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EECS 151/251A ASIC Lab 2: Simulation

Prof. Sophia Shao

TAs (ASIC): Erik Anderson, Roger Hsiao, Hansung Kim, Richard Yan

Department of Electrical Engineering and Computer Science

College of Engineering, University of California, Berkeley

Overview

In the course so far, you have learned the basics of using Verilog to describe hardware at the register-transfer-level (RTL). It might be helpful to take some time to review the Verilog Primer Slides before doing this lab. There are several other Verilog references in the Resources section of the course website that may be helpful.

In this lab, you will start working with Verilog and Verilog simulation. Simulation is very important because it is used at multiple stages of the flow to help verify the functionality of a design. It can help a designer catch functional bugs, as well as physical timing issues. We will focus on RTL, or Register Transfer Level, simulation in this lab, but we will also touch a bit on gate level simulation.

As with the previous lab, we will be using the Cory instructional machines (again, preferably eda-[1-12].eecs.berkeley.edu, but also c111-[1-17].eecs.berkeley.edu).

To begin this lab, get the project files (again, we recommend working in the /home/tmp/<your-eecs-username>/:

cd /home/tmp/<your-eecs-username>/
git clone /home/ff/eecs151/labs/lab2
cd lab2

Submitting Your Jobs

This semester, we will use a new Slurm cluster to manage all the workloads. Due to the high number of students, please do not run any compute-heavy jobs on the eda machines to avoid clogging up the login nodes, such as simulation, synthesis, or layout (Small scripts are okay). For the most part, Hammer (a tool introduced in a later section of this lab) will take care of submitting your jobs to the compute nodes through Slurm. The submission settings can be found at the bottom of the lab2/asap7.yml file:

vlsi.submit.command: "slurm"
vlsi.submit.settings:
  - slurm: {srun_binary: "srun", extra_args: []}

For more information on Slurm, please visit the official documentation.

The course staff will be closely monitoring the usage of our computing resources. If there are any improper uses (e.g., running a long RTL simulation on the eda machines), we may terminate your running jobs and notify you.

**For this class, all make commands should be run on the SLURM cluster.

The ASIC Design Flow

Here is a basic diagram of a VLSI design flow, in an ideal world where everything works the first time around:

At first, it seems like a very straightforward process from a system specification to a chip, right? However, the big piece that is missing from this diagram is simulation, or more broadly, verification. System specifications and architectural designs need to be verified by simulating their behavioral models against the expected system behavior. Functional and logical implementations of the system must also need to be simulated for functional correctness. Only after there is high confidence can we even begin to move onto the circuit (logic synthesis: Lab 3) and physical design (floorplanning, placement, and routing: Labs 4 and 5). We must continue to simulate the design in these stages to ensure that it will meet system specifications. After the layout of the circuit is complete, the circuit must also be verified to meet physical rules imposed by the underlying transistor technology (Lab 6). Only when these processes are all complete can a chip be fabricated, packaged, and tested post-silicon.

Problems found in verification have big implications on the design. More severe issues require the design to be changed earlier in the design flow, causing delays in the design process. Hence, there is essentially a feedback loop from the output of each block in Fig. 1 to any block before it–a number of loops far too great to show in Fig. 1. The fewer loops you take in the design flow, the less time you waste. Therefore, it is imperative that you are well-versed in the mechanics of simulating your designs before even getting around to designing anything!

CAD Tool Aside

You will find that there are several major CAD companies: Cadence, Synopsys, and Mentor Graphics. Each of these companies supplies tools for all parts of the VLSI flow. For instance, Cadence's synthesis suite is called Genus and Synopsys' synthesis suite is called Design Compiler (DC). The instructional machines have licenses available for many of these tools. Oftentimes, designers will mix and match different tools for different portions of the design flow. This is possble because the tools typically can write out to common file formats that can be consumed by other vendors' tools, or they provide utilities such that files can be converted to different formats. For example, a design may use Synopsys vcs for simulation, Cadence Genus and Innovus for synthesis and place-and-route, respectively, and Mentor calibre for DRC and LVS. We will gain experience using some of these different tools in subsequent labs.

