Max CAP and Max Trans violations Why do we check "Max CAP and Max Trans violations" only in Worst case corner, most of the times. Hi Vikram, Let us think when are the max tran and max cap the worse? I hope the slew becomes worse in worst corner rather than in best corner. I am not sure about max cap. Re: Max CAP and Max Trans violations Hi, It's true. we have to calculate the possibility of (worst case)max cap and max trans violation then its easy to design the best skew circuits.. Can you throw some light on this.
Slew can become worse at any corner, it mainly depends on the CAP or Load that it is seeing.
In that case will CAP or load be different in MAX and MIN corners for the same net. Slew will be worst in worst corner. In worst corner, Id current flowing through the cell will be very less due to operating conditions. When Id (drain current or drive) is less, it takes longer time to discharge/charge Cout (output cap of cell) + Cin (input cap of next cell connecting to). Since Id is less, delay is more. This delay is cumulative as it passes from cell to cell and results in worst slew.
I believe Max cap/Max load is constant across the corners. I never heard of operating temperature effecting a capacitance or load value. Correct me if iam wrong and justify your answer. Why the ASIC designer checks the max cap & max trans violations? If you look at the liberty file, the timing value is define in an array, function of cap & trans. If the cap & trans is outside the value define for this array, there is a violation and the tool need to extrapolate the timing value with an internal formula.
But this internal formula is not "perfect" and for huge violation the error/difference with the "real" timing could made an issue. So for each cap & tran violations, the designer must check the possible impact on the timing and fix them if necessary via ECO step. Did you any time fix trans/cap for min corner, while doing Place and route? I fix the max cap/trans violations in min corner when the violation occurs on a signal/net/pin which is timing concern. Other wise, I don't care. So, I'm parsing the list to justify or not an ECO fix or not. This lists are part of our methodology list checks before tapeout. Hi rca,
I want to check with you, let say a net is flagging for max cap violation, which the buffer not able to driver the load of the net, normally we will upsize the buffer, but what will be the impact to a circuit design if we dont fix this max cap violation? How do we really check the impact on this max cap? etc is that to check on RV of the net and so on? The max cap indicates that the output timing arc does not exist in the table in the liberty file and need to be recalculate with the derivative formula. Higher the violation is higher the relative error (on the timing) will be. Then on this particular net you could check the setup/hold time margins you have to be more confident. Of course if the cap violation is relatively small (less than 10%), and the setup/hold has some margin, you could treated this violation has harmless. ---------------------------------------------------------------------------------------------------------------------
maxtran
In the past forum article, there is the related topic, but still here I post the quesion.
The flow I use encounter: 1. optDesign -preCTS there are some maxCap and maxTran violations here.
2. optDesign -drv after fix, there is no maxCap and maxTran violations here 3. CTS 4. reloading timing and set timing derate 5. optDesign -postCTS there is no maxCap and maxTran violations here 6. optDesign -postCTS -hold I found if I set the setuptargetslack and holdtargetSlack too much, there will generate much maxCap and maxTran violations. So I decrease the setuptargetslack and holdtargetSlack . And there is no maxCap and maxTran violations after this step. 7. globalDetailRoute After this step, when I use timeDesign, there existing much maxCap and maxTran violations. Then I use fixDRCViolation to fix them, but after fix them, the setup and hold timing worsen. But if I assure the setup and hold timing, there will be many maxCap and maxTran violations.
Is there some errors that I have made? Or is there some command option that I set wrong? How can I fix such problem.
Hi ,
After routing is performed , instead of using fixDRVviolation command , use optDesign -postRoute -drv option to fix transtion and cap violations. It will help you to much extent with out effecting set up and holld slack. --------------------------------------------------------------------------------------------------------------------soc encounter violations
I am doing a project can anyone help me how to fix total DRVs
After doing post route optimization i got total DRVs as ( max fanout, max cap, max transition)= ( 98,38,0) and real DRVs are (0,0,0) For cap violations,check the drv report & select the pin which has cap violation in the GUI. Now, look for the instance which is driving thsi high cap pin. Try to upsize this instance & you should see some cap violation improvement..
If you want to reduce max_cap violations you can set max_tran to a lower value, re-optimize, then set you max_tran back to original value and run drv check again...they should either be gone or greatly reduced. Note, when I say re-optimize, I mean run drv fixing again. But these max cap violations are external which means it is there in the i/o pad so how can i upsize that and downsize that? --------------------------------------------------------------------------------------------------------------------------------
How to fix the design rule violations in clock tree synthesis using Encounter?
