Fault Modeling of Combinational and Sequential Circuits at Register Transfer Level

International Journal of VLSI design & Communication Systems (VLSICS) Vol.2, No.4, December 2011
DOI : 10.5121/vlsic.2011.2406 61
FAULT MODELING OF COMBINATIONAL AND
SEQUENTIAL CIRCUITS AT REGISTER TRANSFER
LEVEL
M.S.Suma1
and K.S.Gurumurthy2
1
Department of Electronics and Communication Engineering, R.V.College of
Engineering, Bangalore, India
ms_suma@yahoo.com
2
Department of Electronics and Communication Engineering, U.V.College of
Engineering, Bangalore, India
ABSTRACT
As the complexity of Very Large Scale Integration (VLSI) is growing, testing becomes tedious and
tougher. As of now fault models are used to test digital circuits at the gate level or below that level. By
using fault models at the lower levels, testing becomes cumbersome and will lead to delays in the design
cycle. In addition, developments in deep submicron technology provide an opening to new defects. We must
develop efficient fault detection and location methods in order to reduce manufacturing costs and time to
market. Thus there is a need to look for a new approach of testing the circuits at higher levels to speed up
the design cycle. This paper proposes on Register Transfer Level (RTL) modeling for digital circuits and
computing the fault coverage. The result obtained through this work establishes that the fault coverage
with the RTL fault model is comparable to the gate level fault coverage.
KEYWORDS
Automatic test pattern generation (ATPG), fault coverage, fault simulation, stuck-at fault, RTL.
1. INTRODUCTION
VLSI industry is growing as per Moore’s law and integrated circuit designs are accordingly
becoming more and more complex. As a result of this, VLSI testing has become expensive in
terms of cost. Existing gate level fault simulation techniques exhibit poor performance standards
when applied to such designs and are unsuitable for early testability analysis or fault simulations.
Current computer-aided design tools must address the needs for a new generation of integrated
circuits such as systems on chip. Test sequences consist of many thousands of test patterns, which
make gate-level fault simulation inappropriate due to its lengthy computational time. Also test
generation and fault simulation efforts in the post synthesis phase do not contribute to the
improvement in the design. Therefore, we need ATPG tools that reflect new design flows,
especially tools that work at a higher level of abstraction than gate-level.
Many high-level fault models and fault simulation techniques have been proposed. No single
fault model is universally acceptable since no fault model has been developed so far that
comprehensively covers all classes of circuits. The RTL description is at a higher level of

62
abstraction and may not cover all the gate level faults [2].To be widely accepted an RT-level fault
simulator must accept input formats in standard hardware description languages such as VHDL or
Verilog and should show high efficiency. Fault simulation plays a key role in ATPG systems;
including high-level ATPG. Gate-level fault simulation is not appropriate for large systems
because of long runtimes or large memory requirements. RT-level fault simulation may be the
only alternative for estimating the quality of tests generated using high-level ATPG [3]. A high-
level fault model should guarantee fault coverage comparable to the gate-level fault coverage
obtained for the same test sequence.
The fault model proposed by F.Corno, G.Cumani, M.Sonza Reorda and G.Squillero [2] adopts a
particular instantiation of the observability enhanced statement coverage metric in addition to the
single stuck-at bit faults on all assignments targets of the executed statements. The model implies
observability enhanced statement coverage by modeling one of the possible fault classes on
executed statements. This is an incomplete modeling of the various faults associated with the
RTL description of the circuit.
The fault model by Barry W. Johnson is developed via abstraction of industry standard single-
stuck-line (SSL) faults into the behavioral domain. A functional analysis technique was used to
evaluate the effects of the SSL faults on gate-level implementation. Since the gate-level netlist
changes drastically during logic synthesis, the authors in [4] concluded that modeling all possible
gate-level faults at the RTL is highly inefficient.
The RTL fault model and simulation approach proposed by Mao and Gulati [5] uses the single
stuck-at fault for each bit of all variables in the RTL model. The model employs both the RTL
description and functional verification patterns. But their approach required one to run fault
simulation twice, first in an optimistic mode and then in the pessimistic mode and to use the
average of the results to reduce the difference between the RTL and the gate-level fault coverage.
The experimental data shows as much as 10 % error between the actual gate-level fault coverage
and the RTL fault coverage.
Another fault model proposed by Devadas and Ghosh [6] is the Observability Enhanced
Statement Coverage Metric. This model requires that all statements in the RTL description are
executed at least once and that their effects are propagated to at least one primary output. As this
approach can be fruitfully exploited for the test pattern for fault simulation, more accurate results
are needed.
The fault model proposed by Karunaratne et al. [7] does not consider stuck-at faults in the signal
bit values and also not account for these faults. Also the process of locating the RTL faults and
mapping them to the corresponding Gate-Level faults is to be done. It is therefore desirable to
develop the fault model at a higher level of abstraction than the gate level. Fault simulation and
testing at the higher levels of abstraction have a better chance of being integrated well into the
overall design process.
Jose M.Fernandes et al. [8] has proposed a new probabilistic method for controllability evaluation
based on a traitorously selection of registers to form groups. This work needs further optimization
by computing the probabilistic impact of the simultaneous correction of different testability
problems.
Digital circuits are commonly designed at multiple levels of abstraction, including the layout,
transistor, gate, register-transfer (RTL) and behavioral levels. Designers describe circuits in a
hierarchical, top-down fashion, typically using computer-aided design (CAD) tools. To simplify
the design process, designers try to model circuits at a fairly abstract level. Conventional gate-

