Spectre Tech Tips: How to Perform EMIR Analysis in ADE Using Spectre APS?

Spectre Tech Tips is a blog series aimed at exploring the capabilities and potential of Spectre®. In addition to providing insight into the useful features and enhancements in Spectre, this series broadcasts the voice of different bloggers and experts, who would share their knowledge and experience on all things related to Spectre. Enter your email address in the Subscriptions box and click SUBSCRIBE NOW to receive notifications about our latest Spectre Tech Tips posts. How to Perform EMIR Analysis in ADE Using Spectre APS? is the first blog in this series that takes you through the steps required to perform EMIR analysis in ADE using Spectre® APS. Happy Reading!

As an analog or mixed-signal designer, you would be using Spectre APS in the Virtuoso® Analog Design Environment (ADE) for block-level designs. Lately, you might have seen a need for analysing IR drop and EM currents, and you wonder which Cadence tool to use for analyzing potential EMIR problems in your design. Well, you can use Spectre APS to do just that. Spectre APS provides a powerful transistor-level EMIR solution that uses a patented technology and enables you to perform EMIR analysis with high accuracy.

The common simulation flow in circuit design requires you to first perform transient simulations on your pre-layout design. Once the layout is available, postlayout simulations are performed to check the impact of layout implementation. EMIR analysis is just another step in this transient simulation flow. It reuses the Spectre APS and ADE setup, and requires only a few minor additions for defining and enabling EMIR analysis.

Setting up EMIR Analysis Options in ADE

The Cadence EMIR solution is called Voltus-Fi XL, which uses Spectre EMIR as the simulation engine. While Spectre performs the circuit simulation, calculates the IR drop and EM current values, and stores them in a binary database, Voltus-Fi XL evaluates the EM-related techfile information, compares the calculated currents with the current limits in the techfile, and visualizes the EMIR results in the layout editor. The EMIR analysis options can be specified in the Spectre EMIR/Voltus-Fi XL Analysis Setup form, which can be opened by clicking Setup - EM/IR Analysis in Virtuoso ADE Explorer.

By default, Dynamic (transient) EMIR analysis is selected in the form. You can specify the options for EMIR analysis in the Analysis, Solver, and Options tabs, which build the content of the EMIR config file. This is displayed in the Summary Information section at the bottom of the form. The content is written to the emir.conf file, which is included in the Spectre command line with the +emir command-line option, as shown below.

spectre +aps input.scs +emir=emir.conf

The most common EMIR analysis approach is to perform IR drop analysis on power nets and EM current analysis on power and signal nets. Make sure to connect the power nets to a DC voltage source to enable power net detection, and for obtaining the proper voltage reference for IR drop calculation. To set up the power net IR drop analysis:

1. Click the Select button next to the Net Name field in the Analysis tab and select the power net terminals (VDD, VSS) on the DSPF subcircuit instance in the schematic.
2. Select the max check box next to the IR Drop Analysis field.
3. Click Add/Modify.

The analysis statement is added in the Summary Information section.

For EM analysis setup, repeat the above steps, select the DSPF subcircuit instance instead of its terminals, and select the avg and rms options next to the EM Current Analysis field. The related statement is added in the Summary Information section, when you click Add/Modify.

Selecting a Solver

The Spectre EMIR solution provides two methods for analyzing EMIR. The first method is the direct (default) method which provides SPICE-like accuracy by solving the entire circuit, including parasitics in Spectre APS. The second method is the iterated method that couples a linear solver for the parasitics with Spectre APS for circuit simulation. The iterated method provides higher capacity/performance with minimal accuracy loss (<20% for the top IR drop and EM current values). It is recommended to start with the direct method and move to the iterated method after evaluating the accuracy of iterated and direct methods on the target design. To select a solver for EMIR analysis:

1. Click the Solver tab in the Spectre EMIR/Voltus-Fi XL Analysis Setup form.
2. Select the solver from the Solver Method section.

Once the solver method is selected, the related statement is added in the Summary Information section.