Something to look out for is the rise of open source CAD tools and open technologies. You may look at initiatives like OpenROAD and technologies like Skywater130

RTL Simulation

This lab focuses on RTL simulation. RTL simulation is one of the first steps towards checking the baseline functionality of your design. Fixing bugs at the RTL design phase makes subsequent debugging much easier. The resources required and runtime is much lower than gate level simulation. This means it should be used often as you are developing a design. RTL simulation can be used to run unit tests to check a small block, as well as to simulate the behavior of a whole core running software, or beyond.

For this lab, we will be using Verilog code that implements a very simple FIR (Finite Impulse Response) filter. A schematic of the filter is shown below.

There is an input signal and a clock input, and 5 delayed versions of the input are kept, multiplied by different coefficients, and then summed together. The expression for this particular filter is:

y[n] = 1 * x[n] + 4 * x[n − 1] + 16 * x[n − 2] + 4 * x[n − 3] + 1 * x[n − 4]

The input in our example is a 4 bit signed 2’s complement number, and the output is a larger bitwidth signed number to ensure that there is no overflow. The focus of this lab is not the filter design itself, but it serves as a useful example of a digital circuit to implement and test with Verilog code. As such, Verilog code for this FIR filter is provided in the src folder.

Simulation Environment

We will be using Synopsys VCS as our Verilog simulator in this course. VCS works by compiling Verilog modules and generating a simulator binary file. You can then execute that binary file to produce the desired outputs. VCS is one of many Verilog simulators. Others include Cadence Xcelium (formerly Incisive) and the open source Verilator. The framework, mechanisms, and capabilities are quite similar between simulators.

Each CAD tool, including simulators, are highly configurable and can take in tens or hundreds (or more) configuration options. Invoking CAD tools manually and listing options and flags is unsustainable. Thus, whether you're a student pushing through a design or an engineer at a big company, everyone has some kind of automated design flow. This can come in many forms, from a series of TCL files (Tool Control Language, which is the language of choice for ASIC CAD tools) that are pushed around by Makefiles, to more configurable and re-usable design frameworks.

In this course we will use an ASIC design framework developed here at Berkeley called Hammer. Hammer abstracts away tool- (Cadence, Synopsys, Mentor, etc.) and technology- (TSMC, Intel, etc.) specific considerations away from ASIC design. The philosophy of Hammer aims to maximize reusability of design intent between projects that may have differently underlying tool infrastructures and target different process technologies. Documentation about Hammer philosophy and usage is at hammer-vlsi.readthedocs.io.

For this lab, we will be ”implementing” our designs using the ASAP7 7nm Predictive Process Design Kit. We have set up a bare-bones Hammer flow that you will see is largely shared between this lab and subsequent ones. You will execute steps within this flow by using a common Makefile. This flow is distilled from an ASAP7 Tutorial contained in Chipyard, an open-source RISC-V SoC Design Framework also developed here at Berkeley. Chipyard documentation can be found here if are interested in learning more.

Hammer consumes serialized configuration in YAML or JSON format, which are used as intermediate representation (IR) languages between higher-level physical design generators and the underlying scripts that the ASIC design flow tools require. In this lab repository, you will see a set of YAML files (inst-env.yaml, asap7.yml, design.yml, and a few sim-<type>.yml) that contain Hammer IR for our design implementation. Of these files, you will only interact with the sim-<type>.yml files to configure your simulation flow in this lab. The sources for this lab are contained in the src folder, and some special non-Verilog files will be explained later.

The Hammer Python core is installed at /home/ff/eecs151/hammer, along with plugins for Cadence (Genus Synthesis, Innovus Place-and-Route, Voltus Power), Synopsys (VCS Simulator), and Mentor Graphics (Calibre DRC/LVS) tools. Note: The ASAP7 PDK and the Hammer plugins are provided to us solely for educational use. They should NEVER be copied outside of instructional machines under any circumstances, or else we risk losing access in the future!!!