63
level implementation is hard to understand, i.e., poor readability. By modeling circuits at a higher
level, the number of primitive elements in a circuit is reduced, thus making the problem size more
tractable. This allows larger circuits to be handled in less time. The authors [9] conclude that over
1000 times reduction in test-generation time is achievable by performing automatic test pattern
generation (ATPG) at the RTL without any compromise in fault coverage.
2. METHODOLOGY
In this work Verilog Hardware Description Language is used for writing the RTL models.
Although extensive work has been done on Verilog based simulation and synthesis, test
generation and other test related issues are still to explore the capabilities of Verilog. The basic
assumption is that the components are fault free and only their interconnections are affected.
These map to the operators and variables in the RTL descriptions respectively. Gate level
primitives can be instantiated in a model using gate instantiation as these are supported for
synthesis. These primitive gates describe the hardware. Therefore synthesizing a gate primitive
generates logic based on the gate behavior which eventually gets mapped to the target technology
[1].Based on this the single stuck-at fault is modeled. The assumption is also that at most one
fault occurs at a time in the circuit.
The proposed fault model is an improvement over the model given by Karunaratne et al.
[6].Stuck-at faults in the signal bit values was not considered and accounted. Also the process of
locating the RTL faults and mapping them to the corresponding Gate-Level faults was not
implemented.
Figure 1: Design flow with the proposed method

64
The analysis flow for the modeling approach is of two ways as shown in Figure 1. One way
targets on the gate-level fault coverage while the other is on the RTL fault coverage. In the RTL
path, the RTL design description is obtained based on the specification. Since the fault model is at
the RTL, the fault is induced at the input and at the output. This is done by using a buffer for each
bit in all of the variables in the RTL code. These buffers are inserted in the fault free circuit and
should not disturb the functionality of the circuit. As a result, a modified faulty RTL circuit is
obtained. To enable fault simulation the process of generating faulty circuits by inducing faults
into the fault-free circuit is done. For each of the faults a new circuit is created.
Testbench is developed and the simulation is first run on a good circuit and then on each of the
faulty circuits using the commercial simulator. The outputs obtained in each case of the faulty
circuits are compared with the output of the good circuit to determine which faults are detected.
That is the new faulty circuit and the fault free circuit is simulated and the outputs so obtained are
compared. The fault list is tabulated. The ratio of the numbers of RTL faults detected to the total
number of RTL faults gives the RTL fault coverage. At the gate-level, for each RTL description,
gate level netlists are obtained for 65 nanometer target technology using logic synthesis tool and
fault coverage obtained by Tetramax tool. The fault list of both the RTL as well as Gate-level
faults is compared. The effectiveness of our fault model is determined by comparing RTL fault
coverage with the fault coverage obtained at the gate level.
3. RTL FAULT MODEL AND SIMULATION
Test generation plays an important role in the area of digital design. Test generation is a process
of finding input test patterns for detecting possible faults in the circuit. It is difficult to generate
test for real defects due to the diversity of VLSI defects. For generating and evaluating a set of
test patterns, fault models are needed. Widely a good fault model should almost give a true nature
of the behavior of defects and it should also computationally work well in terms of fault
simulation and test pattern generation. It is necessary to propose a fault model, that is a fault
model for how faults occur and their impact on circuits and to do with the business of good and
bad parts, many fault models have been proposed [4], but unfortunately, no single fault model
accurately reflects the behaviour of all possible defects that can occur. As a result, a combination
of different fault models at many instances are used in the generation and evaluation of test
vectors and testing approaches developed for VLSI devices [2]. Developing a test for faults at
higher level of abstraction and then determining the percentage of faults at the lower levels being
covered is a good strategy. Fault models at higher levels result in significant savings in test cost
and test time required for deriving tests.
The most common model used for logical fault is the single stuck-at fault (SSF). In this a fault in
a logic gate gives a favorable outcome in one of its inputs or the output being fixed to either a
logic 0(stuck-at-0) or a logic 1(stuck-at-1).
For our approach 2 to 1 multiplexer is taken as an example.
module mux_if(a,b,sel,z);
input sel;
input a, b;
output z;
reg z;
always @ (a or b or sel)
if (sel)
z=a;
else
z=b;

