ECE429 Lab9 - Tutorial IV: Standard Cell Based ASIC Design Flow

updated by Jia Wang, Aug. 2011, revised by Erdal Oruklu, Spring 2013.

Overview

This tutorial introduces you to the standard cell based ASIC design flow using tools and libraries from various vendors. We will use the OSU standard cell library from FreePDK45 to implement an 8-bit accumulator design. We will first synthesize the design using the Synopsys Design Compiler and then perform place and route using the Cadence Encounter Digital Implementation System. The final layout will be painted in the Cadence Virtuoso platform and the final design will be verified by the Synopsys Formality equivalence checker.

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Table of Contents

1. RTL Simulation

2. Logic Synthesis using Design Compiler

3. Place and Route using Encounter

4. Equivalence Checking using Formality

5. Layout Printing using Virtuoso

1. RTL Simulation

Create and initialize a directory for the project.

source /import/scripts/ece429_enc.cshrc

source /import/scripts/synopsys2012.cshrc

mkdir accu

cd accu

ece429-init-dir

Typically you enter code in Verilog on the Register-Transfer Level (RTL), where you model your design using clocked registers, datapath elements and control elements. You will use Cadence Verilog-XL to simulate your design. You will also need to create a Verilog testbench for your circuit. In this tutorial there are 2 files as follows.

In order to simulate Verilog code, use this command:
    xrun accu_test.v accu.v +access+r

This testbench provides results directly on the screen and also in a waveform database. From the screen we can see that the design behaves as expected as follows.

That is, every 10ns we add 1 to the accumulator. This is expected since in the testbench a clock of 10ns is specified and the input 'in' is connected to a constant 1.

We use the program Cadence SimVision to look at the waveform database that was created by Verilog-XL. Type the following command:
    simvision

Now we need to open the Waveform database. Click on the "Open" symbol, choose the directory "shm.db", which is where the file is located, and double-click on the file "shm.trn" to open it. To see the contents of the waveform database, click on "stimulus":

Now we want to plot our waveforms. We need to select which signals we are interested in. In this case lets look at all waveforms. Select all 4 waveforms on the right and hit the picture-button with the square waves.

We see that the circuit works fine. At every rising clock edge the output changes.

2. Logic Synthesis using Design Compiler

Once you have verified that your Verilog RTL code is working correctly you can synthesize it into standard cells. The result will be a gate-level netlist that only contains interconnected standard cells.

There are template files for all the following steps already prepared for you. We will now copy those templates into our project.
cp /import/scripts/FreePDK45_2011/FreePDK45/osu_soc/flow_ece429/* .

(If you use the machines in the laboratory, you may find that this command does not work. This is because the target folder does not exist in the local machines in 310 laboratory. The command will work if you login to 'uranus' or 'saturn' server. In case you need to use them in 310, you can downlaod them from here: compile_dc.tcl , encounter.conf, encounter.tcl. Put them in your working directory.)

We will use the Synopsys Design Compiler for logic synthesis. Since a hardware design requires not only the Verilog descriptions but also the specifications, we will use a script file to automate the synthesis task. The template file is provided as 'compile_dc.tcl'. Note that dc stands for Design Compiler (DC).

Please open 'compile_dc.tcl' in a text editor. Although you don't need to modify this file for this tutorial, you will need to modify it for the final project so please read the following part carefully. To make it easier to modify the file, all key values are defined in the beginning of the file.

The first few lines will show 

 
#/* All verilog files, separated by spaces         */
set my_verilog_files [list accu.v]

This specifies what Verilog files should be included for our design. It is not necessary to include the testbench as it is not part of the chip.

Then you will see the name of the module that represents the whole design. 

 
#/* Top-level Module                               */
set my_toplevel accu

The name of the clock signal is specified next. It's usually 'clk'. 

 
#/* The name of the clock pin. If no clock-pin     */
#/* exists, pick anything                          */
set my_clock_pin clk

Finally, you specify the desired clock frequency of the circuit. 

 
#/* Target frequency in MHz for optimization       */
set my_clk_freq_MHz 1000

Once you have the script file ready, you can go ahead to synthesize the circuit:
    dc_shell -f compile_dc.tcl

Design Compiler will run for a short time and create substantial amounts of output. When it is finished it will return to the command line. If there is an error it will specify the exact source of the error and the line number in the script that was responsible for the error. Typically there will be no errors. We can verify the initial estimations of area, timing, and power by reading into the following three files 'cell.rep', 'timing.rep', and 'power.rep'.

We can also look at the output of DC. As we said above, it is a gate-level Verilog netlist that only contains interconnected standard cells. The netlist is called 'accu.vh' and you can use any text editor to check its content. Note that the top-level module still has the name 'accu' and the names of the inputs and outputs have not changed. From the outside it is exactly the same circuit as you coded on the RTL level. But on the inside all functionality is now expressed only in terms of standard cells. A post-synthesis simulation can be performed by including Verilog models of the standard cells available from 'gscl45nm.v'. The command line is
    xrun gscl45nm.v accu_test.v accu.vh +access+r
Note how we re-used the original testbench from the RTL level simulation. That is an excellent way to ensure that the gate-level representation matches the RTL level. The simulation results should look similar to before.

3. Place and Route using Encounter

Since we have layouts for all standard cells, we can now place and route them for the final layout. We first use Encounter to place the standard cells and to route the interconnects. The output will be the placement of the cells and the metal interconnects. Further processings are required to obtain the final layout for fabrication.

