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ASIC IMPLEMENTATION OF DIGITAL FILTER DESIGN

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					ASIC IMPLEMENTATION OF DIGITAL
   PULSE SHAPING FIR FILTER



                           By,

                 C.Radhika (09VL03F)
               N.M.Yeshoda (09VL28F)
              Prajakta Panse (09VLF01)


                 Under the guidance of
                 Mr.RAMESH KINI.M
                  Associate Professor.




DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
 NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA, SURATHKAL
           P.O SRINIVASNAGAR, MANGALORE-575025.




                            1
                    Acknowledgment

We would like to take this opportunity to express our deep sense of gratitude
to Mr.Ramesh Kini M.(Associate Professor, E & C, N.I.T.K, Surathkal), our
guide for his help through provoking discussion, invigorating suggestions ex-
tended to us with immense care, zeal to our work.

   We offer our sincere thanks to Dr. Sumam David S.(Head of the Depart-
ment, E & C, N.I.T.K, Surathkal) for providing necessary facilities, valuable
suggestions and support through our study.

    We express our sincere thanks to Mr.Guru Tilak(Lab Assistant), for help-
ing us throughout the course of the Project.




                                     2
                                 Abstract

    In digital telecommunication, pulse shaping is the process of changing
the waveform of transmitted pulses. Transmitting a signal at high modula-
tion rate through a band-limited channel can create inter symbol interference.
Therefore the purpose of pulse shaping filter is to make the transmitted signal
suit better to the communication channel by limiting the effective bandwidth
of the transmission.

    As the fundamental principle of operation is inferred from properties of
FIR filter, pulse shaping filter come with linear phase and inherent stability.
These properties of pulse shaping filter makes it apt for ASIC implementa-
tion. By increasing the order of the filter, smoothness in the intended task
can be achieved to a great level but at the cost of area.Through this project
an attempt is made to minimize area while keeping throughput to an ad-
missible value. The implementation of pulse shaping filter is done in digital
domain using 0.18 micron technology.

    To achieve reduction in the cost and complexity of testing , Build In Self
Test (BIST) is incorporated onto the chip. BIST is designed to test more
structures in parallel thereby reducing the testing duration while providing
good fault coverage.
Contents

1 Introduction                                                           3

2 ASIC Implementation                                                     6
  2.1 Front End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
  2.2 Back End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Results                                                               13




                                    1
List of Figures

 1.1 Zero ISI property of pulse shaping filter . . . . . . . . . . . . .                 4
 1.2 Block diagram of fir filter . . . . . . . . . . . . . . . . . . . . .                5

 2.1   Symmetry of filter coefficients . . . . . . . . . . . .     .   .   .   .   .   .   7
 2.2   The architecture of FIR filter . . . . . . . . . . . .    .   .   .   .   .   .   8
 2.3   Block diagram of control logic . . . . . . . . . . . .   .   .   .   .   .   .   9
 2.4   Combinational Logic for selecting appropriate signal     .   .   .   .   .   .   9

 3.1   Output of FIR filter for rectangular pulse of width= 2 clk cycle                  13
 3.2   Output of FIR in testing mode for boundary scanning . . . . .                    14
 3.3   Output of FIR in testing mode detecting B-S-A-0 . . . . . . .                    14
 3.4   Layout of Control Logic . . . . . . . . . . . . . . . . . . . . .                15
 3.5   Layout of FIR logic . . . . . . . . . . . . . . . . . . . . . . . .              15
 3.6   Layout of Top module of FIR filter . . . . . . . . . . . . . . .                  16
 3.7   Comparison of ideal and practical FIR filter’s frequency do-
       main behavioral . . . . . . . . . . . . . . . . . . . . . . . . . .              16




