Test and Debug in Deep-Submicron Technologies

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Test and Debug in Deep-Submicron Technologies
Ramyanshu Datta, Antony Sebastine, Ravi Gupta, Whitney J. Townsend, and Jacob A. Abraham
Computer Engineering Research Center
The University of Texas at Austin
rdatta,antonys,ravi,whitney,jaa @cerc.utexas.edu
IBM Technical Contact : Dr. Robert Montoye
Abstract
With the scaling of feature sizes into Deep-Submicron
(DSM) values, the level of integration and performance
achievable in VLSI chips increases. A lot of work has been
directed to tackle design related issues arising out of scaling, like leakage mitigation etc. However efforts to enhance testability of such designs have not been sufficient.
It is not viable to overlook testability issues arising out of
these designs because the defect sizes do not scale proportional to the feature sizes. Previously effective fault models like stuck-at appear archaic and are unable to model
faults accurately. This necessitates the need for more detailed models which can more explicitly model the behavior
of faulty DSM chips. Also there is a significant increase in
delay faults in logical paths of integrated circuits. Delay
faults cause the delay of paths in a chip to be larger than
expected resulting in the output of a chip to be deviant from
the expected behavior, in spite of the chip being functionally
correct. Efficient techniques are needed for detecting such
defects in first silicon and eliminating them before the final versions of the chips are shipped. This requires efficient
debug techniques for performance characterization of large
complex integrated circuits in deep-submicron and nanometer technologies. In this paper we present an insight into test
challenges arising out of deep submicron technologies and
effective approaches to tackle the same.
1. Introduction
Tremendous progress in scaling of process technologies
down to the Deep-Submicron (DSM) domain has paved the
way for a significant increase in the level of integration and
performance of modern VLSI chips. The integration of
complex System on a Chip (SoC) is now a reality. Achieving acceptable reliability levels for modern VLSI chips is
a critical issue, making testability a significant factor that
could limit scaling trends if not addressed adequately. This
is especially significant since defect sizes have not scaled
in a manner commensurate with shrinking geometries [1].
Process variations can have a significant influence on a
chip’s failure to meet specified performance. Process variations can be broadly classified into two types [2]: process variations within the same fabrication plant and process
variations between fabrication plants. The former includes
line-to-line, wafer-to-wafer, inter-die (each device on one
die shows uniform variations) and intra-die (process variations not uniform over entire die) variations. These variations result in delays of wires and gates within a chip which
can be about 30% to 40% [2]. The latter pertains to variations caused by factors like usage of different cell libraries
and resulting synthesized circuitry in different foundries,
and this may cause variations of 20%-25% between chips
fabricated in different foundries. [2]. However fabrication of the same product in different plants is not a common practice and certainly never done for high-performance
chips, and hence is not a strong contributor to on-chip process variations. Process variations affect yield, performance
and reliability of processors, and need to be considered for
delay analysis for .13 micron and lower technologies [3].
Device physics factors [24],[25] like random dopant placement in channel, spatially correlated gate length variation,
interconnect thickness variation and processing factors like
die location dependence, optical proximity effect, microloading in etching and deposition lead to uncorrelated variations of process parameters within the same die. There has
been a lot of work done on design related issues in DSM e.g.
