Built-In Self-Test for Multipliers Mary Pulukuri Dept. of Electrical & Computer Engineering Auburn University

M. Pulukuri 9/09VLSI D&T Seminar2 Outline of Presentation  Overivew of multiplier architectures  History of Digital Signal Processor (DSP) Architectures in FPGAs  Overview of Virtex-4 DSP  Prior Testing R&D for Multipliers  Our Approach  Analysis Methodology  Simulation Results  Application to Virtex-4 & 5 DSPs  Summary and Conclusions

M. Pulukuri 9/09VLSI D&T Seminar3 Overview of Multipliers  Array Multiplier  Final product calculated by using an array of full adders & and gates

M. Pulukuri 9/09VLSI D&T Seminar4 Overview of Multipliers  Signed array or Baugh Wooley multiplier  Final product calculated using an array of full adders, and gates & nand gates

M. Pulukuri 9/09VLSI D&T Seminar5 Overview of Multipliers  Modified Booth multipliers  Partial products calculated using the modified booth algorithm Modified booth algorithm uses a binary encoder to calculate partial products using a series of shift operations Modified booth algorithm uses a binary encoder to calculate partial products using a series of shift operations  Summation of partial products done using CLA adders A.R. Cooper, “Parallel architecture modified Booth multiplier” IEEE Proc. Electronic Circuits and Systems, vol. 135, no. 3, pp , 1998

M. Pulukuri 9/09VLSI D&T Seminar6 Overview of Multipliers  Modified Booth/Wallace Tree multipliers  Summation of partial products done using a Wallace Tree Each column of partial products are summed using a multi- stage setup of half and full adders Each column of partial products are summed using a multi- stage setup of half and full adders Each multi-stage adder circuit generates a sum and carry which form the two final partial products Each multi-stage adder circuit generates a sum and carry which form the two final partial products  Two final stage partial products from the wallace tree are added using a CLA adder

M. Pulukuri 9/09VLSI D&T Seminar7 Xilinx FPGA Architectures  4000/Spartan  NxN array of unit cells Unit cell = CLB + routing Unit cell = CLB + routing Fast carry logic in CLBs for adders Fast carry logic in CLBs for adders  Virtex/Spartan-2  MxN array of unit cells Carry logic + AND gate for array multipliers Carry logic + AND gate for array multipliers  4K block RAMs at edges  Virtex-2/Spartan-3  18K block RAMs in array  18x18-bit multipliers with each RAM “based on modified Booth architecture” “based on modified Booth architecture”  Virtex-4/Virtex-5  Added 48-bit DSP cores w/multipliers PC

M. Pulukuri 9/09VLSI D&T Seminar8 Virtex-4 DSP Architecture  2 DSP slices per tile  tiles in 1-8 columns  Each DSP includes:  18x18-bit 2's-comp multiplier (w/o adder)  3-input, 48-bit adder/subtractor P = Z  (X+Y+Cin) P = Z  (X+Y+Cin) Optional accum reg Optional accum reg  User controlled operational modes For X, Y, & Z MUXs For X, Y, & Z MUXs  Configuration bits control other MUXs Pipelining registers Pipelining registers Accumulator register Accumulator register Easily tested Easily tested     X Y Z X Y Z C(48) A(18) B(18) A(18) B(18) P (48) P (48) Inputs for cascading Outputs w/ dedicated routing

M. Pulukuri 9/09VLSI D&T Seminar9 BIST Approach for Virtex-5 DSP Larger multiplier

M. Pulukuri 9/09VLSI D&T Seminar10 Multiplier Architectures  Test algorithm depends on architecture  But architecture is not specified in data sheets Eliminate sequential logic architectures Eliminate sequential logic architectures “Based on modified Booth” “Based on modified Booth”  Multiplier choices include:  Array  Booth  Modified Booth  Modified Booth/Wallace tree Our assumption based on area/performance analysis Our assumption based on area/performance analysis  Our goal: find/develop architecture independent test algorithm(s)