                                     2
Chapter 1

Introduction

Data transmission systems that must operate in a bandwidth-limited envi-
ronment must contend with the fact that constraining the bandwidth of the
transmitted signal necessarily increases the likelihood of a decoding error at
the receiver. Bandwidth limited systems often employ pulse-shaping tech-
niques that allow for bandwidth containment while minimizing the likelihood
of errors at the receiver.
    Generally the digital transmission will be rectangular pulses, whose spec-
trum spans infinite frequency. In many data transmission applications, the
transmitted signal must be restricted to a certain bandwidth due to either
system design constraints or government regulation. In such instances, the
infinite bandwidth associated with a rectangular pulse is not acceptable. The
bandwidth of the rectangular pulse can be limited, however, by forcing it to
pass through a low-pass filter.The act of filtering the pulse not only causes its
shape to change from purely rectangular to a smooth contour without sharp
edges in first symbol period, but it is also spread out over the subsequent
symbol periods. This results into Inter symbol interference(ISI) making sys-
tem less reliable. Hence for doing this pulse shaping, special types of pulse
shaping filters are preferred rather than simple low pass filters.The pulse
shaping filter is a FIR filter, which inherits all properties of FIR filter. The
LTI model of FIR filter can be represented by the difference equation as
                                   N −1
                          y[n] =          h[k] ∗ x[n − k]                 (1.1)
                                   k=0

Where x[n]: current input sample

       x[n-k]: discrete input samples delayed by k sample periods

       h[k]: filter coefficients or taps

                                           3
       y[n] : current output discrete sample

     The impulse response of pulse shaping filters is short enough that very
little energy from one symbol smears into the next symbol.Thereby it does
not cause ISI. Figure 1.1 shows pulse shaping filter’s zero ISI property.




            Figure 1.1: Zero ISI property of pulse shaping filter


    Pulse-shaping filters that are commonly found in communication systems
are:

   • The trivial boxcar filter

   • Sinc shaped filter

   • Raised-cosine filter

   • Gaussian filter

Raised-cosine filter is practical to implement and it is in wide use. The
complex form of raised cosine is the square root raised cosine filter, which
offer s better spectral efficiency. The impulse response of square root raised
cosine pulse shaping FIR filter is given as

                       sin(π Ttc (1 − α)) + 4α Ttc cos(π Ttc (1 + α))
             RC(t) =                                                    (1.2)
                                    π Ttc (1 − (4α Ttc )2 )




                                        4
Where

    α: roll-off factor

    Tc: Chip duration

   The roll off rate and chip duration decide the performance metric of
the FIR filter. With the typical α=0.22 and Tc=0.2674 microseconds filter
coefficients are even symmetric.
   Pulse shaping filter is basically a FIR filter. Hardware realization of it can
be represented as basic Multiplier and Accumulator (MAC) unit as shown in
the Figure 1.2
.




                    Figure 1.2: Block diagram of fir filter


    As the order of the filter increases, the accuracy with which it smoothens
the wave contour becomes superior. But at the same time number of multi-
pliers and adders required increases linearly. Specially higher the number of
multipliers higher is the chip area and thus higher number of multipliers is a
threat to the popularity of pulse shaping filter.
    In this project, implementation of the pulse shaping FIR filter is done
with an aim to reduce the chip area by limiting number of multipliers. In
order to make filter design fault tolerent BIST is incorporated in the chip.
BIST is a Design-for-Testability (DFT) technique, because it makes the elec-
trical testing of a chip easier, faster, more efficient,less costly and reduces
dependence on an external automated test equipment (ATE).In the project,
parallel testing of scan design and combinational design is done to reduce
number of clock cycles required for testing. The functionality of the system
is verified using XILINX and the backend design of the system is done using
CADENCE.


                                      5
Chapter 2

ASIC Implementation

The preliminary concept of pulse shaping filter was validated in MATLAB
to extract filter coefficients with the filter specifications as shown in Table 2.1.