To get started, examine the Makefile in the lab files. Skimming through, you can see that there are rules for the basic VLSI flow steps explored in these labs: simulation, power analysis, synthesis, place-and-route (PAR), design rule checking (DRC), layout vs. schematic (LVS), and translational steps between them. The top section of the Makefile defines variables that point to the YAML and JSON files needed by Hammer.

First, examine the sim-rtl.yml file:

# RTL Simulation Variables
sim.inputs:
  input_files:
    - "src/fir.v"
    - "src/EECS151.v"
    - "src/addertree.v"
    - "src/fir_tb.v"
  timescale: "1ns/10ps"
  options:
    - "-notice"
    - "-line"
    - "+lint=all,noVCDE"
    - "+v2k"
    - "-debug"
    - "-sverilog"
  top_module: "fir"
  tb_name: "fir_tb"
  defines:
    - "CLOCK_PERIOD=1.00"
~

This is a snippet of Hammer IR in the sim namespace. These keys are consumed by Hammer to configure the simulation. Notice that the simulator needs some specifications, such as the files that contain our Verilog source code and our testbench name, some timescale on which to simulate and a clock period, and some options. These will be explained below. Exploring the other YAML files in this lab will give you an idea of what else is needed to define environment, tool, and technology constraints.

It's not critical that you understand right now how Hammer works, but as you work through the ASIC labs, it may be interesting for you to think about why it may be useful and other methods of creating an ASIC flow design framework.

Under the Hood

Now, let’s decipher what happens under the hood when we run a simulation with Hammer. You can find the VCS user guides from the eecs151 class-account home directory for reference: /home/ff/eecs151/labs/manuals/vcsmx_ug.pdf, /home/ff/eecs151/labs/manuals/ucli_ug.pdf, and /home/ff/eecs151/labs/manuals/dve_ug.pdf. To get started, let’s set up our environment. You will need to do this for every terminal in which you use Hammer on the instructional machines:

source /home/ff/eecs151/asic/eecs151.bashrc

You may find it useful to add this command to your .bashrc so you don't need to run it manually every time.

Now, let’s run a basic RTL simulation and look at the terminal output:

make sim-rtl

Here is a summary of what you should see when you scroll through the terminal output after it finishes, from top to bottom:

  1. The first line is the command that the Makefile constructed for you.
  2. This is followed by Hammer debug output, such as the technology and CAD tool used.
  3. The first time you run sim-rtl, Hammer will extract the PDK, compile memories (unused in this lab), and make an auto-build file. You will not see this again unless you clean your working directory.
  4. When it is time to simulate, Hammer issues a subprocess call to the VCS executable. Notice that the arguments passed to VCS are the same as the list of sim options from our sim-rtl.yml file. One important setting here is the timescale option, which sets the timescale for the Verilog testbench. We will discuss what this actually means later on in the lab.
  5. VCS compiles your Verilog code. If your code has errors, they will be shown during this step.
  6. This is followed by a wall of “Lint” messages. Linting catches Verilog mistakes that aren’t actual syntax errors, but cause common bugs. We intentionally used the +lint=all option to trigger these messages. In this case, assigning a number like “1” instead of 16’d1 gives Verilog less information about how you want the bits interpreted, which could cause a design bug that is hard to track down.
  7. The compilation finishes and generates a binary executable. If you look at the sim-rtl phony (target is not a file) rule in the Makefile, the --obj_dir flag to the Hammer executable is the target working directory. sim is the action, so our simulation environment and results are in the build/sim-rundir folder. In that folder, you will see the binary executable: the simv file.
  8. At the end, the simulation should report the observed values with the expected and actual values at each timestep, and they should match. You can even automate the checking of these outputs in either the testbench itself or through parsing the output text directly, but that will be explained in more detail later.

Note: If you rerun make sim-rtl without changing any code, the simulation does not run and the command seems to fail with an error. You can disregard this error as this is simply VCS skipping redundant recompiling and re-running simulation for the same design.