circuit families like Limited Switch Dynamic Logic (LSDL)
optimized for performance, power, area and leakage [4], but
the same is not true for testing. There is an increased need
to focus on testability issues related to DSM chips. With
increasing system complexity, current test and verification
techniques need to be improved. Fault models like stuck-at,
which correctly modelled behavior of defective chips earlier, are not effective. Existing techniques like functional
simulation, formal verification, and static timing analysis
cannot guarantee defect free first silicon. There can be func-
tional design errors, timing errors and design rule violations
[38] which may go undetected before first silicon. This is
because verification is applied only to a model of an Integrated Circuit(IC) and not to actual silicon, and adding
more details to this model cause the computational cost of
these verification methods to become prohibitive. A structured debug technique is then necessary to locate and rectify any design error in first silicon in order to reduce time
to market. Debug of a chip requires analysis of both its internal and external behavior under known stimuli. Faulty
behavior and possible design errors can be located by repeatedly performing this analysis and comparing responses
obtained with a set of expected responses. These results
can assist in fault localization, and the design cycle is repeated to rectify these defects before new silicon is manufactured. Diminishing feature sizes limit the observability of chips making debugging more difficult especially for
timing violations. In order to ensure optimal performance
of DSM chips, their functional correctness as well as timing behavior need to be tested. With shrinking feature sizes,
delay faults in chips have emerged as a significant problem.
Process parameter variations can result in distributed delay
faults in the chip, which cause minor delay faults on multiple gates in a given path to accumulate and result in the
path failing to meet performance specifications [17]. Process parameter variations can be inter-die i.e. those which
influence the entire chip or functional block such that each
device on a chip or a block shows uniform process variations. These are caused by systematic effects like process
gradients over the wafer. Process variations which are not
uniform over the entire die are called intra-die variations.
Timing requirements of modern chips have introduced the
need to test for and detect defects of the order of few picoseconds. DC functional tests like scan tests can detect
static faults such as stuck at, but cannot detect dynamic
faults like delay faults. Also testing for delay defects using
Automated Testing Equipment (ATE) for GHz range processors is very expensive and most testers in test facilities still
run at a few 100 MHz. These factors necessitate on-chip test
infrastructure like Built-In Self Test and Design For Testability as well on-chip timing analysis and delay fault testing
circuitry, for test and debug of modern VLSI chips. Also,
the use of on-chip testing circuitry allows for at-speed testing which is imperative to ensure accurate timing behavior
and performance characterization of chips.
In addition to test challenges associated with DC and AC
faults, DSM chips also suffer from transient errors like Single Event Upsets (SEUs) caused by cosmic neutrons during
normal operation of the chip. It is reported that upto 20 neutrons/sq cm/hr reach the Earth’s surface with an energy level
greater 10MeV. This effect of random errors due to such
hits is tremendous in sub 100nm technologies and supply
voltages below 2.2V [23]. Ever smaller dimensions brings
CLOCK
a) FAULT−FREE RISE
b) SLOW TO RISE 1
c) SLOW TO RISE 2
d) SLOW TO RISE 3
e) FAULT−FREE FALL
f) SLOW TO FALL 1
g) SLOW TO FALL 2
h) SLOW TO FALL 3
i) SETUP VIOLATION 1
j) SETUP VIOLATION 2
Figure 1. Delay Fault Types
with it an ever increasing probability of transient errors occurring within circuits and these cannot be detected using
regular test methods. Detection and debug of such errors
has increased the importance of On-Line Testing, i.e. testing chips during their regular operation. Just as memories
include redundant circuitry today, it may become necessary
to protect datapath circuitry with error detection and even
error correction. Therefore this paper includes a method
for providing for on-line testing during circuit operation as
well as another method which provides for on-line masking
of errors.
This paper is organized as follows. Section 2 reviews
some background on delay faults. Section 3 elaborates on
scan chains and Design for Testability (DFT) schemes. Section 4 provides an overview of Built in Self Test (BIST) and
presents some BIST schemes for multipliers. Section 5 discusses some existing strategies for on chip delay measurement and characterization. In Section 6 we present on-line
testing for error detection and conclude the paper.
