ECE 269 VLSI System Testing Krish Chakrabarty Memory Testing ECE 269 Krish Chakrabarty 1 Density Density Trends Trends 1970 DRAM Invention Intel 1024 bits 1993 1st 256 MBit DRAM papers 1997 1st 256 MBit DRAM samples 1 bit 120 X 10 6 bit ECE 269 Krish Chakrabarty 2 1 Memory Memory Cells Cells Per Per Chip Chip Krish Chakrabarty ECE 269 3 Test Test Time Time in in Seconds Seconds Memory Memory Size Size nn Bits Bits Size n Number of Test Algorithm Operations n 0 06 1 Mb 0 25 4 Mb 16 Mb 1 01 64 Mb 4 03 256 Mb 16 11 64 43 1 Gb 128 9 2 Gb ECE 269 n log2n n3 2 n2 1 26 5 54 24 16 104 7 451 0 1932 8 3994 4 64 5 515 4 1 2 hr 9 2 hr 73 3 hr 586 4 hr 1658 6 hr 18 3 hr 293 2 hr 4691 3 hr 75060 0 hr 1200959 9 hr 19215358 4 hr 76861433 7 hr Krish Chakrabarty 4 2 Outline RAM structure Fault models Opens shorts and stuck at faults Address decoder faults inaccessible cells or multiple cell accesses Coupling and pattern sensitive faults interactions between neighboring cells Standard RAM tests March tests Finite sequence of March elementts i e finite sequence of operations applied to every cell before proceeding to next cell Checkerboard Walking 1s and 0s Galloping pattern of 1s and 0s GALPAT Reference A J Van de Goor Testing Semiconductor Memories Theory and Practice John Wiley 1991 Krish Chakrabarty ECE 269 5 RAM Structure Address buffer Row decoders Address Storage array Sense amplifier Refresh Control DRAM Column decoder Data buffer Control Data In Out ECE 269 Krish Chakrabarty 6 3 Fault Models Fault models are technology dependent Stuck at fault SAF A cell is always in state 0 s a 0 or state 1 s a 1 Transition fault TF Special case of SAF Cell unable to make 0 1 transition when 1 is written to it up transition fault symbol 0 Cell unable to make 1 0 transition when 0 is written to it down transition fault symbol 1 Krish Chakrabarty ECE 269 7 Fault Models w0 w1 w1 S1 w0 State diagram of fault free cell S0 w0 w0 w1 S0 S1 w1 State diagram of s a 1 cell State diagram of s a 0 cell w1 w0 w1 S0 w0 S1 State diagram of cell with 0 fault ECE 269 Krish Chakrabarty 8 4 Fault Models Coupling faults Used mostly for dense DRAMs Cells are close to each other hence interactions are more likely Assumptions Read operation will not cause an error in absence of faults in address decoder Non transition write will not cause an error 2 coupling faults A write operation which generates or transition in one cell changes contents of a second cell ECE 269 Krish Chakrabarty 9 Coupling Faults CF Inversion coupling fault An or transition in one cell coupling cells inverts the contents of a second cell coupled cell Idempotent coupling fault An or transition in one cell coupling cells forces the contents of a second cell coupled cell to a certain value 0 or 1 ECE 269 Krish Chakrabarty 10 5 Coupling Faults w0 i w0 j w0 i w1 j w1 j State diagram of two fault free cells S00 w0 i w0 j S01 w0 i w1 i w1 i w1 j w1 i w0 j S10 w0 j S11 w1 i w1 j State diagram of inversion coupling fault ECE 269 Krish Chakrabarty 11 Coupling Faults State diagram for idempotent coupling fault force to 0 ECE 269 Krish Chakrabarty 12 6 Pattern Sensitive Faults PSF The contents of a cell or ability to change the contents of a cell is influenced by contents of other cells PSF is the general case of k coupling fault Base cell Neighborhood cell ECE 269 Krish Chakrabarty 13 Zero One Tests Write and read 1s and 0s to every cell Also called Memory Scan MSCAN Length of test 4 n O n Detects only SAFs not all TFs CFs or PSFs Step 1 Memory w0 address ECE 269 2 3 4 r0 w1 r1 Krish Chakrabarty 14 7 Checkerboard Tests Write 1 in all odd cells and 0 in all even cells Read and verify Write 0 in all odd cells and 1 in all even cells Read and verify 1 0 0 1 0 1 1 0 Test length 4 n O n odd even odd even even odd even odd odd even odd even even odd even odd ECE 269 1 0 1 0 0 1 0 1 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 Krish Chakrabarty 15 Walking 1s and 0s Write 0 to every cell for i 0 to n 1 do Write 1 in test cell Ci Read and verify all Cj Ci Read Ci Write 0 in test cell Ci i i 1 Repeat with 0 and 1 switched Total number of steps 2 n n 1 n 1 2 2n2 6n O n2 Detects all TFs and CFs GALPAT same as walking 1s and 0s except that each Read Cj is followed by Read Ci Total number of steps 2 n n 1 2 n 1 2 4n2 4n O n2 ECE 269 Krish Chakrabarty 16 8
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