Viewing Waveforms

After completing the simulation, VCS dumps the waveforms to a file: build/sim-rundir/vcdplus.vpd. This file contains simulation history data, and is generated in the Verilog testbench (src/fir_tb.v) with the $vcdpluson and $vcdplusoff system tasks. A system task in Verilog is a function that is created by the compiler to make things easier for you, and usually begins with a $ character. We will not dig deeper into system tasks in this lab, but they are very powerful tools that are worth investing the time in learning.

Let us look at the waveforms in a graphical viewer DVE (Discovery Visualization Environment). (Note: this will require X11 forwarding, so if you ssh from a terminal be sure to use either the -X or -Y flag. If you are using PuTTy, make sure to enable X11 forwarding and have the proper X server software installed. An example of an X server for Windows is a program called VcXsrv, although there are a ton of other options). A faster, smoother, cross-platform alternative would be to use X2Go (recommended option, see Lab 1 for instructions).

cd build/sim-rundir
dve -vpd vcdplus.vpd &

When DVE pops up, you should see the window below:

This window will contain the design hierarchy, the signals in that level, as well as a lot of other buttons and options. One important feature is towards the top left of the window where it has a textfield and next to that says “x10ps”. This represents the units of time that the signals are plotted on. Change this to be in units of nanoseconds. You can add waveforms to a new view by selecting a few of them (In, Out and clk) and the right clicking and going to “Add To Waves” and then clicking either “New Wave View” or “Recent”. Since there are no other wave views open these both accomplish the same thing. If you already had a wave view open, then you can selectively add signals to one wave view or another. A screenshot of adding these waveforms is shown below.

After the window opens, you may need to adjust the zoom settings from “View — Zoom“ or the corresponding buttons on the toolbar. The picture below displays the output of the FIR filter as a step waveform. This can be achieved by setting the radix of the signal to be 2’s complement (right-click — Set Radix — Twos complement) and then changing the Set Draw Style Scheme to be Vector: Analog (right-click — Set Draw Style Scheme — Analog). You then need to change the properties of the signal since the beginning of the waveform does not utilize the full scale. If you right click and go to properties, you can set the Analog Waveform properties to be User, and set the Min to be -32 and the max to be 32. Then you should see something similar to the waveform below.

For those of you who know a bit about FIR filters, this beginning section of the waveform is the filter’s impulse response.

When looking at a waveform viewer like DVE, signals are generally shown in green. Blue lines (annotated as Z) are high impedance (unconnected) nets. In our designs, this typically indicates that something is not wired up correctly.

Red lines (annotated as X) indicate that a net has an unknown value at that time. You may see this at the beginning of the simulation before the registers in the filter have known values. Once they get a known value, the lines turn green.

Testbenches

Testbenches are how you simulate a design. Specifically they setup the inputs and check the outputs of the submodule that you are trying to test. If you look at the fir_tb.v file in the src/ folder, there are a few important parts that you will need to understand in order to write your own testbench. The first important piece is generating the clock waveform. This is accomplished by the following lines of Verilog code:

reg clk;

initial clk = 0;
always #(`CLOCK_PERIOD/2) clk <= ~clk;

This creates a signal named clk, which is initially 0 and toggles every CLOCK_PERIOD divided by 2 (in order to generate a rising edge every CLOCK_PERIOD). This clock period is defined in the sim.inputs.defines key in the sim-rtl.yml file you saw above. After we have a clock waveform, the next step is to setup the inputs. There is an initial begin block that is the body of the testbench that does this work. Let us look at a piece of it:

initial begin
$vcdpluson;
  In <= 4'd0;
  @(negedge clk) In<= 4'd1;
  @(negedge clk) In<= 4'd0;
  .
  .
  .
  @(negedge clk) In<= 4'd13;
  @(negedge clk) In<= 4'd14;
  @(negedge clk) In<= 4'd15;
  $vcdplusoff;
  $finish;
end

The $vcdpluson and $vcdplusoff are system tasks that setup the vpd file generation that we used to look at the waveforms in the previous section. The other lines setup the register In to take on different values after the negative edge of the clock. In this block, the lines are executed after each other, so the next @(negedge clk) call waits until the next negative edge of the clock before executing the code that follows it. This allows us to set up a series of values for the inputs sequentially in time. The reason that we are operating on the negative edge of the clock is that the registers that are sampling the inputs operate on the positive edge, so we want to make sure that the correct value is sampled when that edge occurs. If we were to change the inputs on the same edge, we could cause a hold time violation later when delay gets annotated.