M. Pulukuri 9/09VLSI D&T Seminar11 Modified Booth Test Algorithms  Test algorithm uses 8-bit counter (256 vectors)  “ “Effective Built-In Self-Test for Booth Multipliers” Gizopoulos, Paschalis & Zorian Gizopoulos, Paschalis & Zorian  IEEE Design & Test of Computers pp , 1998  Claim fault coverage ~ 99.8% 4x4 connections to multiplier inputs 4x4 connections to multiplier inputs  Order of the bits does not matter  Algorithm used in Srinivas Garimella’s MS thesis for Virtex-2 multipliers × n n 2n2n Booth encoding n×n multiplier 8-bit counter MSB LSB 4 4 4×4 algorithm

M. Pulukuri 9/09VLSI D&T Seminar12 Modified Booth Test Algorithms  Test algorithm uses 8-bit counter (256 vectors)  “An Effective BIST Architecture for Fast Multiplier Cores” Paschalis, Kranitis, Psarakis Paschalis, Kranitis, Psarakis Gizopoulus & Zorian  Proc. Design, Automation and Test in Europe Conf. pp , 1999  Claim fault coverage ~99.8% 5x3 connections with 5 inputs to 5x3 connections with 5 inputs to Booth encoding  But this was not explicit in paper  Only shown in figure  Order of the bits does not matter  Note that this paper is from 1999 × n n 2n2n Booth encoding n×n multiplier 8-bit counter MSB LSB 5 3 5×3 algorithm

M. Pulukuri 9/09VLSI D&T Seminar13 Modified Booth Test Algorithms  Test algorithm uses 8-bit counter (256 vectors)  “Low Power BIST for Wallace Tree-based Fast Multipliers” Bakalis, Kalligeros, Nikolos, Bakalis, Kalligeros, Nikolos, Vergos & Alexiou  Proc. Int. Symp. on Quality of Electronic Design, pp , 2000  Claim fault coverage > 99% 5x3 connections with 5 inputs to 5x3 connections with 5 inputs to Booth encoding  Specifically stated in paper  But no data to back up claim that 5x3 better than 3x5  Did they just observe it in Zorian paper?  Note that this paper was published a year later than Zorian 8-bit counter MSB LSB × n n 2n2n Booth encoding n×n multiplier 5 3 5×3 algorithm

M. Pulukuri 9/09VLSI D&T Seminar14 Modified Booth Test Algorithms  Test algorithm uses 8-bit counter (256 vectors)  But which side is Booth encoding? Xilinx does not specify Xilinx does not specify  Our original approach Run 5x3 algorithm Run 5x3 algorithm  256 vectors and run 3x5 algorithm and run 3x5 algorithm  512 vectors Include 4x4 if fault coverage improves Include 4x4 if fault coverage improves  768 vectors Additional algorithms only require multiplexers to change inputs Additional algorithms only require multiplexers to change inputs  Use same 8-bit counter × n n 2n2n Booth encoding n×n multiplier 8-bit counter MSB LSB 5 3 5×3 algorithm 3 5 3×5 algorithm

M. Pulukuri 9/09VLSI D&T Seminar15 Methodology for Analysis  Multipliers evaluated  Unsigned array  Signed array – Baugh Wooley  Modified Booth Carry look-ahead adders sum partial products in every stage Carry look-ahead adders sum partial products in every stage  Modified Booth Wallace Tree Carry look-ahead adder sums final stage partial products Carry look-ahead adder sums final stage partial products Carry select adder sums final stage partial products Carry select adder sums final stage partial products Ripple carry adder sums final stage partial products Ripple carry adder sums final stage partial products

M. Pulukuri 9/09VLSI D&T Seminar16 Methodology for Analysis  Designed 8-bit models of the multipliers  Fault model: Collapsed single stuck-at gate level faults  Exhaustive testing To determine undetectable faults To determine undetectable faults  Test algorithms evaluated 4×4 4×4 5×3 5×3 3×5 3×5 5×3 & 3×5 5×3 & 3×5 4×4, 5×3 & 3×5 4×4, 5×3 & 3×5

