3. Flow for IR & EM Analysis with VAVO/VAEO
December 2009 3
Contents
1 Flow Overview.....................................................................................................................4
2 Setting up QRC for IR/EM Analyses..................................................................................5
3 Simulation Setup ...............................................................................................................7
3.1 Spectre ..........................................................................................................................7
3.2 Ultrasim.........................................................................................................................8
4 Running VAVO/VAEO .........................................................................................................9
4.1 IR Drop Analysis ..........................................................................................................9
4.2 EM Analysis ...............................................................................................................10
4. Flow for IR & EM Analysis with VAVO/VAEO
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1 Flow Overview
This application note describes the recommended flow for doing an IR Drop and Electromigration (EM)
analysis using the Virtuoso Analog VoltageStorm Option (VAVO) and the Virtuoso Analog ElectronStorm
Option (VAEO). VAVO enables you to verify the integrity of your power and ground networks in analog
blocks within the Cadence Analog Design Environment (ADE). VAEO enables you to find signal nets that
could fail electromigration specifications in analog blocks.
The basic steps to running VAVO/VAEO are as follows:
- Create schematic and testbench schematic
- Create layout
- Run Assura LVS (match is necessary!)
- Run QRC with specific options (see QRC setup section), creating a QRC extracted view
- Create a config view of the testbench schematic using HED, and specify the cells under analysis
to use the extracted view created in previous step.
- Open ADE from within configured testbench schematic
- Create or load state with appropriate analyses and options (see section on Simulation Setup)
- Run simulation
- From top-level schematic, descend into the cell under analysis into extracted view.
- Launch VAVO/VAEO from Tools->VoltageStorm menu. A new menu EM & IR will appear
- Run IR drop or EM analysis tools (see sections on IR drop and EM analysis)
Schematic Layout
Assura LVS
QRC (R/RC)
Create simulation netlist
from config view
including
QRC extracted cellview
Testbench schematic
QRC extracted cellview
Create Simulation files
Run Spectre or UltraSim
Run VAVO/VAEO
Display results in
QRC extracted cellview
5. Flow for IR & EM Analysis with VAVO/VAEO
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2 Setting up QRC for IR/EM Analysis
VAVO & VAEO uses a parasitic extracted view created by QRC for IR/EM analyses. QRC extraction flow
does require specific options to be enabled for this, which are as described below:
Note: A full treatment of setting up QRC is beyond the scope of this document. Familiarity with these
flows is assumed.
In the Extraction Tab:
- Extraction Mode should be set to RC decoupled or R Only.
-
- VAVO/VAEO will work with RC coupled or RLCK, but these will simply slow down the simulation
without adding any more accuracy.
- Max Fracture Length can be left at infinite (default).
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In the Netlisting tab:
- Parasitic Resistor Models must be set to ‘Include As Comment’. This saves the layer information
for the resistors, which is important for the EM and IR drop analyses.
- Parasitic Resistance Width must be saved so that width can be computed for current density
calculation. QRC can output Silicon widths or Drawn widths. So based on the foundry EM rules
the corresponding option should be selected.
- Parasitic Resistance Length must be saved so that length can be computed for current density
calculations having Length based rules. It is recommended to have this enabled for EM rules that
are based on only width as well, as this also forces fracturing on changes in metal width, which is
important for accurate EM.
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- Parasitic Temperature Coefficient should be set, to print temperature coefficients which are used
to determine the effect of temperature on resistance. The expected use model is to define the
temperature during simulation. The presence of the TC1/TC2 temperature coefficients in the
extracted view allows Spectre/Ultrasim to directly consider the effects of temperature on the
parasitic resistance. This avoids having to re-run QRC to create parasitic extracted views for
specific temperature corners. When this option is enabled, the Temperature option in the
Extraction tab will be disabled to avoid double counting the effect.
Note: The QRC techfile needs to have the TC1/TC2 temperature coefficients for this to work as
mentioned above.
- Add Explicit Vias must be enabled so the via resistors are kept separate from the metal resistors.
Once this setup is done, QRC should be run.
Note: One can also use the Mentor Calibre LVS- QRC flow to create a DFII Extracted View of the design.
For information on how QRC reads the Calibre input data and performs extraction, see “Running QRC
with Calibre® Input” in the QRC Extraction Users Manual.