The above code sets up the values sequentially, but you can do this in a much cleaner way by reading values from a file. Take a look at the file src/fir_tb_file.v, parts of which are copied below:

initial begin
  $vcdpluson;
  repeat (26) @(negedge clk);
  $vcdplusoff;
$finish;

end

initial begin
    $readmemb("../../src/data_b.txt", Out_correct_array);
    $readmemb("../../src/input.txt", input_array);
end

assign Out_correct = Out_correct_array[index_counter];
assign In = input_array[index_counter];

always @(negedge clk) begin
    $display($time, ": Out should be %d, got %d", Out_correct, Out);
    index_counter <= index_counter + 1;
end

This testbench file uses text files to pull both the inputs and expected outputs, as well as displays what the output is and what it thinks the correct output should be.

This is done using the $readmemb command, which reads a file into a verilog memory that you instantiate in the testbench. By then using a counter to loop through both the input and the desired output, the testbench simply needs to run for the right number of cycles, which is shown using the repeat syntax inside the first initial begin block. Finally, this testbench uses $display, which is verilog’s print statement to print out information about the waveforms to the console so that you do not need to look at the waveforms. More sophisticated testbenches can be created this way so that you do not have to look at the waveforms to gather the necessary information, allowing you to automate your testing procedures. While this is a small enough design that you could in theory debug using only the waveforms, the project later in this course will be much more complicated so learning how to build automated testbenches will be very important. You can run this automated testbench yourself by replacing fir_tb in sim-rtl.yml with fir_tb_file, and running make sim-rtl again. Don't forget to change the testbench name too.

Question 1: Conceptually translating between waveforms, Verilog, and schematics

When asked to write Verilog, include the module definition. There are multiple correct solutions, so we will accept any solution that works.

Remember to include a short explanation of each answer (about 2-4 sentences) with your responses to the lab questions.

You may find it helpful to actually write and simulate each question to get practice writing Verilog and testbenches, as well as interpreting simulation results and waveforms.

a.) Using the provided Verilog code and input waveforms, submit the equivalent logic gate schematic and draw/sketch the output waveforms. Note that the initial conditions of X and Z are given.

module dut (
  input A, B, clk,
  output reg X, Z
);
  always @(posedge clk) begin
    X <= B;
    Z <= (Z & X) | A;
  end
endmodule

b.) Using the schematic below and input waveforms, submit the equivalent Verilog code and a sketch of output waveforms.

c.) Using the input and output waveforms provided below, submit the equivalent schematic and Verilog code. Hint: Use 1 flip-flop and 1 logic gate only. A is an input, X and Y are outputs.

Question 2: Writing a testbench

a.) Using the Verilog code in Question 1a, write a testbench that emulates the input waveforms, including initial conditions, then simulate the block using VCS and compare to your previous answer. Submit your Verilog testbench and a screenshot of the simulation waveforms showing all of the input and output pins. Hint: Make sure to keep your testbench separate from your DUT code. Instantiate your DUT within your testbench rather than duplicating functionality.

Gate-Level Simulation

The RTL design of the FIR filter, fir.v, conceptually describes hardware, but cannot be physically implemented as-is because it is purely behavioral. In the real world, a CAD tool translates RTL into logic gates from a particular technology library in a process called synthesis. In Lab 3, you will learn how to create this file yourself, but for now we have provided the synthesized gate-level netlist at src/post-syn/fir.mapped.v.

