Vlsi Design of Low Transition Low Power Test Pattern Generator Using Fault Coverage Circuits

IOSR Journal of Electronics and Communication Engineering (IOSR-JECE)
e-ISSN: 2278-2834,p- ISSN: 2278-8735.Volume 10, Issue 6, Ver. I (Nov - Dec .2015), PP 36-42
www.iosrjournals.org
DOI: 10.9790/2834-10613642 www.iosrjournals.org 36 | Page
Vlsi Design of Low Transition Low Power Test Pattern Generator
Using Fault Coverage Circuits
1
B.Praveen Kumar , 2
M.Ravindra Kumar , 3
Rajesh.Ch
1,2,3
(Electronics & Communication Engineering, Pydah College of Engineering & Technology, India)
Abstract: Now a day’s highly integrated multi layer board with IC’s is virtually impossible to be accessed
physically for testing. The major problem detected during testing a circuit includes test generation and gate to
I/O pin problems. In design of any circuit, consuming low power and less hardware utilization is an important
design parameter. Therefore reliable testing methods are introduced which reduces the cost of the hardware
required and also power consumed by the device. In this project a new fault coverage test pattern generator is
generated using a linear feedback shift register called FC-LFSR which can perform fault analysis and reduces
the total power of the circuit. In this test, it generates three intermediate patterns between the random patterns
which reduces the transitional activities of primary inputs so that the switching activities inside the circuit under
test will be reduced. The test patterns generated are applied to c17 benchmark circuit, whose results with fault
coverage of the circuit being tested. The simulation for this design is performed using Xilinx ISE software using
Verilog hardware description language.
I. Introduction
Test Pattern generation has long been carried out by using conventional Linear Feedback Shift
Registers (LFSR’s). LFSR’s are a series of flip-flop’s connected in series with feedback taps defined by the
generator polynomial. The seed value is loaded into the outputs of the flip-flops. The only input required to
generate a random sequence is an external clock where each clock pulse can produce a unique pattern at the
output of the flip-flops.
The number of inputs required by the circuit under test must match with the number of flip-flop outputs
of the LFSR. This test pattern is run on the circuit under test for desired fault coverage. The power consumed by
the chip under test is a measure of the switching activity of the logic inside the chip which depends largely on
the randomness of the applied input stimulus. Reduced correlation between the successive vectors of the applied
stimulus into the circuit under test can result in much higher power consumption by the device than the
budgeted power. A new low power pattern generation technique is implemented using a modified conventional
Linear Feedback Shift Register [1].
Need for using BIST technique
Today’s highly integrated multi-layer boards with fine-pitch ICs are virtually impossible to be accessed
physically for testing. Traditional board test methods which include functional test, only accesses the board's
primary I/Os, providing limited coverage and poor diagnostics for board-network fault. In circuit testing, an-
other traditional test method works by physically accessing each wire on the board via costly "bed of nails"
probes and testers. To identify reliable testing methods which will reduce the cost of test equipment, a research
to verify each VLSI testing problems has been conducted. The major problems detected so far are as follows:
1.1 Test Generation Problems
The large number of gates in VLSI circuits has pushed computer automatic-test-generation times to
weeks or months of computation. The numbers of test patterns are becoming too large to be handled by an
external tester and this has resulted in high computation costs and has outstripped reasonable available time for
production testing.
1.2 The Gate to I/O Pin Ratio Problem
As ICs grow in gate counts, it is no longer true that most gate nodes are directly accessible by one of
the pins on the package. This makes testing of internal nodes more difficult as they could neither no longer be
easily controlled by signal from an input pin (controllability) nor easily observed at an output pin (observe
ability). Pin counts go at a much slower rate than gate counts, which worsens the controllability and observe
ability of internal gate nodes.