                        Table 2.1: filter specifications


        Parameter                              Design values
        Symbol rate                          3.84 Msymbols/sec
     Cut-off frequency                         Cut-off frequency
    Sampling frequency         Interpolation factor ∗ symbol rate= 15.36MHz
       Chip duration                           0.2604 micro sec
 Stop band attenuation(dB)                           -52


    As fixed point arithmetic is much faster than floating point arithmetic,
coefficients were quantized to integer form. After quantization, performance
of FIR filters was compared with non-quantized filter in time and frequency
domain. It has been observed that the acceptable performance in terms of
stop band attenuation and pass band ripple is obtained with the filter order
of 32. Hence the order of filter is chosen as 32 and coefficients have been
extracted. For further details refer Appendix.
    After extracting filter coefficients, hardware was described in behavioral
VHDL using ISE simulator to ensure that the code is independent of any
target architecture, therefore can be synthesized using any libraries. The
simulated VHDL code was synthesized using Synopsys Design Micro vision
Analyzer (Design Compiler) using VTVT TSMC 180 standard cell digital
libraries. The Verilog netlist was imported into CADENCE Silicon Ensemble

                                      6
where floor planning, place and route were carried out. The system design
is done in two stages namely front end and back end.


2.1     Front End
Pulse shaping FIR filter is basically a MAC unit with memory. The simplest
FIR filter implementation requires number of multipliers equal to the order
of the filter. Therefore with the order of 32, filter implementation requires 32
multipliers thereby consuming lot of chip area. So the design has been done
with the goal of area optimization maintaining acceptable speed of operation.
The property of even symmetry of the filter coefficients has been exploited
to achieve the goal as shown in the 2.1.

                 Figure 2.1: Symmetry of filter coefficients




    In order to achieve area optimization fundamental block of 16-bit FIR
filter was designed and utilization of multipliers is done over two system
clock cycles. Input is presumed 8-bit unsigned number. Coefficients are
shifted by 7 bits to left (multiplication by 127d ) in order to achieve higher
precision. For filter coefficients signed-magnitude representation is used.
    The system is broadly divided into three subunits namely,
   • Control Logic

                                      7
   • Fir Logic
   • Boundary Scanner


                  Figure 2.2: The architecture of FIR filter




    The architecture is shown in the figure 2.2. Cont logic performs the func-
tion of routing appropriate data sequence to fir logic. In addition to that
it has all components of BIST like test vector gen, test mode gen, reset gen
etc. The fir logic block contains main 16-tap FIR filter, adder and latch to
get final convolution sum. Boundary scanner unit is included as an add-on,
to test input and output pins.
    The figure 2.3 shows components of cont logic. In cont logic, system clock
is divided using T-Flip-flop. This signal is used to control the data given to 16
bit FIR filter. AND-OR combinational logic is used as multiplexer to route
appropriate (Xout /Data) signal to dout pin. Output of the T-Flip-flop is
also given to the pin n.
    Built in self unit (BIST) is integrated within cont logic to endow with
testability measures. Design is optimized to trim down area and time over-
head. Testing of combinational and sequential design is done separately and
simultaneously. Test mode can be enabled by setting Test/normal pin of
the input to high. Fault collapsing and fault dominance has been exploited
to find out minimum number of test vectors for approximately 95% fault
coverage.

    For fault collapsing and fault equivalence logically equivalent model of
combinational logic (figure 2.4) is used. From this circuit test vectors needed
to find out all faults are found. These test vectors are listed in table 2.2

                                       8
                 Figure 2.3: Block diagram of control logic




      Figure 2.4: Combinational Logic for selecting appropriate signal




    This necessitates the use of 6 clock cycles for testing entire circuit. In
the test mode, address generator unit is activated to generate address for
extracting test vectors. Test vectors along with expected output are saved in
ROM. In each clock cycle one vector is retrieved and applied to combinational
logic and second test vector is given out as dout signal for testing sequential
logic. Output is compared with expected result using XOR gate. Output of
XOR gate is used as feedback to address generator. Thus as soon as fault is
detected test enable signal goes low disabling address generator. This puts
into practice an idea of fault dropping and reduced power consumption. At
the end of the test mode, whatever data stored in sequential circuit need
to be flushed out. Thus reset gen unit is used to generate monoshot when
test mode pin makes high to low transition. This signal is inturn given out