To simulate using the gate-level netlist, you simply need to make a few changes to the input constraints to Hammer. Take a look at sim-gl-syn.yml. You will notice that a few things have been added, including a level:"gl" option, and timing_annotated: true option, two JSON files, and a Standard Delay Format (SDF) file. Hammer consumes the two JSON files in order to generate a Unified Command-Line Interface (UCLI) script that tells VCS to force the synthesized flip-flops into a valid initial state before starting the simulation. This is required because Verilog simulators cannot simulate with unknown of ’x’ valued inputs. Theoretically, a real design should work regardless of initial conditions, but we will not do that in this lab due to time constraints. The SDF file is an output from the synthesis tool that annotates delays according to the synthesized gates.

Under the hood, Hammer has already included the Verilog models of the standard cells from the ASAP7 PDK. You will learn more about these standard cells in the next lab, but just know that they are required because the gate-level has instances of the technology’s standard cells everywhere, and VCS must also know the Verilog definition of those cells. The extra options in the new VCS section of the Makefile are simply to deal with these standard cell models.

Now, return back to the lab2 root directory, run the gate-level simulation, and reload the waveforms after it is finished. Notice that in this invocation we are overriding the SIM_RTL_CONF Makefile variable from the command line (later labs will use a different make rule):

make sim-rtl SIM_RTL_CONF=sim-gl-syn.yml

To understand what we will see in the waveforms, open src/post-syn/fir.mapped.sdf, and go to line 259.

(CELL
    (CELLTYPE "INVx1_ASAP7_75t_SL")
    (INSTANCE add0.g833)
    (DELAY
        (ABSOLUTE
          (PORT A (::0.0))
          (IOPATH A Y (::11) (::7))
        )
    )
)

The above text describes the delay for a cell of type INVx1_ASAP7_75t_SL for the instance add0/g833. The format of the delay is minimum:typical:maximum, which refer to different operating regions that will be discussed in more detail in future labs. Note that this SDF file only specifies maximum delays, which is generally what we want because we need to simulate the worst-case conditions (more on that in future labs). For this specific instance that means that there will be a delay of either 11ps or 7ps, depending on whether the data is transitioning from low to high or from high to low. We know that these delays are in picoseconds because of the declaration on line 12 of the SDF file.

Remember that previously we mentioned the timescale option. This is passed to VCS as a -timescale flag with the value 1ns/10ps, which means that a delay of 1 would correspond to an actual delay of 1ns, with a simulation step resolution of 10ps.

To tell the simulator about these delays, we must use the +sdfverbose -sdf max:fir:<path/to/fir.mapped.sdf> VCS options (auto-generated by Hammer). Other VCS flags that Hammer auto-generates for gate-level simulation are +neg_tchk and -negdelay. In regular RTL-level simulation, all the aforementioned flags are replaced by +notimingcheck and +delay_mode_zero instead.

Now, examine the waveforms in DVE. Notice that the waveforms look similar to, but not exactly the same as the results from simulating the RTL-level simulation of the design. Let’s see why. By default, the logic gates behave ideally. Depending on the operating conditions of the chip—voltage, process variation, temperature—the delay through a gate will be known. CAD tools do this calculation for you, and annotate the delay onto the gates using an SDF file like the one you just saw.

Configure the waveforms so that you see at least the clk and delay_chain0 signals (hint: you may need to go down to the dut level of hierarchy in the left pane). Zoom into the first rising edge of delay_chain0, which should be around the 2.5 ns mark. Recall that an RTL-level sim, logic gates behave ideally. This means that the flip-flop output delay_chain0 would change state (given an input that has changed) perfectly synchronously to the rising edge of clk. However, you will see here that the transition edge of delay_chain0 is some amount of time after the rising edge of clk. This delay was annotated in the SDF as the flop’s clk-q time (IOPATH CLK Q, for rising and falling edges) and properly simulated in VCS.

Try looking at some other signals and think about why some signals have more delay than others. Also try out some of the other options in the wave viewer to try and figure out what is going on. If you get stuck on anything that you are trying to do, you can look up the Synopsys DVE User Guide, which has a significant amount of information about DVE, in the eecs151 home directory: /home/ff/eecs151/labs/manuals/dve_ug.pdf.

Question 3: Calculating delays from the DVE window

a.) Calculate the delay of the first flip-flop in the chain (delay chain0) relative to the input pin clk at 2.5 ns. What is the delay, and how were you able to get it from the viewer? Can you correlate this with a particular line in the SDF file?