Vlsi Design of Low Transition Low Power Test Pattern Generator Using Fault Coverage Circuits
II. Testing Schemes
Power dissipation is a challenging problem for today’s Sys-tem-on-Chips (SoCs) design and test. The
power dissipation in CMOS technology is either static or dynamic. Static power dissipation is primarily due to
the leakage currents and contribution to the total power dissipation is very small. The dominant factor in the
power dissipation is the dynamic power which is consumed when the circuit nodes switch from 0 to 1. During
switching, the power is consumed due to the short circuit current flow and the charging of load capacitances [1].
Automatic test equipment (ATE) is the instrumentation used in external testing to apply test patterns to
the CUT, to analyze the responses from the CUT, and to mark the CUT as good or bad according to the
analyzed responses. External testing using ATE has a serious disadvantage, since the ATE (control unit and
memory) is extremely expensive and cost is expected to grow in the future as the number of chip pins increases.
As the complexity of modern chips increases, external testing with ATE becomes extremely expensive. Instead,
Built-In Self-Test (BIST) is becoming more common in the testing of digital VLSI circuits since it overcomes
the problems of external testing using ATE. BIST test patterns are not generated ex-eternally as in case of ATE
[3].
BIST perform self-testing and reducing dependence on an external ATE. BIST is a Design-for-
Testability (DFT) technique makes the electrical testing of a chip easier, faster, more efficient and less costly.
The important to choose the proper LFSR architecture for achieving appropriate fault coverage and consume
less power. Every architecture consumes different power for same polynomial. Applications of LFSR: Pattern
generator, Low power testing, Data compression, and Pseudo Random Bit Sequences (PRBS).
2.1. Bist Architecture
A typical BIST architecture consists of
• TPG - Test Pattern Generator
• TRA – Test Response Analyzer
• Control Unit
As Shown in “Fig. 1”
“Fig.1 Test Pattern Generator In BIST”
It generates test pattern for CUT. It will be dedicated circuit or a micro processor. Pattern generated
may be pseudo random numbers or deterministic sequence. Here we are using a linear feedback shift register for
generating random number. The Architecture for LFSR is as shown in “Fig. 2”

“Fig.2 The Architecture for LFSR”
Tapping can be taken as we wish but as per taping change the LFSR output generate will change and as
we change in no of flip-flop the probability of repetition of random number will reduce. The initial value
loading to the LFSR is known as seed value.
B. Test Response Analyzer (TRA)
TRA will check the output of MISR and verify with the input of LFSR & give the result as error or not.
C. BIST control unit (BCU)
Control unit is used to control all the operations. Mainly control unit will do configuration of CUT in
test mode/Normal mode, feed seed value to LFSR, and control MISR & TRA. It will generate interrupt if an
error occurs. You can clear interrupt by interrupt clear signal.
D. Circuit under Test (CUT)
CUT is the circuit or chip in which we are going to apply BIST for testing stuck at zero or stuck at one
error.
III. Low Transition Pattern Generations
The technique of inserting 3 intermediate vectors is achieved by modifying the conventional LFSR
circuit with two additional levels of logic between the conventional flip flop outputs and the low power outputs
as shown in “Fig: 3” The first level of hierarchy from the top down includes logic circuit design for propagating
either the present or the next state of flip flop to the second level of hierarchy [4]. The second level of hierarchy
is a multiplexer function that provides for selecting between the two states (present or next) to be propagated to
the outputs as low power output. Minimal at best consisting of few logic gates.
“Fig.3 Low Power Test Pattern LFSR”
In the simulation environment the outputs of the flip-flops are loaded with the seed vector. The
feedback taps are selected pertinent to the characteristic polynomial xg
+x+1. Only 2 inputs pins namely test
enable and clock are required to activate the generation of the pattern as well as simulation of the design circuit.
It is also noteworthy here that the intermediate vectors in addition to aiding in reducing the number of
transitions can also empirically assist in detecting faults just as good as the conventional lsfr pattern [1][5]
description of the technique to produce low power pattern for bit the following is a description of a low power
test pattern generation technique as depicted in the 9 –bit lsfr. The feedback taps are designed for maximal
length lsfr generating all zeros and all one’s as well.