65
endmodule
The above RTL description is a fault free module. Faulty module is created such that the
functionality will remain same as the fault free module .This is done by inserting the buffer for
each of the ports.
The faulty module appears as seen below.
timescale 1ns/1ps
module mux_iffaulty(a,b,sel, z);
input a,b,sel;
output z;
wire a_fault, b_fault, sel_fault;
reg z_fault;
buf A1 (a_fault,a);
buf B1 (b_fault,b);
buf S1 (sel_fault, sel);
buf Z1 (z, z_fault);
/*always @ (a_fault,b_fault,sel_fault)
begin
if(sel_fault)
z_fault= a_fault;
else
z_fault=b_fault;
end */
/* sel stuck at 1
always @ (a_fault,b_fault,sel_fault)
begin
if(1'b1)
z_fault= a_fault; // test pattern (sel, a, b) :
001 ,010
else
z_fault=b_fault;
end */
/* a stuck at 1
begin
if(sel_fault)
z_fault= 1'b1;
else // test pattern 100
z_fault=b_fault;
end
*/
// output stuck at 1
begin
if(sel_fault)
begin
z_fault= a_fault;
z_fault=1'b1;
end
else // test pattern 100,000 (X00)
begin
z_fault=b_fault;
z_fault=1'b1;
end
end
endmodule

66
To these faulty and fault free modules fault simulation is performed with the reduced number of
test patterns for each of the faults. The outputs obtained in each case of the faulty circuits are
compared with the output of the good circuit to determine which faults are detected, finally to
obtain the fault coverage. A single pattern can detect many faults or a single fault. Many patterns
can detect many faults or a single fault. The challenge in testing is to obtain a minimal number of
test patterns which guarantees high fault coverage of a circuit with known set of faults. The
simulated waveform for the output signal remainder stuck at ‘1’ is as shown in Figure. 2.
Figure 2: Simulated waveform for the output signal stuck at ‘1’
4. RESULTS
At the writing of this paper, we have tested our approach on combinational logic circuits and
sequential circuits. The results obtained by applying our approaches to the RTL design
descriptions and their corresponding Gate-level descriptions have been tabulated in table 1. At the
gate-level, the gate-level netlist is created for each of the circuit used. Fault coverage is obtained
for the scan inserted gate-level netlists. From the results it can be observed that the RTL Fault
Coverage obtained by the proposed fault modeling methodology has a close match to the Gate-
Level Fault Coverage for the tested digital circuits.
Table 1. RTL versus Gate-Level Fault Coverage

67
5. CONCLUSIONS
With the progress of semiconductor technology testing of VLSI circuits becomes more and more
difficult and at the same time cost is also increasing. Therefore it is important to achieve high
fault efficiency with low cost. With this approach RTL designer can have an estimation of the
achieved fault coverage before doing synthesis and also it is possible for the designer to locate
faults at a higher level of abstraction. At present our approach is applied to combinational logic
circuits and few sequential logic circuits. Further we would like to extend the approach to
complex sequential circuits such that there is a close match to the gate level fault coverage and
hence reducing the impact on time to market.
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Environment”, IEEE VLSI Test Symposium.
[5] Weiwei Mao, Ravi K Gulati,( 1996)”Improving Gate Level Fault Coverage by RTL Fault Grading”,
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[14] Yiwen Shi,Wan-Chan Hu and Jennifer Sworak,(2010) “Too Many Faults,Too Little Time On
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Technology

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