How can we fix the design rule violations in clock tree synthesis especially using encounter? while doing timing analysis you have option in the form where in one can optimize drvs especially. enable that switch. May be that can fix the problem to an extent. The Transition viol is related to setup time. i,e to say, tht wen u fix the setup violations, the Transition violation gets fixed up to an extent
I assume that fixing DRC (as u mentioned ) is for the clock path !!
The CTS specification file has to be written properly to fix the violations. the options like 1) buffer transition
2) sink transition 3) MaxSkew 4) NoGating 5) NO Buffer ..... all have to be provided exactly.
cos the options like "buffer type" and "buffer transition" values define the length & delays of the net(in Clock path) which the tool takes while performing the DRC checks ... !!! hi, the first thing is to check why encouter not fix such vios. as I remember, false_path can lead to this hi i give an example i have met i use astro to do cts
the command window displays like CTS: Total number of transition violations = 2 CTS: Total number of capacitance violations = 0
i only know there are 2 VIOs even dont know what and where are them
so how should i do to fix them i dont use astro, but the ability to generate cts reports should be similar. violations should show in the beginning.
What are constraints given to PD for CTS....
Hi Priya_j, DRV violation include three things 1.Max Cap 2.Max Tran 3.Max Fanout First, one should fix Max cap, that you can do by including Max Cap statement in cts specification file.You assign cap value for a buffer by using this statement.Cap value you decide by looking int SDC. Even you have Max Fanout statement in cts spec file,you give fanout value by looking into SDC and libraries. After Max Cap is fixed mostly you will not have trans violation. try fixDRCViolation -maxCap -maxTran -maxFanout after CTScommand in encounter console As everyone has stated, if your are getting drv violations during cts then you need to tighten your cts constraints. What do you have set for transition/slew? --------------------------------------------------------------------------------------------------------------------
max cap violation
CAN ANY ONE EXPLAIN ME ABOUT THE MAX CAP AND MAX TRANS VIOLATIONS THAT OCCUR IN DESIGN AND HOW THEY ARE FIXED BY TOOLS ? There are different ways to fix max cap and max trans violations. I have listed few of them
1. cloning is done ( if possible ) 2. upsizing the cells 3. buffer insertion CAN U SPECIFY THIS WITH AN EXAMPLE OR ANY DOCUMENT RELATED TO THAT fix max transition violation
Slew violations (Max trans viol): maximum transition time for a net is the longest time required for its driving pin to change logic values .
Max cap violations(Load violations) The maximum capacitance is a pin-level attribute used to define the maximum total capacitive load that an output pin can drive. That is, the pin cannot connect to a net that has a total capacitance (load pin capacitance and interconnect capacitance) greater than or equal to the maximum capacitance defined at the pin.
DC/Back end tools will find the max value from the library. If design is having more than the limit on lib for particular net, its a violation.
How to fix them : 1) Size down the sink 2) buffer the net 3) resize source and lot more .. Thanks ,
But i have few more ques , like if u upsize the buffer the driving strength will increase and it can drive a better load ( so max trans fixed on that net ) , so is this heplful in fixing max cap violations also , or tool have to do cloning and decloning for fixing max cap violations.
And how does downsizing of sink helps, means if i dwnsize the sink i ll probably decrase the i/p cap seen by the previous driving pin , so is this fixing max cap violations by doing this. sink max tran
i am clueless ..
what is cloning? upsizing n down sizing og buffer r also used for fixing timing violation .. DRC r also fixed with same??? i am nt clear with this max_cap violation
Cloning is basically grouping the similar cells together , that r talking with similiar domain , upsizing and down sizing r used for fixing timing violaitons , that i correct , but they can also be used to fix max trans violations as if we upsize a buffer , it can drive a better load and max trans can be fixed , but to fix max cap this is also used but dnt know how ? Also for fixing max cap violations tool jus divide the load by adding buffer and instead of one buffer driving larger load we have more no of buffers driving that particular load but it is distributed. Re: MAX CAP AND MAX TRANS VIOLATIONS
Hi, I know that buffering will help in solving the slew violations, but there should be limit on number of buffers added. what is the limit? what is the relation between the slew and the number of buffers added? MAX CAP AND MAX TRANS VIOLATIONS
according to me you are free to buffer as long as that particular path still meets timing(setup).