2. Delay Faults
Delay faults are a category of faults which cause an otherwise defect free (i.e. logical operation correct) chip to
malfunction at a desired clock rate. Delay faults could be
due to increased interconnect or path resistance, crosstalk
induced delay, excessive voltage drop on supply nets, substrate and thermal noise, resistive opens and process variations. Process variations have a significant effect on timing behavior of chips built using 0.13 m and lower technologies [3], [17]. Process variations can be inter-die i.e.
those which influence the entire chip or functional block
such that each device on a chip or a block shows uniform
process variations. These are caused by systematic effects
like process gradients over the wafer. Process variations
which are not uniform over the entire die are called intradie variations. Figure 1 shows some ways delay faults manifest themselves. Figure 1a is a fault free rising transition
on a path. In 1b the signal transition starts to rise at the
same time as the fault free signal, but by time the signal
reaches half of its supply voltage (which is assumed to be
the threshold value required by a circuit to detect a rise or
fall), the assertion edge of the clock has passed, and incorrect value is captured into a latching element clocked by this
clock. In 1c the transition itself takes place after the clock
assertion edge and causes a wrong value to be latched in.
Figure 1d shows a case where the fault causes the signal to
be delayed so much that it misses the next assertion edge of
the clock too. Figures 1 e-h show the corresponding cases
for falling transitions. Figures 1 i and j cause setup time violations of latching elements and cause indeterminate values to be latched in. Each of the transitions in b-d and f-h
require different kind of tests [27]. Delay Fault Testing determines the correctness of the circuit at the specified speed.
A CUT that is functionally correct at a particular frequency
may fail at a higher frequency. Higher propagation delays
of signals may cause wrong values to be latched at outputs.
Delay fault testing detects such temporal defects in a CUT,
and has gained significance in face of aggressive timing requirements of modern day ICs. However issues pertaining
to delay testing are quite different from those that pertain to
functional test strategies like stuck-at test. Unlike stuck-at
fault testing, delay testing is closely tied to the test application strategy, and knowledge of test application methodology is a prerequisite to test generation [18]. Additionally
delay testing requires generation of two patterns (one for
initialization and second for the required transition) in order to detect a fault. The fault models used for delay fault
testing are also quite different from the conventional fault
models like stuck-at. Delay fault models can be primarily
categorized into 5 types [19], [13], [21], [33], [12]
1. Transition fault model
2. Gate delay fault model
3. Line delay fault model
4. Path delay fault model
5. Segment delay fault model
SCANOUT
mode
COMBINATIONAL
LOGIC
COMBINATIONAL
LOGIC
SCANIN
Figure 2. Scan Chain
The transition delay fault model [19] assumes that delay
faults affect only a single gate in the CUT and are classified into slow-to-rise and slow to-fall faults. These faults
increase or decrease the nominal delay of the gate. In this
fault model the delay is assumed to be ob The gate delay
fault model[13] assumes that the delay fault is lumped at
one gate in the circuit. However unlike the transition fault
model it does not assume that an increase in delay due to the
fault will affect the performance irrespective of the propagation path. Instead it assumes that only long paths through
the fault model will cause performance degradation. Limitations of this fault model are similar to that of the transition
fault model since the assumption that a single gate is affected by a delay fault is overly optimistic.The advantage is
that the number of faults is linear with the number of gates
in the circuit.
Line delay fault model [21] is a variation of the Gate delay fault model in that it tests a rising/falling delay fault on
a given line in the circuit, and propagates the fault through
the longest sensitizable path in the circuit. This fault model
can detect some distributed delay defects on the propagation paths but since only one propagation path through each
line is considered it may fail to detect some defects.
A path is defined as an ordered set of gates and a path delay fault model is considered faulty if the delay of any of its
paths exceeds a specified limit. The delay of the path is the
sum of the delays of the gates and interconnections on that
path. The path delay fault model [33] is more appropriate to
DSM technology designs, since it can model statistical process variations. However the disadvantage of using the path
delay fault model is that the number of paths whose delay
needs to be measured can become exponential for real size
designs.
