  Limitations:  Works best when analog and digital functions are implemented on separate chips.  Devices must be designed for backdriving protection.  Not applicable to system-on-a-chip (SOC).  Disadvantages:  High cost and inflexibility of test fixture.  System test must check for timing.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)4 PCB vs. SOC  Tested parts  In-circuit test (ICT)  Easy test access  Bulky  Slow  High assembly cost  High reliability  Fast interconnects  Low cost  Untested cores  No internal test access  Mixed-signal devices PCB SOC

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)5 SOC: Core-Based Design  Cores are predesigned and verified but untested blocks:  Soft core (synthesizable RTL)  Firm core (gate-level netlist)  Hard core (non-modifiable layout, often called legacy core)  Core is the intellectual property of vendor (internal details not available to user.)  Core-vendor supplied tests must be applied to embedded cores.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)6 Partitioning for Test  Partition according to test methodology:  Logic blocks  Memory blocks  Analog blocks  Provide test access:  Boundary scan  Analog test bus  Provide test-wrappers (also called collars) for cores.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)7 Test-Wrapper for a Core  Test-wrapper (or collar) is the logic added around a core to provide test access to the embedded core.  Test-wrapper provides:  For each core input terminal  A normal mode – Core terminal driven by host chip  An external test mode – Wrapper element observes core input terminal for interconnect test  An internal test mode – Wrapper element controls state of core input terminal for testing the logic inside core  For each core output terminal  A normal mode – Host chip driven by core terminal  An external test mode – Host chip is driven by wrapper element for interconnect test  An internal test mode – Wrapper element observes core outputs for core test

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)8 A Test-Wrapper Wrapper test controller Scan chain to/from TAP from/to External Test pins Wrapper elements Core Functional core inputs Functional core outputs

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)9 References  Test Wrapper:  B. Nadeau-Dosti, Design for At-Speed Test, Diagnosis and Measurement, Springer,  System Test:  R. Rajsuman, System-on-a-Chip: Design and Test, Artech-House,  M. L. Bushnell and V. D. Agrawal, Essentials of Electronic Testing for Digital, Memory & Mixed-Signal VLSI Circuits, Springer, 2000.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)10 Overhead of Test Access  Test access is non-intrusive.  Hardware is added to each I/O signal of block to be tested.  Test access interconnects are mostly local.  Hardware overhead is proportional to: (Block area) – 1/2

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)11 Overhead Estimate Rent’s rule: For a logic block the number of gates G and the number of terminals t are related by t = K G  where 1 ≤ K ≤ 5, and  ~ 0.5. Assume that block area A is proportional to G, i.e., t is proportional to A 0.5. Since test logic is added to each terminal t, Test logic added to terminals Overhead = ──────────────────── ~ A –0.5 A

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)12 DFT Architecture for SOC User defined test access mechanism (TAM) Module 1 Test wrapper Test source Test sink Module N Test wrapper Test access port (TAP) Functional inputs Functional outputs Func. inputs Func. outputs SOC inputs SOC outputs TDI TCK TMS TRST TDO Instruction register control Serial instruction data

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)13 DFT Components  Test source: Provides test vectors via on-chip LFSR, counter, ROM, or off-chip ATE.  Test sink: Provides output verification using on-chip signature analyzer, or off-chip ATE.  Test access mechanism (TAM): User-defined test data communication structure; carries test signals from source to module, and module to sink; tests module interconnects via test-wrappers; TAM may contain bus, boundary-scan and analog test bus components.  Test controller: Boundary-scan test access port (TAP); receives control signals from outside; serially loads test instructions in test-wrappers.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)14 Related Topics  IEEE 1500 standard  Core Test Language (CTL):  Test configuration(s)  Test interfaces  Test data (stimuli and responses)  Reference  Y. Zorian and A. Yessayan, “IEEE 1500 Utilization in SOC Design and Test,” Proc. International Test Conf., November  E. J. Marinissen, R. Kapur, M. Lousberg, T. McLaurin, M. Ricchetti, and Y. Zorian, “On IEEE P1500 Standard for Embedded Core Test,” J. Electronic Testing: Theory and Applications, vol. 18, no. 4-5, pp , August 2002.

Spring 07, Jan 25ELEC 7770: Advanced VLSI Design (Agrawal)15 Summary  Functional test: verify system hardware, software, function and performance; pass/fail test with limited diagnosis; high ( ~100%) software coverage metrics; low ( ~70%) structural fault coverage.  Diagnostic test: High structural coverage; high diagnostic resolution; procedures use fault dictionary or diagnostic tree.  SOC design for testability:  Partition SOC into blocks of logic, memory and analog circuitry, often on architectural boundaries.  Provide external or built-in tests for blocks.  Provide test access via boundary scan and/or analog test bus.  Develop interconnect tests and system functional tests.  Develop diagnostic procedures.  Test scheduling to minimize test time and test power.