Trace_Impedance-Coupling_Check_QandA

Trace Impedance/Coupling Check Q&A Product Version 16.6July 2013Document Updated on: June 20, 20132013 Cadence Design Systems, Inc. All rights reserved.Cadence Design Systems, Inc. (Cadence), 2655 Seely Ave., San Jose, CA 95134, USA.Open SystemC, Open SystemC Initiative, OSCI, SystemC, and SystemC Initiative are trademarks or registered trademarks of Open SystemC Initiative, Inc. in the United States and other countries and are used with permission.Trademarks: Trademarks and service marks of Cadence Design Systems, Inc. contained in this document are attributed to Cadence with the appropriate symbol. For queries regarding Cadence’s trademarks, contact the corporate legal department at the address shown above or call 800.862.4522.All other trademarks are the property of their respective holders.Restricted Permission: This publication is protected by copyright law and international treaties and contains trade secrets and proprietary information owned by Cadence. 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Cadence does not warrant that use of such information will not infringe any third party rights, nor does Cadence assume any liability for damages or costs of any kind that may result from use of such information.Restricted Rights: Use, duplication, or disclosure by the Government is subject to restrictionsas set forth in FAR52.227-14 and DFAR252.227-7013 et seq. or its successor.Table of Contents1Introduction (4)1.1Purpose (4)1.2Audience (4)2Trace Check Workflow (5)2.1Do I have to use the trace check workflow? (5)3Impedance/Coupling Calculations (6)3.1Trace coupling parameters (6)3.2What is total coupling index? (6)3.3Is trace-to-trace coupling included in impedance calculation? (9)3.4Is trace-to-shape coupling included in impedance calculation? (9)3.5Can trace impedance be determined only thru co-planar cooupling? (9)3.6Are differential trace impedance and coupling check results available? (9)3.7Is trapezoidal cross-section considered in the trace check? (10)3.8Is via impedance/coupling included in the trace check? (10)3.9Is via delay calculated in the trace check? (10)3.10What happens to trace segments without a reference plane (10)3.11Do I need to fix reference discontinuities? (10)4Trace Check Options (11)4.1What are the advantages for running trace check by net groups? (11)4.2Is check by XNets available when using the first two options (11)4.3Why are impedance/coupling plots only available when check by net groups? (11)4.4Is coupling between different net groups included (11)4.5Is coupling for not enabled nets included? (11)5Nets and Net Groups (12)5.1How are net groups formed? (12)5.2Can I use Net Manager to select a few nets? (13)5.3Why is the ground net disabled in the trace check? (14)6View Trace Check Results (15)6.1After viewing the impedance overlay in the layout, how do I change it to the normal layout view? (15)7Trace Check New features (16)7.1ASI 16.61 new features (16)1 Introduction1.1 PurposeThis document provides answers to common questions regarding the trace impedance/couplingchecker in the Allegro Sigrity tools.1.2 AudienceThis document is intended for Allegro Sigrity users who have questions regarding the traceimpedance/coupling checker.2 Trace Check Workflow2.1 Do I have to use the trace check workflow?Yes. It is highly recommended to follow the Trace/Impedance Coupling Check workflow,which controls Net Manager for enabled nets, and gets results ready for viewing and reporting.3 Impedance/Coupling Calculations3.1 Trace coupling parametersYou can set up trace coupling parameters for the impedance/coupling check. The trace couplingparameters will be used as a threshold. The trace couplings below this threshold will be ignored.In order for trace coupling to be included,•The trace near end coupling coefficients must be larger than the given value, and•The coupled trace segment length must be longer than the NEXT saturation length. The saturation length is calculated using the rise time asL saturation=t r∗vThe smaller the coupling % and rise time thresholds, the more trace segments will be consideredas coupled lines. This results in a longer simulation time.3.2 What is total coupling index?The total coupling index can be better explained starting with maximum couplings.Maximum coupling at net levelMaximum coupling to a trace at the net level is reported in the Net Coupling Summary table.Maximum coupling at trace segment levelIn the Coupling Coefficient Plot (collapsed) plot, for each trace segment of a given victim net,the maximum couplings are reported. For most cases, these maximum couplings come frommultiple aggressor nets.In the max coupling screen capture shown below•Max Coupling is selected•for each trace segment, only the aggressor with the max coupling coefficient is shownTotal couplings at trace segment levelThe total coupling is the couplings to a given victim net from all other enabled nets in the coupling calculation. The result is shown in the Coupling Coefficient Plot (collapsed) plot.In the total coupling screen capture shown below,•All Coupling is selected•for each trace segment, all aggressors are shownTotal coupling index at net levelThe total coupling index for a given net is calculated from all its aggressors, as shown in theCoupling Coefficient Plot (collapsed) plot with All Couplings selected, using the followingequation:Total coupling index =���(% coupling ∗coupling lengtℎ )eacℎ aggressor segment �eacℎ victim segmentThe total coupling index is an indication of how tightly a given net is coupled to other netsincluded in trace check simulation.3.3 Is trace-to-trace coupling included in