                                      9
         Table 2.2: Fault equivalence and summery of test vectors


   Test vector        All Equivalent Faults         Expected       Faulty
     (abcd)b                                         output        output

       0101          a-sa1,c-sa1,e-sa1,fsa1,g-sa1        0             1


       0011             c-sa0,d-sa0,f-sa0,g-sa0          1             0


       1110               a-sa0,b-sa0,e-sa0              1             0


       1010                  b-sa1,d-sa1                 0             1




as reset out signal.
    Fir logic contains 16 bit FIR filter unit employing 32 registers of size 8
bit, 16 multipliers of 8∗8 and 32 adders/ subtracters. Additional pin ’n’ is
provided which is connected to single bit signal and can have value either
’0’ or ’1’. As per value on this pin traversing path for coefficient array is
selected. During first clock cycle ’n’ goes to ’0’ thereby outputs partial con-
volution sum. In second clock cycle ’n’ goes to ’1’ and outputs second partial
convolution sum. Latch is used at the output to hold first partial convolution
sum for one clock cycle. So at the end of second clock cycle adder adds up
both partial convolution sums to give final output. This output has been
latched again and fed to divider. Divider unit divides the result by 7 bit
while preserving sign of the output. So final output is 12 bit signal in signed-
magnitude with MSB representing sign.

    In test mode, scan chain of flip-flops is formed. This chain is configured
in such a way that multiplexer is included for every alternate flip-flop in scan
chain. Thus in 4 clock cycles all vectors will be out. This output data of scan
chain is compared with test vector so as to detect any fault if present.This
signal is then ORed with input signal sd to generate SCAN OUT signal. At
the end of the test mode, reset signal goes high and clears all flip-flops in
scan chain thereby reducing 4 clock cycles.
    In boundary scan mode, both internal modules are disconnected from


                                      10
data input pins and input pins are directly connected to output pins to
verify I/O functionality.


2.2     Back End
The synthesis process begins by converting the vhdl description of the logic
into a netlist using generic library gates. The important input to the syn-
thesis tool is the timing constraint .tcl file. The timing constraints have
been described in the Synopsys Design Constraint format. Here the clock,
the latency of the clock, the input and output delays is provided as inputs.
Once the constraints have been set, the code is compiled with appropriate
mapping effort ensuring no area, delay constraints violation. Then the post
synthesized net list is generated. In addition to the actual synthesized gate
level net list, it also generates several constraint reports as


   • Area

   • Power

   • Delay

***********************************************************
Report: area
Design: fir 32 ru
***********************************************************

 Number of ports:          24
 Number of nets:           189
 Number of cells:          32
 Number of references:     29
 Combinational area:       509625.406250
 Noncombinational area:    75118.203125
 Total cell area:          584710.18750

   This report gives the rough estimate of area (excluding routing area) re-
quired to do the back end design. Similarly the power analysis report gives
the amount of power dissipated, from which an estimate of driving current
can be obtained to decide the power ring metal width in the back end design.



                                     11
*********************************************************
Report: power-analysis effort medium
Design: fir 32 ru
*********************************************************
 Operating Conditions: nom pvt
 Library: vtvt tsmc180

 Global Operating Voltage          1.8V
 Power-specific unit information:
 Voltage Units                     1V
 Capacitance Units                 1.000000ff
 Time Units                        1ps
 Dynamic Power Units               1mW (derived from V,C,T units)
 Leakage Power Units               1mW
 Cell Internal Power               5.4245 mW (66%)
 Net Switching Power               2.7922 mW (34%)
 Total Dynamic Power               8.2167 mW (100%)
 Cell Leakage Power                1.2977 micro W



The post synthesized net list from Synopsys is used for back end layout
design. The tool used for placement and routing is SOC ENCOUNTER.
   In layout design modular approach of coding is enforced to the fullest by
doing separate layout design for cont logic and fir logic. These designs have
been saved as .lefs and imported to top module. Table 2.3 gives summery of
backend performance metric got from SYNOPSYS AND CADENCE SOC.