M. Pulukuri 9/09VLSI D&T Seminar17 Multiplier Total Faults Test Algorithm # faults detected (effective fault coverage) Exhaust4×45×35×33×53×5 5×3 & 3×5 5×3, 3×5 & 4×4 Unsigned array (100) 1621 (98.60) 1644 (100) Signed array (100) Mod-Booth (100) 2180 (99.27) 2168 (98.72) 2179 (99.23) 2182 (99.36) 2193 (99.86) Mod-Booth Wall-Tree CLA (100) 2061 (98.61) 2068 (98.95) 2070 (99.04) 2071 (99.09) 2074 (99.23) Mod-Booth Wall-Tree CSA (100) 2215 (98.75) 2217 (98.84) 2218 (98.89) 2222 (99.06) 2228 (99.33) Mod-Booth Wall-Tree RCA (100) 1937 (98.73) 1944 (99.08) 1947 (99.24)

M. Pulukuri 9/09VLSI D&T Seminar18 Application to Virtex-4 & 5 DSPs  In Virtex-4 & 5 DSPs  Final stage carry look-ahead adder (CLA) separated from the multiplier  5×3 & 3×5 give the same fault coverage for the multiplier alone  Separate test algorithm for the CLA Run both 5×3 and 3×5 to test for bridging faults on the cascade routing between adjacent slices Run both 5×3 and 3×5 to test for bridging faults on the cascade routing between adjacent slices Mode (Test)First 256 ccsSecond 256 ccsThird 256 ccsFourth 256 ccs 00 (multiply)P = A×B P = A×B+CP = A:B+C

M. Pulukuri 9/09VLSI D&T Seminar19 Summary and Conclusion  If the architecture of the multiplier is not known:  3×5 algorithm gives best overall fault coverage for most multipliers Contradicting the claim of the authors who proposed 5×3 Contradicting the claim of the authors who proposed 5×3  Running 3×5 & 5×3 gives better fault coverage for all multipliers  Running all three algorithms: 3×5, 5×3 and 4×4 test algorithms provides the best fault coverage for all multipliers Architecture independent testing Architecture independent testing  Virtex-4 & Vritex-5 multipliers  Original approach was 3×5 and 5×3  Better approach would be 3×5 and 4×4

M. Pulukuri 9/09VLSI D&T Seminar20 Summary and Conclusion  For multipliers in Virtex-2 FPGAs  Adder not separated from the multiplier Run both 3×5 and 5×3 algorithms Run both 3×5 and 5×3 algorithms  These give highest fault coverage for multiplier & CLA  The 3×5 and 4×4 BIST algorithm should be applied to multipliers in  Spartan-3A Similar to multipliers in Virtex-4 Similar to multipliers in Virtex-4  Spartan-6 Similar to multipliers in Virtex-4 Similar to multipliers in Virtex-4  Virtex-6 Similar to multipliers in Virtex-5 Similar to multipliers in Virtex-5  If only 2 algorithms can be applied Best results if all 3 can be applied Best results if all 3 can be applied

M. Pulukuri 9/09VLSI D&T Seminar21 Summary and Conclusion  Area overhead for different approaches  In addition to 8-bit counter  Maximum area overhead for N-bit multiplier: One test algorithm: 2N 2:1 multiplexers One test algorithm: 2N 2:1 multiplexers Two test algorithms: 2N 3:1 multiplexers Two test algorithms: 2N 3:1 multiplexers  1 additional counter bit for control All three test algorithms: 2N 4:1 multiplexers All three test algorithms: 2N 4:1 multiplexers  2 additional counter bits for control  This is worst case since synthesis tools may reduce multiplexers Particularly in case of two and three test algorithms Particularly in case of two and three test algorithms  Due to counter duplicate bits to same multiplexers  Regardless, this is an area efficient BIST approach Paper almost finished for JETTA Letter or Trans. IE Corr. Paper almost finished for JETTA Letter or Trans. IE Corr.  Brad is using 3×5, 5×3 & 4×4 algorithms in test bench for multipliers in Output Response Analyzer (ORA) for mixed signal BIST

M. Pulukuri 9/09VLSI D&T Seminar22