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Lecture 23 Design for Testability (DFT): Full-Scan

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Lecture 23 Design for Testability (DFT): Full-Scan

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Lecture 23Design for Testability (DFT): Full-Scan • Definition • Ad-hoc methods • Scan design • Design rules • Scan register • Scan flip-flops • Scan test sequences • Overheads • Scan design system • Summary VLSI Test: Bushnell-Agrawal/Lecture 23
Definition • Design for testability (DFT) refers to those design techniques that make test generation and test application cost-effective. • DFT methods for digital circuits: • Ad-hoc methods • Structured methods: • Scan • Partial Scan • Built-in self-test (BIST) • Boundary scan • DFT method for mixed-signal circuits: • Analog test bus VLSI Test: Bushnell-Agrawal/Lecture 23
Ad-Hoc DFT Methods • Good design practices learnt through experience are used as guidelines: • Avoid asynchronous (unclocked) feedback. • Make flip-flops initializable. • Avoid redundant gates. Avoid large fanin gates. • Provide test control for difficult-to-control signals. • Avoid gated clocks. • Consider ATE requirements (tristates, etc.) • Design reviews conducted by experts or design auditing tools. • Disadvantages of ad-hoc DFT methods: • Experts and tools not always available. • Test generation is often manual with no guarantee of high fault coverage. • Design iterations may be necessary. VLSI Test: Bushnell-Agrawal/Lecture 23
Scan Design • Circuit is designed using pre-specified design rules. • Test structure (hardware) is added to the verified design: • Add a test control (TC) primary input. • Replace flip-flops by scan flip-flops (SFF) and connect to form one or more shift registers in the test mode. • Make input/output of each scan shift register controllable/observable from PI/PO. • Use combinational ATPG to obtain tests for all testable faults in the combinational logic. • Add shift register tests and convert ATPG tests into scan sequences for use in manufacturing test. VLSI Test: Bushnell-Agrawal/Lecture 23
Scan Design Rules • Use only clocked D-type of flip-flops for all state variables. • At least one PI pin must be available for test; more pins, if available, can be used. • All clocks must be controlled from PIs. • Clocks must not feed data inputs of flip-flops. VLSI Test: Bushnell-Agrawal/Lecture 23
Correcting a Rule Violation • All clocks must be controlled from PIs. Comb. logic D1 Q Comb. logic FF D2 CK Comb. logic Q D1 Comb. logic FF D2 CK VLSI Test: Bushnell-Agrawal/Lecture 23
Scan Flip-Flop (SFF) Master latch Slave latch D TC Q Logic overhead MUX Q SD CK D flip-flop Master open Slave open CK t Normal mode, D selected Scan mode, SD selected TC t VLSI Test: Bushnell-Agrawal/Lecture 23
Level-Sensitive Scan-Design Flip-Flop (LSSD-SFF) Master latch Slave latch D Q MCK Q SCK D flip-flop SD MCK Normal mode Logic overhead TCK MCK TCK Scan mode TCK SCK t VLSI Test: Bushnell-Agrawal/Lecture 23
Adding Scan Structure PI PO SFF SCANOUT Combinational logic SFF SFF TC or TCK Not shown: CK or MCK/SCK feed all SFFs. SCANIN VLSI Test: Bushnell-Agrawal/Lecture 23
Comb. Test Vectors I2 I1 O2 O1 PI PO Combinational logic SCANIN TC SCANOUT N2 N1 S2 S1 Next state Present state VLSI Test: Bushnell-Agrawal/Lecture 23
Comb. Test Vectors I2 Don't care or random bits I1 PI SCANIN S1 S2 TC 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 O2 O1 PO SCANOUT N2 N1 Sequence length = (ncomb + 1) nsff + ncomb clock periods ncomb = number of combinational vectors nsff = number of scan flip-flops VLSI Test: Bushnell-Agrawal/Lecture 23
Testing Scan Register • Scan register must be tested prior to application of scan test sequences. • A shift sequence 00110011 . . . of length nsff+4 in scan mode (TC=0) produces 00, 01, 11 and 10 transitions in all flip-flops and observes the result at SCANOUT output. • Total scan test length: (ncomb + 2) nsff + ncomb + 4 clock periods. • Example: 2,000 scan flip-flops, 500 comb. vectors, total scan test length ~ 106 clocks. • Multiple scan registers reduce test length. VLSI Test: Bushnell-Agrawal/Lecture 23