Segment delay fault model is a way around this problem
[12] . It makes the assumption that delay affects several
gates in the local region of occurrence, and also that the
buf_low
buf_low
buf_low
buf_low
buf_low
buf_low
buf_low
buf_low
y
D
Q
CLK
RESET
D
Q
CLK
RESET
Q0
D
Q
CLK
RESET
Q1
D
Q
CLK
RESET
Q2
D
Q
D
CLK
RESET
Q3
Q4
Q
CLK
RESET
D
Q5
Q
CLK
RESET
D
Q6
Q
CLK
RESET
x
buf
reset
buf
buf
buf
buf
buf
buf
buf
Figure 3. Vernier Delay Line
segment delay fault is large enough to cause a delay fault
on all paths that include the segment. The upper bound on
the length of the segment is the maximum number of gates
in the longest path.
Transition, gate and line delay fault models are used to
represent delays lumped at gates whereas path and segment
delay fault models are used to represent delays distributed
across the chip.
this scan also has the disadvantage of greater power dissipation as generally there is more switching during scan than in
normal operation. Thus it is common to use a slow clock for
scan to reduce the average power dissipation. Partial scan
where only some of the flops are converted to scan flops is
also sometimes used in sequential circuits in place of full
scan techniques.
4. Built in Self Test
3 Scan Chains and Design for Testability
Design for Testability (DFT) is a technique to reduce difficulty of testing by adding/modifying some hardware on
chip. The scan DFT methodology [7] is a standard DFT
practice followed by industry. In this technique the sequential storage elements (flip-flops/latches) are connected in
a manner that allows two modes of operation. In normal
mode i.e. when the chip is performing it regular function the
storage elements take their stimulus from a combinational
logic and the response feeds into a combinational logic. In
test mode the storage elements are reconfigured as one or
more shift registers, and each such configuration is known
as a scan chain. The stumuli vector which need to be applied in test mode can be shifted in serially into the scan
chain. The chip is then allowed to function in normal mode
and the responses for a test vector are captured in the storage elements. The response obtained can be shifted out and
compared with golden reference responses in order to test
the chip for functional correctness.
The use of scan design has two penalties. Firstly and
foremost there is an area overhead due to the added scan
flops. There is also the performance overhead caused by
the on-path multiplexors in the scan flops. Besides the area
overhead of the scan flops themselves added routing furhter
adds to the area. In typical designs both the area and performance overhead due to scan insertion is about 5 to 10%. As
scan chains have no logic in the scan path there is a chance
of a race condition if the skew on the second clock signal is
large so has to be taken in designing scan chains. Besides
Built-In Self Test (BIST)[22] refers to techniques and circuit configurations that enable a chip to test itself. In this
methodology, test patterns are generated and test responses
are analyzed on chip. Pseudorandom patter generator logic
was shown to be able to greatly reduce test data volume by
sifting out the easily detectable faults [30]. Test application and test data compression techniques using embedded
processors [15] or reusing on-chip processors [11] [31] [14]
have been suggested. BIST techniques have gained acceptance for testing complex digital designs. These techniques
involve modification of the hardware on the chip such that
the chip has the capability to test itself. BIST offers various advantages over other testing techniques including the
use of automatic test equipment (ATE) [16]. First, the test
circuitry is incorporated on chip and no external tester is required. Second, the test can be performed at normal clock
rate. Third, the tests can be performed even after the chip
has been incorporated in the system which can enable periodic testing. The two disadvantages of BIST are area overhead and performance penalty. Incorporation of the self test
capability requires addition of hardware to the chip. This
increases the silicon area required to implement the chip
hence increasing the cost of manufacturing chips. Also,
the hardware added to the circuit increases the delays of
the normal circuit path decreasing the speed at which the
circuit can be used during its normal operation. The use
of BIST architectures for embedded fast multiplier cores is
the best solution [26]. It permits at speed testing, provides
very high fault coverage and drives down the testing cost.
Q7
Various Design-for-Testability techniques have been proposed, but each approach assumes specific implementations
of the multiplier cells. Some techniques provide designs for
generalized implementations, but the test sets of these approaches are meant to be externally stored and applied to
the multiplier under test and the output response externally
evaluated. The need for a testing scheme that works for
all multiplier cell implementations without requiring a high
controllability and observability is evident.