Now that we are simulating with delay, we can see that to preserve logic functionality, we must not toggle the clk into the circuit too quickly. When this happens, there is a ’setup’ violation and the FIR filter will not function correctly.

Question 4: Creating and fixing setup time violations

a.) Edit the "CLOCK PERIOD=1.00" in sim-gl-syn.yml to make the clock period 0.5ns and simulate again. Does it still work? Why or why not? Run it again at increasingly larger clock periods (but less than the initial 1.0ns) and report the clock period you found where it successfully meets timing. Hint: correct functionality means that the expected and actual outputs match. Look at how the testbench checks for functionality. Note: in the waveform window, you can do File — Reload Databases to show the new waveforms after each simulation run.

Re-run the design with the clock period set to 1.0ns again. Save the vpd file as a different name, such as vcdplus_orig.vpd. After you have renamed the previous vpd file, change the SDF file in sim-gl-syn.yml to point to fir.mapped_hold.sdf rather than fir.mapped.sdf. Then, re-run the gate-level simulation.

This will run the simulation with a different delay file, which intentionally has an error in it. You should now have two different vpd files, and you can load them in the DVE waveform viewer to see the differences. Add all of the delay_chain signals for both vpd files. Zoom into the clock edges near 2.7 ns into the simulation. There should be a significant difference between the two vpd files, and one of them will have a signal that is incorrectly getting captured on the wrong cycle. This is an exaggerated case of a hold time violation, which occurs when a specific delay path is too small relative to another.

Question 5: Fixing hold times

Setup times can be fixed by increasing the clock period, but hold times cannot, because the capturing edge relationship in the hold violation do not change with clock period. Later, you will learn how the CAD tools do this for you, but in this problem you will manually identify the error in the SDF and fix it.

a.) Explain the differences between the waveforms in the two vpd files. What signal(s) are different and why? Show what in the SDF is causing this, and make a best guess at what could cause this to happen.

b.) Modify the src/post-syn/fir.mapped_hold.sdf file to fix the hold time without reverting what you found in a). You are only allowed to change a delay value in one register. Submit the following:

i) Which delay you changed (show the original and fixed value)

ii) Why did you change that register’s delay?

iii) Simulation waveforms showing correct output and the text printout of the simulation showing that the results are correct.

iv) Since when designing in reality you can’t actually hack SDFs to fix hold, give your best guess at what would be inserted/removed from a gate-level implementation of this design that would accomplish your hold fix.

Power Anlaysis

Power is arguably the most important metric in modern chip design, as mobile applications continue driving the demand for SoCs and custom digital hardware. Therefore, a robust analysis of power consumption for a given testbench (or workload, benchmark) is something that designers must simulate. Power analysis results can influence all levels of design in the ASIC flow.

Normally, the most accurate power analysis results come from simulating on a post-place-and-routed design (Labs 4 and 5). For now, we have provided the place-and-routed (P&R) outputs and post-P&R simulation outputs in src/post-par-sim.

To perform power analysis with Hammer, we must specify a few more things. Take a look at sim-gl-par.yml. In addition to the things added in sim-gl-syn.yml, there is a new namespace power which contains keys that specify Switching Activity Interchange Format (SAIF) files, Standard Parasitic Exchange Format (SPEF) files, and a layout database. The layout database and SPEF files are generated from the P&R tool (saved for future labs). Skimming through the SPEF files, you can glance the words CAP and RES everywhere; these are annotations of the parasitic capacitances and resistances caused by physical layout and connections of logic gates. The SAIF file is dumped from a post-P&R gate-level simulation, and contains a somewhat cryptic annotation of how often nets in the design switch and requires the sim.inputs.saif keys in Hammer. A time window over which switching activity is measured is helpful for generating representative traces, such as for workloads that only run after a processor core has booted up. For this lab, the power outputs have been generated in advance.