Multiple Scan Registers • Scan flip-flops can be distributed among any number of shift registers, each having a separate scanin and scanout pin. • Test sequence length is determined by the longest scan shift register. • Just one test control (TC) pin is essential. PI/SCANIN PO/ SCANOUT Combinational logic M U X SFF SFF SFF TC CK VLSI Test: Bushnell-Agrawal/Lecture 23
Scan Overheads • IO pins: One pin necessary. • Area overhead: • Gate overhead = [4 nsff/(ng+10nff)] x 100%, where ng = comb. gates; nff = flip-flops; Example – ng= 100k gates, nff = 2k flip-flops, overhead = 6.7%. • More accurate estimate must consider scan wiring and layout area. • Performance overhead: • Multiplexer delay added in combinational path; approx. two gate-delays. • Flip-flop output loading due to one additional fanout; approx. 5-6%. VLSI Test: Bushnell-Agrawal/Lecture 23
Hierarchical Scan • Scan flip-flops are chained within subnetworks before chaining subnetworks. • Advantages: • Automatic scan insertion in netlist • Circuit hierarchy preserved – helps in debugging and design changes • Disadvantage: Non-optimum chip layout. Scanout Scanin SFF4 SFF1 SFF1 SFF3 Scanin Scanout SFF3 SFF2 SFF4 SFF2 Hierarchical netlist Flat layout VLSI Test: Bushnell-Agrawal/Lecture 23
Optimum Scan Layout X' X SFF cell IO pad SCANIN Flip- flop cell Y' Y TC SCAN OUT Routing channels Active areas: XY and X'Y' Interconnects VLSI Test: Bushnell-Agrawal/Lecture 23
Scan Area Overhead Linear dimensions of active area: X = (C + S) / r X' = (C + S + aS) / r Y' = Y + ry = Y + Y(1--b) / T Area overhead X'Y'--XY = -------------- x 100% XY 1--b = [(1+as)(1+ -------) – 1] x 100% T 1--b = (as + ------- ) x 100% T y = track dimension, wire width+separation C = total comb. cell width S = total non-scan FF cell width s = fractional FF cell area = S/(C+S) a = SFF cell width fractional increase r = number of cell rows or routing channels b = routing fraction in active area T = cell height in track dimension y VLSI Test: Bushnell-Agrawal/Lecture 23
Example: Scan Layout • 2,000-gate CMOS chip • Fractional area under flip-flop cells, s = 0.478 • Scan flip-flop (SFF) cell width increase, a = 0.25 • Routing area fraction, b = 0.471 • Cell height in routing tracks, T = 10 • Calculated overhead = 17.24% • Actual measured data: Scan implementation Area overhead Normalized clock rate ______________________________________________________________________ None 0.0 1.00 Hierarchical 16.93% 0.87 Optimum layout 11.90% 0.91 VLSI Test: Bushnell-Agrawal/Lecture 23
ATPG Example: S5378 Original 2,781 179 0 0.0% 4,603 35/49 70.0% 70.9% 5,533 s 414 414 Full-scan 2,781 0 179 15.66% 4,603 214/228 99.1% 100.0% 5 s 585 105,662 Number of combinational gates Number of non-scan flip-flops (10 gates each) Number of scan flip-flops (14 gates each) Gate overhead Number of faults PI/PO for ATPG Fault coverage Fault efficiency CPU time on SUN Ultra II, 200MHz processor Number of ATPG vectors Scan sequence length VLSI Test: Bushnell-Agrawal/Lecture 23
Automated Scan Design Behavior, RTL, and logic Design and verification Rule violations Scan design rule audits Gate-level netlist Combinational ATPG Scan hardware insertion Combinational vectors Scan netlist Scan sequence and test program generation Chip layout: Scan- chain optimization, timing verification Scan chain order Design and test data for manufacturing Mask data Test program VLSI Test: Bushnell-Agrawal/Lecture 23
Timing and Power • Small delays in scan path and clock skew can cause race condition. • Large delays in scan path require slower scan clock. • Dynamic multiplexers: Skew between TC and TC signals can cause momentary shorting of D and SD inputs. • Random signal activity in combinational circuit during scan can cause excessive power dissipation. VLSI Test: Bushnell-Agrawal/Lecture 23
Summary • Scan is the most popular DFT technique: • Rule-based design • Automated DFT hardware insertion • Combinational ATPG • Advantages: • Design automation • High fault coverage; helpful in diagnosis • Hierarchical – scan-testable modules are easily combined into large scan-testable systems • Moderate area (~10%) and speed (~5%) overheads • Disadvantages: • Large test data volume and long test time • Basically a slow speed (DC) test VLSI Test: Bushnell-Agrawal/Lecture 23