Lab 3: Timing Constraint Specification

ECSE 325 – Digital Systems

Lab 3: Timing Constraint Specification and Timing Analysis using TimeQuest
Overview
In this lab, you will learn how to specify timing constraints and perform static timing analysis of
the synthesized circuit using the TimeQuest timing analyzer. You will also learn two techniques
to reach timing closure for a time-critical circuit.
Digital Filters
Digital filters are a very important digital signal processing (DSP) primitives. Digital filters have
two uses: signal separation and signal restoration.
• Signal separation is needed when a signal has been affected by interference, noise, or other
signals. For example, imagine a device for measuring the electrical activity of a baby’s heart
while still in the womb. The raw signal will likely be corrupted by the breathing and heartbeat
of the mother. A filter can separate these signals so that they can be individually analyzed.
• Signal restoration is used when a signal has been distorted in some way. For example, an
audio recording made with poor equipment may be filtered to better represent the sound
as it actually occurred. Another example is the deblurring of an image acquired with an
improperly focused lens, or a shaky camera.
FIR Filters
The most straightforward way to implement a digital filter is by convolving the input signal with
the digital filter’s impulse response. Filters carried out by convolution are called Finite Impulse
Response (FIR) filters. An FIR filter is a filter that provides a finite-period response to any finite
length input. For a causal discrete-time FIR filter of order N, each value of the output sequence
is a weighted sum of the most recent input values:
�(�) = &�! ∗ �(� − �)
"
!#$
where x(n) and y(n) are input and output signals, respectively. The weights are denoted by bi .
D D D D D D
D
b0 b1 b2 bN-2 bN-1
x
y
Root Square Mean Error
Root Mean Square Error (RMSE) is the standard deviation of the residuals (prediction errors).
Residuals are a measure of how far from the regression line data points are; RMSE is a measure
of how spread out these residuals are. In other words, it tells you how concentrated the data is
around the line of best fit. Root mean square error is commonly used in signal processing.
���� = 0∑ (�2! − �!) "&' %
!#$
�
Where
• �2! is the estimated value
• yi is the actual value
• N is the number of samples
Implementation of FIR Filters
In this lab, you will implement a bandpass FIR filter with order 25 (25-tap FIR filter) to restore a
sine wave corrupted by white noise. You are provided with the text files containing the filter
inputs and weights in floating-point format. Given the block diagram of an N-tap filter above,
implement the 25-tap FIR filter in VHDL while representing the filter input, output signals and
weights in the fixed-point representations (1,15), (2,15) and (1,15), respectively.
Note: Use constant parameters to represent the weights.
entity gNN_FIR is
port( x : in std_logic_vector (15 downto 0); -- input Signal
 clk : in std_logic; -- clock
 rst : in std_logic; -- asynchronous active-high reset
 y : in std_logic_vector (16 downto 0) -- output signal
 );
end gNN_FIR;
FIR Filters - Simulation
Once you have implemented your design in VHDL, you need to verify its functionality by writing
a testbench code. In the testbench code, you will read the given test vector in your testbench
and obtain an output test vector. The obtained output signal must match with the given output
signal.
When you design and validate the VHDL codes, show them to the TA. 
Using the TimeQuest Timing Analysis
To ensure a properly working circuit, the designer must take into consideration various timing
constraints. In class we saw that for a register to correctly store an input value, the input must
be held stable for a period (called the setup time) before the clock edge, and also for a period
(called the hold time) after the clock edge.
Whether a circuit meets these timing constraints can only be known after the circuit is
synthesized. After synthesis is done, one can analyze the circuit to see if the timing constraints
(setup and hold times) are satisfied using the term slack. Slack is the margin by which a timing
requirement is met or not met. It is the difference between the required time and the arrival
time. A positive slack value indicates the margin by which a requirement was met. A negative
slack value indicates the margin by which a requirement was not met.
In Quartus II, timing analysis is done using the TimeQuest Timing Analyzer. Please read pages 7-
6 through 7-12 of the document ”Altera TimeQuest Timing Analyzer.pdf” (we put a copy in the
Documentation folder on MyCourses) before proceeding to the next part of the lab.