3.1 Algorithm For Low Power Lfsr
The first step is to generate it, the first vector by enabling (clocking) the first 4 bits of the LFSR and
disabling (not clocking) the last 4 bits .this shifts the first 4 bits to the right by one bit .the feedback bits of the
lsfr are the outputs of the 8th
and the first flip-flop.the output of the 8th
flip-flop is 1 and the output of the first
flip-flop is 0. The exclusive-or of the 8th
flip-flop (logic 1 in this case) and the first flip-flop (logic 0 in this case)
is input (1 exor 0 =1) into the first d flip flop the new pattern in the first four bits of the lsfr is 1010.note that the
shaded register is clocked along with the first 4 bits of the lsfr.
So the input of the shaded flip-flop is the output of the 4th
flip-flop which in this case is 0. Also note
that prior to the first clock, the input of the shaded register was the seed value of the 4 th
at the output of the 4 th
flip-flop which in this case is 0. So after the first clock this value of 0 will now appear at the output of the
shaded flip-flop. In other words the value of the 4 th
output is stored in this shaded register and is used in the next
few steps. The first 4 shifted bits of the LFSR and the last 4 Th
un-shifted bits (i.e. the seed value) are propagated
as ti (1010 1011) to the final outputs. Next few steps involve generating the 3 intermediate patterns from it.
These patterns are defined as Tk1, Tk2 and Tk3 shown in “Fig. 4.”below flow.Tk1 is generated by
maintaining (disabling the clock to the first 4 bits)the first four bits of the LFSR outputs(as is from it) as the
final first four low power outputs 1010.note that the clock to the last four bits of the I.fsr is also disabled.
The last four bits however are the outputs from the injector circuits. The injector circuit compares the
next value (the inputs of the D –flip-flop) with the current value (the output of the D flip-flop). According to Ti,
the outputs(current values) of the last 4 bits of the LFSR are 1011. The next values are the values at the inputs of
the D-flip-flops which in this case are 0101. Compare the current values (1011) bit by bit with the next values
(0101). if the values bit by bit are not the same then use the random generator feedback R (in this case is logic
1)as the bit value as shown in the schematic above. If however both values bit by bit are the same then
propagate that bit value to output as opposed to the R bit. This bit by bit comparison gives us the last four bits of
Tk1 to be 1111.therfore Tk1=10101111.next step is to generate Tk2.shift the last 4 flip-flops to the right one bit
but do not shift the first 4 flip –flops to the right. The clock to the first 4 bits the shaded flip-flop is disabled .the
clock to the last 4 bits is enabled. Propagate the outputs of the flip-flops of the entire LFSR as opposed to the
outputs of the injection circuits to the outputs (low power). The injection circuits are disabled .as in Tk1,
maintain the first four LFSR outputs (1010) as the low power outputs. Again from Tk1, the inputs of the last
four d flip-flops from the previous step (generating Tk1) are 0101.also note that the output of the shaded register
is 0 from the previous step (generating Tk1).therefore the input of the 5 th
flip-flop is a 0. The outputs of the last
4flip-flops are 0101resulting in Tk2=10100101.
The 3rd
intermediate vector Tk3 is generated via disabling the clock to the entire LFSR. Propagate the
first 4 outputs from the injection circuit as the first 4 low power outputs and maintain the last 4low power
outputs the same as Tk2. generating injection circuit outputs for Tk3 is conceptually the same as explained
above in generating Tk1current values (the outputs of the flip-flops) of the first four flip-flops are compared
with the next values (the inputs of the flip-flops) of the flip-flops therefore the logical feed forward value of r is
1. The feedback value from the first flip-flop is also 1as per the current values above. The exclusive or of two
ones is a 0. Therefore the input to the first flip-flop is a 0 which is also the next state of the first flip-flop. Hence
the next values are 0 for the first flip-flop and 101 for the 2nd
3 rd
and4 rd
flip-floprespectively.the next values are
0101.the first four outputs from the injection circuit are 1111. The last 4 outputs are the same as Tk2 which are
0101 resulting in the 3rd
and final intermediate vector Tk3=1111 0101.generating Ti2 is quite similar to
generating it. As in Tk3 the outputs of the last four LFSR flops are 0101.the outputs of the first 4 flip-flops of
the LFSR are the current values which are 1010.therefore the seed vector for generating Ti2 is 1010 0101.shift
the first four bits of the LFSR plus the shadedflip-flop.do not clock the last four flip-flops. Propagate the outputs
of the entire LFSR to the final low power outputs. The output of the 8th
flip-flop from the previous step
(generating Tk3) is a 1 and the output of the first flip-flop from the previous step(generating Tk3) is also a 1 .
The exclusive or of the output of the 8th
flip-flop and the first flip-flop is 0 .therefore the input to the
first flip-flop will be a 0.
The inputs to the 2nd
,3 rd
and 4th
the shaded flip-flops are 1010.these are also the current values from the
previous step (generating Tk3).shifting the first four flip-flops of the LFSR to the right by one bit results in 0101
as the outputs of the first four flip-flops. Therefore Ti2 generated is 0101 0101.