3 Simulation Setup
Using a testbench schematic, a simulation is done on the QRC created extracted view within the ADE
environment. A Transient and/or DCop simulation must be done for EM/IR drop analyses.
Note: A full treatment of setting up ADE for Spectre or Ultrasim simulation is beyond the scope of this
document. Familiarity with these flows is assumed.
3.1 Spectre Setup
With Spectre simulation, all voltages must be saved for VAVO/VAEO. Saving currents is not necessary
and will significantly slow down simulation time. This simulation data will then be used by VAVO/VAEO to
generate the IR drop and EM information.
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3.2 Ultrasim Setup
You may use UltraSim simulation results as input to VAVO/VAEO instead of Spectre if desired. Ultrasim
is suggested for larger designs and should run faster.
In general we recommend the following options for Ultrasim + VAVO flow:
a) Simulation mode ‘A’ without Ultrasim UPS (standard Ultrasim setup for smaller designs, <5k devices)
In the Outputs Tab:
- Turn on ‘Select All Node Voltages’ and ‘Preserve All Nodes’.
- Set “Hierarchical Depth” to the depth of the extracted view in the schematic hierarchy.
b.) Ultrasim UPS with default ‘MS’ as the recommended simulation mode
With the Ultrasim UPS-VAVO/VAEO flow, Ultrasim calculates EM and IR related data for each resistor
and each node and this data is saved in a database. VAVO/VAEO reads in this database. Previously
EM/IR calculations were done in VAVO/VAEO after loading in Ultrasim’s waveform data.
Now the handshaking between Ultrasim and VAVO/VAEO is through a much smaller database. For
Ultrasim, now one does not need to save node voltages, this will speed up simulation. The speedup on
VAVO/VAEO side is even more significant since all the calculations are done by Ultrasim.
To enable this in Ultrasim, one needs to have the following command in the netlist.
.usim_emir format=[vavo]
To add this option in the netlist do the following:
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Click on the Simulation-Options-Analog tab in ADE. You can specify the period between which EM/IR
results need to be computed. Please refer to Ultrasim User Manual for further details of this command.
Once this setup is done, simulation should be run.
4 Running VAVO/VAEO
To run VAVO/VAEO, from the top-level schematic one need to descend into the cell under analysis into
extracted view (avoid opening the extracted view directly). Launch VAVO/VAEO from Tools-
>VoltageStorm menu. A new menu ‘EM & IR’ will appear on the Virtuoso Window.
4.1 IR drop Analysis
Do an EM&IR->IR drop analysis, and the following form comes up:
10. Flow for IR & EM Analysis with VAVO/VAEO
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- Select the simulation directory where the results are stored
- Select the analysis (transient/DCop) that interests you
- Select the type of IR drop that interests you (Peak, Average, RMS or Peak AC)
- Select one or more pins or internal signals. It is recommended to select only one or two at a time,
or the data becomes too large to view reasonably.
- Click OK.
- After a couple of minutes (depending on design size) the following form will come up:
Points to note:
- The results can be saved into a file and read in later, saving the recalculation.
11. Flow for IR & EM Analysis with VAVO/VAEO
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- The different colours represent the IR drop from 0 to the maximum found divided into 1-10
different bins (default 6).
- You can open the layout window next to the extracted, and then click Get Layout Window
Number. After that, the zooms in the extracted view will also zoom in the same area in the layout
allowing quick fixing of the problem areas directly in the layout.
4.2 EM Analysis
Do an EM&IR->ElectroMigration Power Analysis (for nets with pins) or ElectroMigration Signal analysis
(for all other nets), and the following form comes up:
- Select the Assura directory and the Simulation directory.
- Select the analysis (transient/dcOp) and the start and stop time (within the range of the transient
that was simulated)
- Select the type or types of analysis (Average,RMS, Peak,Peak-AC)
- Select one or more pins/nets. It is recommended not to select more than one or two at a time.
- Click OK
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- After a few minutes (depending on size of design), the following form will come up:
Points to note:
- The results can be saved into a file and read in later, saving the recalculation.
- The list can be sorted by clicking on the field name.
- You can open the layout window next to the extracted, and then click Get Layout Window
Number. After that, the zooms in the extracted view will also zoom in the same area in the layout
allowing quick fixing of the problem areas directly in the layout.
Please refer to the ‘Virtuoso Analog VoltageStorm and ElectronStorm’ User Guide present in the IC
5141/IC61 installation for further information on VAVO/VAEO.