We evaluated the effectiveness of various BIST scehemes for multipliers and implemented them on 8x8, 16x16
Wallace as well as Dadda multipliers. Two different implementations were done, differing only in the kind of PG. One
used a constant size counter [26]and the other used an LFSR
of the size of the multiplier input. An 8 bit counter/LFSR
was used to generate the inputs and the product driven into
an MISR. The selection between the test patterns and the
input patterns is done using a multiplexer.
5. On-Chip Delay Measurement Strategies
Sevaral methods have been proposed for on-chip delay
measurement and delay fault testing. In [8] a sampling circuit is proposed based on two clocks which are 180 phase
shifted with each other. However the method requires ratioed capacitors and fails if skews happen in transitions of
the two clocks. Such behavior is expected due to existence
of clock skews. Also this sampling circuit can detect only
delay faults where the the delayed transition on a path occurs after a the sampling time and hence is rendered ineffective. Franco and McCluskey [9]propose a DFT technique
to detect delay faults using transient switching currents in
CMOS inverters. However this scheme has a low noise margin which hinders its fault detection capability. It is also
a dynamic circuit hence suffers from high power overhead
due to switching. In [27] a DFT technique based on capacitor voltage level has been proposed. The scheme requires
determination of a threshold voltage and is too complex for
practical use.
Several methods have been proposed for on-chip delay
measurement based on digitizing short intervals of time.
These include a shift register/fast counter based Time-toDigital Converter (TDC) [32], oscillator based TDC [28]
and various CMOS tapped delay line configurations [6],
[28],[20],[10], [29]. Analog methods based on voltage ramp
generation have also been proposed [34] where the the voltage on a capacitor is proportional to the time difference
between two rising edges. Delay line configurations are
beneficial [6] in that they do not require anything more
than a standard digital CMOS process, advantages of which
are lower cost, lower power dissipation, higher integration level, higher noise margins and a large set of existing
Computer Aided Design (CAD) tools. However minimum
achievable resolution of a TDC based on a single delay line
is limited by the minimum gate delay in the technology it is
implemented in. This can be overcome by using a balanced
delay line called a Vernier Delay Line (VDL) [6], [10], [29],
based on the Vernier principle. The Vernier principle is derived from a measurement tool called Vernier caliper. This
tool measures the length of an object by placing it between
its two jaws. One of the two jaws is movable and the other
is fixed. As the movable jaw slides an indicator mark shows
the distance between the jaws on a calibrated scale. VDL
has two delay lines, and measures range of delay between
two signals. The basic scheme for the Vernier Delay Line
(VDL) is shown in Figure 3 [6] [10] [29].
It consists of two delay buffer chains with the delay of individual buffers in the lower chain (t ) greater than the delay of individual buffers in the upper buffer chain (t ).
The first arriving signal is fed to the input x of the lower
buffer chain and the late arriving signal is fed to the input
y of the upper buffer chain. As x and y propagate through
their respective delay chains, the time difference between
the two signals is reduced in every stage by an amount
which equals the difference in delay of individual buffers
in the respective chains. This is basically the resolution of
the VDL, i.e.
Edge triggered latching elements at every stage are clocked
by propagating x and latching in the value of propagating y
when the latch is transparent. The stage/latch number n in
the delay line where x catches up with y indicates the range
of time difference between x and y signals. The event of x
catching up with y is indicated by the presence of the first
’1’ among the flops in the delay line. All subsequent stages
will latch in a ’1’. The range of the delay difference between the two signals is then given as,
In theory, any difference between the two signals can
be measured by making the resolution as low as possible.
