Signoff 介紹——LVF

mzsm 2022-06-22 發(fā)布于湖北

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本公眾號【讀芯樹：duxinshu_PD】主要介紹數(shù)字集成電路物理設(shè)計相關(guān)知識，才疏學(xué)淺，如有錯誤，歡迎指正交流學(xué)習(xí)。

這是集成電路物理設(shè)計的第五個系列【signoff】的第十二篇文章，本篇文章主要介紹LVF相關(guān)內(nèi)容：

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什么是LVF?

LVF = Liberty Variation Format
隨著工藝的不斷發(fā)展，對variation的建模從單一的derate factor到LVF （varriation包含在timing library中）。
LVF數(shù)據(jù)一般應(yīng)用于22nm及以下工藝節(jié)點，這是因為隨著工藝特征尺寸的降低，variation對cell的影響越來越大，這需要更準(zhǔn)確的數(shù)據(jù)對variation進行建模。當(dāng)工藝到7nm-5nm時，variation對cell nominal delay的影響可能占到50%以上。
LVF特性涉及l(fā)ib_cell的SPICE 網(wǎng)表的Monte Carlo分析，LVF使用SPICE仿真的近似結(jié)果，這可以縮短時間，但需要保證LVF的數(shù)據(jù)要接近silicon實際結(jié)果，以快速實現(xiàn)時序收斂。
LVF是對Liberty Format的擴展，加入了statistical varitaiton信息。
LVF包含timing informatin: cell delays, transition times, setup/hold constraints；sigma informatin: early/late statistical variation values；

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為何引入Moments到LVF?

在先進工藝超低電壓下，cell的delay會表現(xiàn)強非高斯分布（strongly non-Gaussian），表現(xiàn)出均值偏移（mean-shift）和偏斜效應(yīng)（skewness）。
為了更準(zhǔn)確地對statistical distribution進行精確建模，需要使用non-Gaussian。
Mean-Shift Extension: the different between the nominal value expressed in Liberty and the mean value of the distribution.
Standard Deviation Extension: representing the standard deviation of the distribution.
Skewness Extension: the third-order moment of a distribution and provides a measure of the symmetry of the distribution.

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HM Library

HM = hold margin
Conventional Timing Library: xxx_{ccs/escs}.lib
HM Timing Library:
File name: xxx_hm_{ccs/ecsm}.lib
For every hold/removal/non_seq_hold arc, constraint value is u+3sigma.
For libraries do not ccontain sequential cells, TSMC do not provide HM timing library.
Used in SBOCV flow to replace fixed hold margin set in clock uncertainty or embedded in .lib by user.

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LVF Library exmaple

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Constraint Variation

Delay/Transition variation is included with following formats
a) Early/Late ocv_sigma tables
b) Moment (mean_shift, std_dev and skewness) tables
Hold constraint variation is included in one of two formats
a) 3sigma variation are added to hold constraints tables
b) variation is stored in ocv_sigma tables
setup constraint variation is included with separate setup ocv_sigma tables