STIL ScanStructures Application in ATE Domains

1
STIL ScanStructures - Application in ATE Domains

• ...and Concerns
• Greg Maston
• Synopsys

2
Purpose

• STIL ScanStructure information is being applied
  at Test, with mixed reviews
• This presentation identifies
• the concerns
• what does work
• what is required to allow it to work
• and when it doesnt work

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Intent of STIL ScanStructures

• To maintain information about the internal scan
  constructs of a design
• To support accelerated-simulation contexts by
  bypassing serial scan loading

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Caveat

• As stated in the STIL spec, the ScanStructures
  data was never intended to go to test. Last
  sentence of the first paragraph of clause 20
• The ScanStructures are not required for
  translation of STIL to a testers format, since
  the scan data is in tester ready format.

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Reason to take this data to Test

• Provide for at-tester identification of the
  scancell that captured the failing situation
• Identify scan-based failures by more than vector
  offset information

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Mapping Output Failures to ScanCells

• Assumes a direct (or identifiable) relationship
  of scan-out-measures by Vector, to scan cell order

ScanChain A
// start Shift
ScanCells
V 205, so0
main.seg1.regb_16
Scan cells listed in reverse order...
V 206, so0
main.seg1.regb_15
V 207, so0
main.seg1.regb_14
V 208, so1
main.seg1.regb_13
V 209, so0
main.seg1.regb_12
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STIL Requirements to support this application

• The EFFECTIVE SCAN START of the scan operation
  must be definitively known
• The EFFECTIVE SCAN CHAIN related to this failure
  must be identified
• The EFFECTIVE STATE of the cell at the failure
  may be desired

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Effective Scan Start

• The scan operation is commonly the application of
  the Vectors inside a STIL Shift block
• But scan shifts may occur before or after the
  Shift block as well, as single or multiple
  Vectors (in-line expansion of the scan operation
  or partial flattening of scan data).

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Effective Scan Chain

• Either
• The failing output signal is uniquely identified
  in the scan chain definitions of the design
• Or
• The ScanChain statement must be present in the
  pattern sequence before a scan operation starts,
  to identify which chain(s) are in effect for the
  next scan operation.

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Effective Scan State

• If the "actual state of the scancell" is desired,
  then it is critical that the scan inversion data
  be properly and completely represented in the
  ScanCell block in order to derive the proper
  state.

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Mapping Scenarios

• Scenario 1
• Clearly delineated scan application
• Unambiguous scan chain reference
• Scenario 2
• Reverse the order of the clock and output-strobe
  timing
• Scenario 3
• Hidden scan

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Scenario 1
ScanStructures ScanChain A ScanOut so
ScanCells ...
Waveforms _ins 10 0ns U/D _outs
HLX 20ns h/l/x _clks P 30ns U
45ns D
Procedures load_unload WFT TP Shift
V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 1
ScanStructures ScanChain A ScanOut so
ScanCells ...
1. Identify scan
Waveforms _ins 10 0ns U/D _outs
HLX 20ns h/l/x _clks P 30ns U
45ns D
Procedures load_unload WFT TP Shift
V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 1
ScanStructures ScanChain A ScanOut so
ScanCells ...
1. Identify scan 2. Identify Start
Waveforms _ins 10 0ns U/D _outs
HLX 20ns h/l/x _clks P 30ns U
45ns D
Procedures load_unload WFT TP Shift
V se1 ckP so...
Pattern load_unload (soHHHL...
15
Scenario 1
ScanStructures ScanChain A ScanOut so
ScanCells ...
1. Identify scan 2. Identify Start 3. Identify
Chain
Waveforms _ins 10 0ns U/D _outs
HLX 20ns h/l/x _clks P 30ns U
45ns D
Procedures load_unload WFT TP Shift
V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 1
ScanStructures ScanChain A ScanOut so
ScanCells ...
// start Shift
V 205
V 206, so1/0
1. Identify scan 2. Identify Start 3. Identify
Chain 4. Map Failure to cell
Waveforms _ins 10 0ns U/D _outs
HLX 20ns h/l/x _clks P 30ns U
45ns D
Procedures load_unload WFT TP Shift
V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 2
ScanStructures ScanChain A ScanOut so
ScanCells ...
Only change flip the order of events on the
clock and output measures
Waveforms _ins 10 0ns U/D _clks
P 20ns U 35ns D _outs HLX
40ns h/l/x
Procedures load_unload WFT TP Shift
V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 2
ScanStructures ScanChain A ScanOut so
ScanCells ...
Consequential change add a Vector to capture
first scan output
Waveforms _ins 10 0ns U/D _clks
P 20ns U 35ns D _outs HLX
40ns h/l/x
Procedures load_unload WFT TP V
so Shift V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 2
ScanStructures ScanChain A ScanOut so
ScanCells ...
1. Identify scan
Waveforms _ins 10 0ns U/D _clks
P 20ns U 35ns D _outs HLX
40ns h/l/x
Procedures load_unload WFT TP V
so Shift V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 2
ScanStructures ScanChain A ScanOut so
ScanCells ...
1. Identify scan 2. Identify Start
Waveforms _ins 10 0ns U/D _clks
P 20ns U 35ns D _outs HLX
40ns h/l/x
Note the scan starts in the cycle before the
Shift
Procedures load_unload WFT TP V
so Shift V se1 ckP so...
Pattern load_unload (soHHHL...
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Scenario 3 Hidden Scan
1. Got Milk ?
Procedures load_unload WFT TP V
so V so V so V so ...
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Scenario 3 Hidden Scan
1. Identify scan ?
Procedures load_unload WFT TP V
so V so V so V so ...
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STIL Requirements to support this application

• Convention to identify start of scan operation
• ScanStructure ScanChain definitions uniform to
  this convention
• ScanStructure ScanChain definitions logically
  correct (to interpret state)
• Use of the ScanChain statement in the patterns if
  ambiguous scan references from scan-out Signals

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Possible Conventions

• Use STIL Shift statement to identify scan
• Pass all scan data on a single argument and
  define complete scanchains
• Pass data internal to the Shift in an argument
  scan operations external to the Shift are not
  seen as scan and scanchain descriptions contain
  only the chain-internal-to-the-Shift

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More Caveats for this application

• ScanStructures are an optional construct
• Limited contexts that require this data
• Some contexts will not define this data
• ScanStructures define a large amount of data
• over half of a STIL test when present, for
  scancell names only

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Caveats on the ScanChain ref stmt

• ScanChain ref statement supports single chain
  name only
• Scope of application is identified as next set
  of statements...

Procedures load_unload WFT TP
ScanChain A ScanChain B Shift V m0
se1 ckP so1 ScanChain C ScanChain
D Shift V m1 se1 ckP so3
Ambiguity between accumulation and reset behaviors
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Final Caveat

• This functionality is not diagnosis
• The identified scancell (most probably) didnt
  fail
• It captured a failure
• Identifying the failing element requires
  additional diagnostic operation

Make sure the customer understands what this
information is not!
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Conclusion

• STIL ScanStructures can be used at test
• Provides additional fail information
• Does not trace to causative fail
• Additional requirements on this data
• Uniform definition with application
• Identifying constructs if shared-IO
• Conventions (usage model) required to interpret
  this data