impedancecalculation?No.3.4 Is trace-to-shape coupling included in impedancecalculation?Shapes directly above and below a trace are used as references in impedance and couplingcalculations.Trace couplings to shapes next to a trace on the same layer is called co-planar coupling. In order toinclude co-planar coupling in the impedance calculation, you need to turn on the co-planar options.3.5 Can trace impedance be determined only thru co-planarcooupling?For traces without reference planes, they may depend on the ground shapes on the same layer assignal return. Trace check impedance cannot be determined using co-planar coupling.3.6 Are differential trace impedance and coupling checkresults available?Starting from ASI 16.61, differential impedance and coupling are available. In order to getdifferential trace check results, differential nets need to be classified when forming net groups.3.7 Is trapezoidal cross-section considered in the trace check?Yes. You should use workflow step Check Stackup to set up trapezoidal angles.3.8 Is via impedance/coupling included in the trace check?No.3.9 Is via delay calculated in the trace check?No.3.10 What happens to trace segments without a reference planeTrace segments without a reference plane will be reported in the trace check, but no impedanceand coupling results for the corresponding segments will be available.If you manually add floating planes in the layout, the trace check results will be affected.3.11 Do I need to fix reference discontinuities?It depends. Generally speaking, reference discontinuities are bad for signal integrity. But it is hardto tell whether or not it needs to be fixed without a time domain simulation.4 Trace Check Options4.1 What are the advantages for running trace check by netgroups?There are three trace check options.The major advantages for the Check by NetGroup option are:•Detailed and interactive results are available in both GUI and the trace check report•The impedance and coupling check results are bundled together by net groups, making it easier for comparison•XNets made up of two nets are considered in the trace check4.2 Is check by XNets available when using the first twooptionsNo. XNets made up of two nets are only recognized by the Check by NetGroup option.4.3 Why are impedance/coupling plots only available whencheck by net groups?In order to show interactive results, the Check by NetGroup option uses the Tx and Rxcomponents information indicated by the user. This will automatically handle XNets and createnet groups.The impedance/coupling plots are shown for a given net group, plotted from the Tx component toone of the selected Rx components.With the other two trace check options (Check all signal nets and Check all enabled signalnets), net group information is not used in the trace check. The plots are not available becausethere is no component to start or to end plots.4.4 Is coupling between different net groups includedIf multiple net groups are included in one trace check simulation, couplings among nets in all netgroups are included.4.5 Is coupling for not enabled nets included?No.5 Nets and Net Groups5.1 How are net groups formed?To make it easier to check trace impedance and coupling, net groups are generated automatically.The automatically generated net groups are based on the selected Tx and Rx components whengenerating net groups. The net groups are named by the rule:InterfaceName_TxCompName_RxCompNamesFor example, below is a topology of a memory subsystem:• A group of data signals connect controller CPU1 and memory U0• A group of data signals connect controller CPU1 and memory U1• A group of data signals connect controller CPU1 and memory U2• A group of data signals connect controller CPU1 and memory U3• A group of DDR signals connect controller CPU1 and all four memory components U0 – U3In this case, if•CPU1 is selected as the Tx component•all four DRAMs are selected as Rx components•no net interface is definedfive net groups will be automatically generated as shown below, with NG used as default interfacenames:If the interface names for nets in net group 5 are defined as:•data nets to DRAM U0 with interface name Data0•data nets to DRAM U1 with interface name Data1•data nets to DRAM U2 with interface name Data2•data nets to DRAM U3 with interface name Data3•control nets with interface name Ctrl•address and command nets with interface name Addcmd•clock nets with interface name clkseven net groups will be generated as shown below.Net groups can be renamed using the right-click menu as shown below.5.2 Can I use Net Manager to select a few nets?Yes, but remember to select option 2 in workflow step Setup Trace Check Parameters forchecking only enabled nets.5.3 Why is the ground net disabled in the trace check?After trace check parameters are set up, signal nets to be included in the trace check are enabled inNet Manager. Ground/power nets are not included because they will be used as references, butare not being checked for impedance and coupling.6 View Trace Check Results6.1 After viewing the impedance overlay in the layout, how do Ichange it to the normal layout view?After viewing Impedance Overlay in Layout, you can click workflow step Impedance Overlayin Layout to toggle between Impedance Overlay in Layout and Normal Layout View.Similarly, after viewing Coupling Coefficient Overlay in Layout, you can click workflow stepCoupling Coefficient Overlay in Layout to toggle between Coupling Coefficient Overlay inLayout and Normal Layout View.7 Trace Check New features7.1 ASI 16.61 new features•Differential trace impedance/coupling check•Trace impedance/coupling violation table and highlight in layout•Trace delay。