           Table 2.3: Summary of backend performance metric

                      Parameter           Design values
                     Supply voltage           1.8V
                    Power Dissipation      8.2167mW
                       Core Area            1 sq.mm
                    Clock Frequency          125KHz
                      Gate Density             9K




                                    12
Chapter 3

Results

The functionality of the 32 tap fir filter is verified using XILINX ISE Simu-
lator.The design is tested for different data vectors namely

   • Delta input

   • Step input

   • Rectangular pulse

   Figure 3.1 shows results of convolution of rectangular input [FF FF]h of
width equal to 2 clock cycles with filter coefficients.




Figure 3.1: Output of FIR filter for rectangular pulse of width= 2 clk cycle


   As chip is designed for good testability, on chip BIST is incorporated.
This testing is done in two phases namely

   • Boundary Scan


                                    13
   • Single stuck at faults and Scan chain in shift registers mode

  Figure 3.2 shows result for boundary scanning. In this mode of testing,
whatever data fed at the input reflects at the output. In second phase of




     Figure 3.2: Output of FIR in testing mode for boundary scanning


testing, design is tested for single stuck at fault. Figure 3.3 shows result of
B-S-A-0.




       Figure 3.3: Output of FIR in testing mode detecting B-S-A-0


   After verifying functionality in XILINX, synthesis was done using SYN-
OPSYS and netlist has been extracted. This netlist is imported to CA-
DENCE SOC ENCOUNTER for Backend design.
   As the entire design is done in modular form, first banckend design of
cont logic and fir logic is done. Layout of cont logic is as shown in figure 3.4
while layout of fir logic is shown in the 3.5.



                                      14
                    Figure 3.4: Layout of Control Logic




                      Figure 3.5: Layout of FIR logic

   In the design of top module along with technology lef file, lef for these
two base modules are also imported.After importing lef files carefully place-
ment of the two blocks is done for optimum utilization of area. Layout top
module obtained is shown in the figure 3.6

    Output of FIR filter is rechecked for precision by using MATLAB.The
result is compared by plotting Fast fourier transform (FFT) of ideal and
actual characteristics as shown in Figure 3.7




                                    15
              Figure 3.6: Layout of Top module of FIR filter




Figure 3.7: Comparison of ideal and practical FIR filter’s frequency domain
behavioral



                                   16
Conclusion

   The concept of even symmetric pulse shaping FIR filter was substantiated
through the project. It is an attempt to optimize the design for area by
minimizing the number of multipliers. On-chip testability is adjoined for
good fault coverage while keeping area and delay overhead optimum. The
precision of the filter is retained to an acceptable level.




                                   17
Appendix

The filter coefficients after truncating the decimal part of the coefficients
obtained from MATLAB are

                      Table 3.1: Filter Coefficients

              Sampling instance     Value of coefficient
                     1                       -1
                     2                       1
                     3                       0
                     4                       -2
                     5                       -1
                     6                       3
                     7                       1
                     8                       -5
                     9                       -6
                     10                      3
                     11                     12
                     12                      2
                     13                     -22
                     14                     -22
                     15                     28
                     16                     100


   The above 16 coefficients are symmetric w.r.t the zeroth sampling in-
stance.




                                  18
Bibliography

[1] R. Veljanovski, J. Singh and M Faulkner., ”ASIC And DSP Implementa-
    tion Of Channel Filter For 3G Wireless TDD System”

[2] Ken Gentile., ”Digital Pulse-Shaping Filter Basics”

[3] M. L. Bushnel and V. D. Agarwal, ”Essentials of Testing for Digital,
    Memory and Mixed-Signal VLSI Circuits” Boston, 2000.




                                    19

				
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