Additional Setup Required for EMIR Analysis

You need to perform the following additional steps to set up EMIR analysis:

DSPF File Setup

The EMIR flow requires a DSPF file to be extracted for the block to be analyzed with EMIR. In addition to the parasitic R and C elements, and devices, such as MOSFET and BJT, the DSPF file contains layer, coordinate, and width/length/area information, which is required for EMIR analysis. However, before including the DSPF file in the simulation, it is recommended that you run the spfchecker utility on the DSPF file to ensure that the DSPF file is free of errors. To run the spfchecker utility:

1. Click the Options tab in the Spectre EMIR/Voltus-Fi XL Analysis Setup form.
2. Click Add and browse to the location where the DSPF file is located to add the file.
3. Click Run to run the utility.

The errors, if any, are reported in the <netlist>.dspf.chklog file. All reported errors must be fixed; otherwise, you may spend hours in EMIR simulation just producing garbage.

The spfchecker utility may also add a few additional options in the Summary Information section. These options are recommended for better mapping between DSPF and schematic netlist.

To include the DSPF file in the simulation:

1. Click Setup – Simulation Files in the ADE Explorer window.
2. In the Parasitic Files (DSPF) section, click Click here to add a file and browse to the location where the file is located to add the file.

In the netlist, the file is included with a dspf_include statement.

Technology File Setup

If the technology file (most common format: ICT) is provided by the foundry and includes the current limits for the EM current analysis, you can include this file in EMIR simulation. If enabled, in addition to reporting the IR drop voltage and the EM current values, it compares the EM currents with the current limits per layer segment, and reports errors where the limit is exceeded.

To include a technology file in the simulation:

1. Click the Analysis tab in the Spectre EMIR/Voltus-Fi XL Analysis Setup form.
2. Add the file in the EM Tech File field located in the EM Rule Check section.

EMIR Analysis Reports

Once the DSPF file and EMIR configuration are set up, run the simulation in ADE Explorer. In addition to the regular ADE output, like voltage or current waveforms, EMIR simulation provides the following additional EMIR text reports:

• DSPF subckt pin current summary (*.rpt_pin)
  • For each pin, maximum, average, and RMS current are reported.
• Power net IR drop report (*.rpt_ir)
  • For each power net tap node, the IR drop voltage value is reported. The reference voltage is the voltage of the connected voltage source.
• Power and signal net EM current report (*.rpt_em)
  • For each net resistor segment, the average and RMS current are reported.
  • If a technology file is provided, the current is compared with the given current limit for the layer segment, and a pass/fail result with percentage is reported.

These reports can be accessed by clicking Results - EM/IR Data - Report in Virtuoso ADE Explorer.

Alternatively, the IR drop and EM current information can be graphically displayed using Voltus-Fi XL in the layout environment. The layout view can be accessed by clicking Results - EM/IR Data - Layout Analysis. For more information, refer to Voltus-Fi Custom Power Integrity Solution XL User Guide.

Related Resources

This blog introduces you to EMIR analysis and focusses on the basic Spectre EMIR flow using the ADE environment. The Voltus-Fi XL/Spectre EMIR flow provides many advanced features, such as static EMIR, Static Power Grid Solver (SPGS) point-to-point resistance checking, power gate handling, signal net IR drop, differential IR drop, what-if analysis, and self-heating analysis. In addition, Spectre EMIR analysis provides powerful options for optimizing EMIR accuracy and performance. For more details on the advanced features or accuracy/performance optimization, refer to the following:

• Spectre Classic Simulator, Spectre Accelerated Parallel Simulator (APS), and Spectre Extensive Partitioning Simulator (XPS) User Guide
• IC6.1.7: Voltus-Fi EMIR Analysis Workshop- The DSPF flow (RAK)
• Spectre EMIR workshop in the Spectre installation (.../tools.lnx86/spectre/examples/EMIR_workshop)

For more information on Cadence products and services, visit www.cadence.com.