From the ”File/New” menu item, select ”Synopsys Design Constraints File”. This ”.sdc” file is
where we can specify various timing constraints for our design.
We will add a single constraint specifying the clock period. To add the constraint, type the
following text into the window, and save the file with the name ”gNN testbed.sdc” where NN is
your group number. The constraint below tells the timing analyzer that the clock period is 20 ns.
Recompile your design. The Timing Analyzer will read the .sdc file and use the constraint
information when doing its analysis.
After the compilation is finished, look at the timing summaries for the Slow Model and the Fast
Model. Pay attention to the Fmax Summary (which gives the maximum clock speed) and the
Setup and Hold summaries (which give the slack amounts for the setup and hold constraints).
Find the maximum achievable frequency of your design and show it to the TA. Does your design
meet the specified timing constraint (i.e., the clock period of 20 ns)?
Timing Violation
In case of any timing violation, Quartus recommends solutions for each violated path in your
design. Denote violated paths in your design and show them to the TA.
Click on the ”Report recommendations for this path” to open up TimeQuest Timing Analyzer and
get more information on the violated paths. Under the ”Recommendation Summary”, you can
find possible solutions to fix the timing violations.
Now, you apply learn two common solutions to resolve the timing issues of the violated paths.
• Reduce levels of combinational logic for the violated paths
• Redesign architecture and rearrange registers
Reducing Combinational Logic Levels
One way to reduce the levels of combinational logic of the violated paths is to reduce the bitwidth
of signals. Use any programming language of your own choice and the given data to find the
minimum bitwidth required to keep the quantization root mean square error (RMSE) below 0.27.
Modify your VHDL code to perform the computations with the new bitwidth. Then, recompile
your design and use the Timing Analyzer to find the maximum frequency. Does the new design
meet the specified timing constraint (i.e., the clock period of 20 ns)? Finally, verify your design
using your testbench. Show the obtained results to the TA.
Redesign Architecture and Rearrange Registers
Another solution to increase the frequency of systems is redesigning architecture and
rearranging registers. Using the broadcasting form, which is the result of register rearrangement
and design modification, is a common approach for hardware implementations of FIR filters. The
broadcasting form of FIR filters is shown below.
First, show why the broadcasting form works by writing down equations for both the regular and
broadcasting forms and include them in your report. Second, implement the broadcasting form
of the FIR filter in VHDL while representing the filter’s input, output signals and weights in the
fixed-point representations (1,15), (2,15) and (1,15), respectively. Then, recompile your design
and use the Timing Analyzer to find the maximum frequency. Does the new design meet the
specified timing constraint (i.e., the clock period of 20 ns)? Finally, verify your design using your
testbench.
Show the obtained results to the TA.
Lab Report
At the end of the lab, you should know how to prepare and compile a VHDL code and understand
how it maps to FPGAs. You are required to submit a single PDF file that:
D
D
bN-1 bN-2 bN-3 b1 b0
x
D D D D D y
• is written in the standard technical report format
• documents every design choice clearly
• is organized for the grader to easily reproduce your results by running your code
• contains the code that is well-documented and easy to read
• should not cause the struggle for a grader to understand
Necessary parts
• An introduction in which the objective of the lab is discussed
• The VHDL code for both filter designs, with explanation and comments
• Discussion on the resource utilization of your design (i.e., how many registers are used
and why? What changes would you respect in the resource utilization if you increased the
bit size of the counter?)
• Testbench VHDL code to verify your design
• Discussion on the resource utilization of your design (i.e. how many registers are used and
why? What changes would you respect in the resource utilization if you increased the bit
size of the counter?)
• Content of SDC file, screenshots of timing violations (if any), reported maximum clock
frequency 
Grading Sheet
Group Number:
Name 1:
Name 2:
Task     Grade     /Total     Commentsadadsasdasda    
VHDL    for    25-tap    FIR /20    
VHDL for broadcast /30
Testbench    VHDL     /35
Maximum    frequency /15