Signal Integrity Analysis in Static Timing Analysis

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What What is Cross Cross Talk Talk analysis nalysis / Signa Signall Inte Integrit grity y analy analysis sis in Stat Static ic Timing Timing Anal Analysis ysis ? - Tech Techn nolog ology@T y@Td dzire

Technology@Tdzire

STA, TECHNOLOGY

WHAT IS CROSS TALK ANALYSIS / SIGNAL INTEGRITY ANALYSIS IN STATIC TIMING ANALYSIS ? DECEMBER 8, 2012 | GAYATHRI | 12 COMMENTS

Let me explain here the basics the basics of Cross talk analysis /Xtalk analysis based on primetime as the STA tool. Crosstalk is the undesirable phenomenon, caused by the cross coupling cross coupling capacitance between metal metal wires in a chip. When the technology shrinks, the effect of effect of coupling coupling capacitance also increases. increases. Please refer to my other post on “coupling capacitance” for fo r the same. Imagine two metal wires W1 and W2. The signal which is passing through through W2 can get affected by affected by the electrical interference of the signal which is passing through W1. Here we tell W1 as the aggressor and W2 as the victim. We can think of two main effects of cross coupling capacitance here. 1. When W2 signal is having a transition transition,, this transition time can be affected by the signal

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What is Cross Talk analysis / Signal Integrity analysis in Static Timing Analysis ? - Technology@Tdzire

will make W2 signal transition faster. If the transition at W1 is in the opposite direction as W2 signal

transition, it will make W2 signal transition slower. ie, the net delays of the

victim net will change with respect to the signal transition in the aggressor net. This should be taken care in the timing analysis for proper timing closure. But keep in mind, that the victim net will get affected, only if the transitions in both the nets happen in the same timing window. Also the delay change in the victim depends on the type of transition (rise or fall) in the aggressor net.See the below figures for more understanding.

2. When W2 is having a constant signal, and when W1 signal is having a transition, it

produces a spike at the W2 signal. This glitch is called crosstalk noise. This is also referred to as bump violation.This glitch if wide and large enough, can get propagated through the logic, and can create logical failure. The bump violations in clock

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How to fix the Crosstalk violations ? These crosstalk violations can be fixed either by upsizing (increasing the drive strength) of the victim, or by downsizing (decreasing the drive strength) of the aggressor. Noise/Bump violations can be fixed by changing the spacing between the nets at the physical design stage.

Some notes on PTSI Signal integrity comes as an added feature of primetime as primetime SI. If you enable the switch si_enable_analysis you can analyse the crosstalk effects. If you enable crosstalk primetime will by default do the timing in on chip variation mode. We need to read the spef which contains the coupling capacitance information into PTSI with the command

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PTSI will calculate the delay changes in the nets due to the cross coupling and will dump the timing reports incorporating these. report_timing ‐crosstalk_delta will provide the delay change value, due to the aggressor nets in the timing reports. You can declare the number of iteration cycles to calculate the arrival window of the aggressor nets. PTSI as the first iteration, assumes that the aggressor nets can toggle any time in a timing window. This is referred to as the infinite arrival window of the nets. This gives the most impact on the victim nets and this analysis is unnecessarily pessimistic. But this first iteration consumes very less time. For the second iteration it calculates the net delays and more accurately calculates the arrival window of aggressor nets. This consumes more time, but will be less pessimistic. Idealy with 2 iteration cycles PTSI gives good result on signal integrity.

How do setting the clock group relationship affect the crosstalk analysis in PTSI/PrimeTimeSI? You can set the clock group relationships as 1. Asynchronous – Means the clocks are totally asynchronous to each other. So for crosstalk

analysis the infinite window arrival is considered between these two. ie, aggressor can change any time in the timing window. 2. Physically exclusive – Means both the clocks will not be physically present at the same

time. So for crosstalk analysis, the effects between these two clocks are neglected. 3. Logically exclusive – Means both the clocks can be present physically at the same time,

but logically they don’t have interactions with each other. ie, timing paths between these clock groups are not valid. But crosstalk effects are computed as normal. Copyright secured by Digiprove © 2013 Rajesh Govind SHARE THIS: 

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12 Comments

tdzire

•

Coupling capacitance can be formed that way too. But it won't be that prominent as between the wires in the same layer. Between different metal layers, the interfacing area will be less due to the orthogonal structure and the distance between layers also is more. Gayathri

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pavanrocks

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can we use Shielding to reduce the crosstalk effect.. and while doing shielding we put two metal wires parallel around the net to which we want to do shielding and we connect the two wires to either VDD or VSS, In which case we shud use VDD or VSS, based on wat considerations we select either VDD or VSS.... I hope you got my question.. •

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tdzire

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Yes, shielding is done for reducing cross talk noise. As per my understanding, Vss is more stable than Vdd and the shielding lines should be connected to Vss. •

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pavanrocks

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hello mam.. can you explain me the purpose of endcap cells,decap cells and filler cels.. from wer do dey take this and wats their funtionality •

tdzire

•

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Yes Cross talk effects can take place between any metal layers or in the same metal layer itself. But its effect is prominent when it is in the same layer. •

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pavanrocks

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thanks mam for the clarification •

pavanrocks

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hello guys i have gone through the post it gives good information, a small doubt, weather the crosstalk effects takes place between same metal layers or differnt metal layers or both.. •

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Manohar •

Hello, I have a question about coupling capacitance again....will the capacitance be formed between any metal layers ?? For example can it be formed between similar metal layers M1-M1. I understand it can be formed between different metal layers M1-M2, M2-M3 etc... Also can the capacitance formed between M1-M5 M2-M6 like that ? •

Manohar •

Thank You

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Gayathri

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When capacitance increase, the time to get charged and discharged will increase. Hence the transition gets affected to that extend. •

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Manohar •

Hello Gayathri, Thanks for the post. I understand the concept of crosstalk analysis, but i just wanted to know how does the coupling capacitance make the transition of the victim faster or slower ? Can you please tell me the concept behind the effect of capacitance. •

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Havisha Tiruvuri

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thanks for posting technology blogs •

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Signal Integrity Analysis in Static Timing Analysis

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