Similar to logic synthesis using DC, we will use a set of script files to automate the placement and route task. All the commands for placement and route are in the template file 'encounter.tcl'. It includes commands to input the gate-level netlist, floorplan the chip, place the cells, route the cells, and to verify and output the result in GDSII format, which is the most popular format for IC layouts. The circuit to be placed and routed is specified in the file 'encounter.conf'.

Similar to 'compile_dc.tcl', although you don't need to modify 'encounter.conf' for this tutorial, you will need to modify it for the final project.

The only place you need to change are the lines 

 
# Specify the name of your toplevel module
set my_toplevel accu

This variable 'my_toplevel' will be used to deduct various file names. So it should be the same as in 'compile_dc.tcl'.

Now we are ready to run Encounter. The command line is
    encounter -init encounter.tcl

Encounter will run for a while and create substantial amounts of output. We can obtain more accurate timing by reading into the file 'timing.rep.5.final'.

The outputs of Encounter include a GDSII stream 'final.gds2' and an equivalent Verilog model 'final.v' of the placed circuit. Similar to 'accu.vh', the top-level module in 'final.v' still has the name 'accu' and the names of the inputs and the outputs are not changed. From the outside it is exactly the same circuit as you coded on the RTL level. But on the inside additional buffers are introduced to reduce interconnect delays and enhance clocking robustness. A post-P&R simulation can be performed with:
    xrun gscl45nm.v accu_test.v final.v +access+r
The simulation results should look similar to before.

 

Please type "reportGateCount -limit 0" in the encounter shell to get the area report of your design. For example, we can get the following output for the accu design:

 

Gate area 2.8158 um^2
[0] accu Gates=56 Cells=30 Area=158.6 um^2

"Gate area 2.8158 um^2" states the area of the reference gate.
"Gates=56" is the equivalent gate count, computed by round(158.6/2.8158).
"Cells=30" is the actual number of gates and cells, e.g. inverters, NAND gates, flip-flops, etc.
"Area=158.6 um^2" is the area.

 

To get the power report, please type "report_power" in the encounter shell. (The unit is mW.)?o:p>

4. Equivalence Checking using Formality

While post-synthesis and post-P&R simulations may help you detect errors during logic synthesis and place and route due to either bugs in the EDA tools or careless manual modifications, they cannot guarantee to find all errors if there is any. We will again rely on equivalence checking to formally prove the correctness of logic synthesis and place and route steps.

Since both logic synthesis and place and route output circuits in Verilog, we cannot use Formality ESP as introduced in Tutorial III since it validates a SPICE netlist against a Verilog model. Actually, Formality ESP is an extension to Synopsys Formality that validates two Verilog models. Therefore, we will use Formality directly for this tutorial. Note that although we have three Verilog models of the design -- 'accu.v', 'accu.vh', and 'final.v', we only need to validate 'accu.v' against 'final.v' since 'accu.vh' can be treated as an intermediate result.

Start Formality:
    fm_shell -gui

You will see a guided interface to perform equivalence checking that is very similar to Formality ESP.

Skip the step '0. Guidance'. For the step '1. Reference', click 'Verilog...' in '1. Read Design Files' to choose 'accu.v' and then 'Load Files' to load it.

Click '3. Set Top Design', choose 'accu', and then click 'Set Top'. You will see the green check to the side of '1. Reference' indicating the step is completed correctly.

Move to the step '2. Implementation'. Choose and load 'final.v'.

Now, since 'final.v' uses the cells from the standard cell library, we need to let Formality know the functionality of the cells. This is achieved by load 'gscl45nm.v' as a technology library. Click the 'Options' button to bring up the Set Verilog Read Options dialog box. Switch to the 'Library Type' tab and choose 'Read technology library files into library'.

Click 'OK'. Choose and load 'gscl45nm.v'.

Click '3. Set Top Design', choose 'accu', and then click 'Set Top'.

Skip '3. Setup'. For '4. Match', . Click 'Run Matching' to match the inputs/outputs/DFFs of the two designs. All of them should be matched by their names.

Move to the step '5. Verify'. Click 'Verify'. If the two designs are the same, you will see a dialog box showing 'Verification succeeded!'. Then '6. Debug' will show all output ports and DFFs as 'Passing Points'.

Have a try to modify the functionality of 'accu.v' and see how Formality can detect the discrepancies.

5. Layout Printing using Virtuoso

The GDSII stream 'final.gds2' contains only the placement of the cells and the metal interconnects. It doesn't contain any layout details inside the cells, e.g. wells and poly. Therefore, we need to combine this stream with cell layouts to obtain the final layout for fabrication.

We will use the Virtuoso platform for this task. Start Virtuoso in the directory 'accu'. Select the library 'OSU_stdcells' in the library manager. It contains all the cell layouts we are going to use. Click Edit→Copy to copy the cells in 'OSU_stdcells' to a new library 'accu'.

Click 'OK' when asked to confirm the location of the library 'accu'. Then you should be able to see the cells are copied to 'accu' in the library manager.

Now, bring up the Virtuoso Log window and click File→Import→Stream. You should see the Virtuoso XStream In dialog box. Click 'Show Options' to show 'StreamIn Options'. At the top section, select 'final.gds2' for 'Stream File', 'accu' for 'Library', and 'NCSU_TechLib_FreePDK45' for 'Attach Technology Library'. For 'StreamIn Options', switch to the 'Layers' tab. Load 'gds2_virtuoso.map' as the 'Map File', which translates GDSII layers into a human readable format.

'Hide Options' and then click 'Translate' to import the stream. When it finishes, there should be no error. In the library manager, you should see a new cell 'accu' in the library 'accu' with a single 'layout' view. Open it and zoom in. You will see the placed cells and the metal interconnects in the design.

Change the 'Display Stop Level' to 1. The final layout is shown as follows.