However in practice minimum resolution is limited by factors like mismatch of transistors, delay mismatch due to
loading, length of the delay line. In deep submicron designs
additional factors like process variations, noise, crosstalk
etc also affect the resolution. Additionally the range of the
VDL i.e. the maximum time difference between two signals
that can be measured is limited to N*t , where N is the total number of stages in the VDL. Improvements have been
proposed for improving both resolution of delay lines [29]
and increasing the range of the VDL using Delay Locked
Loops [6]. DL has two delay lines, and measures range of
delay between two signals. VDL schemes suffer from overhead of reading out delay values and inability to test paths
for all possible transitions. A Modified Vernier Delay Line
Addend
Augend
1−out−of−3
encoding
1−out−of−3
encoding
Intermediate
Sum
&
Carry
Final
Decoder
Sum
Sum
Figure 4. Block Diagram for Error Detecting Addition
(MVDL) that can be used to characterize critical path delays was proposed in [5]. This scheme overcame the overheads of VDL by applying an efficient readout scheme and a
scheme to handle all possible worst case transitions. Existing VDL schemes can measure delay between only two incoming pulses or rising transitions. However in delay fault
testing, the worst case delays in paths could be due to other
different types of transitions. A scheme to modify the VDL
in order to handle all possible transitions on input and output is presented in this section. Flops with set-reset capability are used to handle such transitions. Experimental results for delay measurement using MVDL closely matched
with those obtained using a well known commercial timing analysis tool [35]. Reading out of the values stored in
the delay line is a crucial task and most proposed solutions
for reading out the latched VDL values involve tremendous
hardware overhead like a separate asynchronous read-out
architecture using registers [6]. We have resolved this issue
in the MVDL by using a readout scheme which has minimal
hardware and pin overhead.
6 On-Line Testing
The continual march to ever smaller dimensions brings
with it an ever increasing probability of transient errors occurring within circuits. Just as memories include redundant
circuitry today, it may become necessary to protect datapath circuitry with error detection and even error correction
in the future. Described in this section is a method of error
detection applicable to extremely fast multiplication and division circuits, followed by a more general error correction
method applicable to many arithmetic circuits.
The redundancy required to detect errors often imposes a
delay penalty on the circuit it protects. In [36], a technique
was described that provides error detection while minimizing the delay impact through the use of signed digit arithmetic. These signed digits are represented using 1-outof-3 code words, thus the code word set represents the digit set . Both signed magnitude
and two’s complement operands can be converted into this
signed digit representation. 1-out-of-3 checkers are used
to detect errors in the circuit during the computatation. A
16
16
4 bit 4:1 mux
4 bit 4:1 mux
4
ci
4 bit adder
4
co
2:1 mux
4
ci
4 bit adder
4
co
ci
4 bit adder
4
1 bit reg
co
4 bit voter
1 bit voter
4
Figure 5. 16-Bit Quadruple Time Redundancy
Adder
block diagram is shown in Figure 4.
Error correction may be required in addition to error
detecting circuits. However, one of the most straightforward methods of providing error correction within a circuit, Triple Modular Redundancy (TMR), requires a hardware redundancy overhead of more than 200%. Quadruple
Time Redundancy (QTR) employs the same fault masking
methodology as TMR but uses a combination of time redundancy, as well as hardware redundancy, to achieve this
effect. QTR divides the functional portion of an arithmetic
circuit into fourths. Three of these smaller portions are instantiated in a TMR configuration. Multiplexors are used to
apply one-fourth of the operands to the circuit in each iteration. After four iterations, the entire output is available. For
64-bit addition QTR requires only 32% hardware overhead
and 30% delay over a non-redundant circuit, compared to
a hardware overhead of 256% for a similarly sized TMR
circuit [37]. The block diagram for a 16-bit QTR adder is
shown in Figure 5.
7. Conclusions and Future Work
In this paper we have presented an insight into existing
test and debug practices in Deep Submicron technologies.
Techniques for on chip subnanosecond capture of signals
for delay fault testing are presented. Future research will
focus on optimizing critical path selection for delay test and
debug, and test generation techniques for delay fault testing.
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