Under the hood, Hammer uses Cadence Voltus to analyze power consumption. It maps the annotated switching activity onto the layout database, taking into account the circuit parasitics. Voltus dumps reports in the build/power-rundir directory. There are results for static and active power analysis. The static power analysis (in staticPowerReports folder) by default assumes an average switching activity factor of 0.2 (nets switch 20% of the time), while the active power analysis (in activePowerReports folder) uses the information from the SAIF. Depending on the testbench, there could be a large difference between the static and active estimates.

Open build/power-rundir/activePowerReports/PVT_0P77V_0C.hold_view.rpt and scroll to the bottom of the file. The total power is grouped into 3 types: internal, switching, and leakage power.

  • Internal power: power dissipated inside logic gates. Usually caused by momentary short-circuiting as transistors are switching.

  • Switching power: power dissipated charging and discharging load and parasitic capacitances/resistances.

  • Leakage power: power dissipated in logic gates when they are not switching. Logic gates have finite resistance between power and ground even when they’re totally static!

Below that first table is a breakdown into types of cells. In our FIR, we have a couple of sequential cells (delay chain flip-flops) but many more combinational cells (adder tree), hence it is reasonable that our power is dominated by combinational logic.

Question 6: Analyzing Power Reports

a.) Also open up build/power-rundir/activePowerReports/PVT_0P63V_100C.setup_view.rpt. What is the most obvious difference in the power numbers compared to the hold view file? What do you think is the dominant factor contributing to this difference?

b.) Compare the reports with SAIF annotation you just viewed against the analogous reports in build/power-rundir/staticPowerReports. What would you estimate is the effective switching activity factor of the testbench we have been using? Note: switching power is linearly proportional to switching activity.

Closing Thoughts

Proficiency in simulation and understanding what considerations go into verifying your design at every stage of the ASIC flow is indispensable. In this lab, we have only skimmed the surface of the methods by which designers validate, verify, and debug their designs. RTL simulation in VCS is simply a form of functional validation; basically, you assure yourself that you build the right thing. However, there is an entire field of verification, where advanced algorithmic techniques and higher levels of abstraction can fully exercise any design and ensure that vanishingly few bugs make it to silicon. In essence, you assure yourself that you built the thing right. Correspondingly, there are many CAD tools that accelerate these tasks and are heavily used in industry. We encourage you to take EE219C if you are interested in this topic.

Within the ASIC design flow itself, there are additional things that need to be simulated. The most obvious thing to simulate is power consumption, which you have seen here. Findings from these simulations directly inform design decisions at all levels of the stack. Digging even deeper, CMOS transistor physics increasingly rears its ugly head. Even the wires that connect transistors now don’t stay intact if signals are constantly traveling through them (called electromigration). Wires are so small now, that even the way that power is distributed across a chip (IR drop) can greatly affect circuit performance. Transistors themselves even have defects and wear out over time, causing premature failures if left unaccounted for (reliability). There are tools that simulate all of these effects. Take EE241B if you interested in this topic.

Lastly, industry chips must also undergo simulations post-manufacturing to find out if there are any manufacturing-induced defects and performance limitations. Designing for test (DFT), extra circuitry to correct errors and enable/disable functionality, and on-chip monitoring of circuit parameters (e.g. temperature, timing) must also be simulated to maximize product yield.

Lab Deliverables

Lab Due: 11:59 PM, 1 week after your registered lab section.

  • Submit a written report with all 6 questions answered to Gradescope

NOTE: Gradescope has multiple assignments for the various lab sections. Please only submit to your registered section.

Acknowledgement

This lab is the result of the work of many EECS151/251 GSIs over the years including: Written By:

  • Nathan Narevsky (2014, 2017)
  • Brian Zimmer (2014) Modified By:
  • John Wright (2015,2016)
  • Ali Moin (2018)
  • Arya Reais-Parsi (2019)
  • Cem Yalcin (2019)
  • Tan Nguyen (2020)
  • Harrison Liew (2020)
  • Sean Huang (2021)
  • Daniel Grubb, Nayiri Krzysztofowicz, Zhaokai Liu (2021)
  • Dima Nikiforov (2022)
  • Roger Hsiao, Hansung Kim (2022)