Fig.4 C17 Bench Mark Circuit
IV. Simulation Results
The design is verified by performing behavioral simulation in “Fig.5”,”Fig.6” and “Fig.7” to do a high
–level or behavioral simulator is used, which executes the design by interpreting the verilog code like any other
programming language, i.e. regardless of the target architecture .at this stage, FPGA development is much like
software development signals and variables may be watched procedures and functions may be traced and
breakpoints may be set. The entire process is very fast, as the design is not synthesized, thus giving the
developer a quick and complete understanding of the design. The downside of behavioral simulation is that
specific properties of the target architecture namely timing and resource usage are not covered.
“Fig.5 RTL Schematic of LT’LFSR.”
“Fig.6 Simulation Report of LT-LFSR with Fault”

“Fig.7 Simulation Report Of LT-LFSR Without Fault”
V. Synthesis Result
Synthesis is the process of translating verilog HDL to a net list ,which is built from a structure of
micros, e.g. adders, multiplexers and shift registers chip synthesizers perform optimizations especially hierarchy
flattening and optimization of combinational paths. Specific cores like RAMS or ROMS are treated as black
boxes. Recent tool can duplicate registers, perform re-timing or optimize this result according to given
constraints shown in “Fig. 9”, “Table 1” and “Table 2”
“Fig.8 Synthesis Results For LT-LFSR.”
Existing results Proposed result
Logic Utilization Used(Available) Used(Available)
Number of slice Flip-flops 251(3,840) 18(4,800)
Number of 4 input LUTs 12(3,840) 16(2,400)
Number of occupied slices 45(1,920) 9(600)
Number of slices
containing only related
logic
45(21) 9(20)
Number of slices
containing unrelated logic
0(21) 7(20)
Number of bounded IOBS 64(173) 14(102)
Power consumption 14 mw 12.19 mw
Fault coverage 100% 100%
“Table I. Synthesis And Par Report”

Circuit Frequency(
MHZ)
Total
power(
mw)
Dynamic
power(mw)
Quiescent
power(mw)
C17 139 12.19 0.96 11.23
“Table Ii. Power Analysis Report”
VI. Conclusion
The proposed approach shows the concept of reducing the transitions in the test pattern generated; the
transition is reduced by increasing the correlation between the successive bits. The simulation results shows that
how the patterns are generated for the applied seed vector. This paper presents the implementation with regard
to verilog language. Synthesizing and implementation (i.e. Translate, Map And Place And Route) of the code is
carried out on Xilinx- project navigator, ISE12.li suite .the power reports shows that the proposed low power
LFSR consumes less power (12.19mw) during testing by taking the benchmark circuit C17.in future there is a
chance to reduce the power somewhat more by doing modifications in the proposed architecture.
References
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Symposium, Vol.5, pp.5278 - 5281, 23-26 May 2005.
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[3] M.Aubertin,Member,IEEE, J. M. Pierre Langlois, Member, IEEE, Munshin,Member,IEEE “A New Approach to Design Fault
Coverage Circuit with Efficient Hardware Utilization for Testing Applications” IEEE transactions on very large scale integration
(vlsi) systems, vol. 21, no. 5, may 2013
[4] Mehrdad Nourani, Mohammad Tehranipoor, Nisar Ahmed,„„Low- Transition LFSR for BIST-Based Applications,‟‟14th Asian
Test Symposium, pp. 138- 143, 18-21 Dec. 2005.176
[5] M. ElShoukry, C. P. Ravikumar, and M. Tehranipoor, “Partial gating optimization for power reduction during test application,” in
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[6] N. Basturkmen, S. Reddy and I. Pomeranz, “A Low power Pseudo-Random BIST Technique,” in Proc. Int. Conf. on Computer
Design (ICCD‟02), pp. 468-473, 2002
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[8] S.Wang, “Generation of Low Power Dissipation and High Fault Coverage Patterns for Scan-Based BIST,” in Proc. Int. Test Conf.
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Vlsi Design of Low Transition Low Power Test Pattern Generator Using Fault Coverage Circuits