On Timing Closure: Buffer Insertion for Hold-Violation Removal

Pei-Ci Wu

Martin D. F. Wong

On Timing Closure: Buffer Insertion for Hold-ViolationRemoval

DAC’14

Outline

Introduction Preliminaries Linear Programming Based

Optimization Bottom-up Buffer Insertion Experimental Results Concluding Remarks

Introduction

Timing closure, which is to satisfy the timing constraints, is a key problem in the physical design

Setup (long-path) constraints ensure that the signal transitions do not arrive

too late hold-time (short-path) constraints

ensure that the signal transitions do not arrive too early

Typically, hold violations are addressed after setup optimization has been performed.

Discrete cell sizes (i.e. discrete buffer sizes for hold optimization) in modern industrial designs

Cell libraries specified for the setup constraints and the hold-time constraints are usually different in modern industrial designs

Preliminaries

Negative setup slacks and negative hold slacks indicate setup violations and hold violations

TNS the absolute value of the total negative setup

slacks of all the pins in PO THS

the absolute value of the total negative hold slacks of all the pins in PO

TNS must not be worsen during hold-violation removal

Given: a design and a buffer library,

find a buffering solution such that: THS and the cost of buffering (i.e. area and

power consumption) are both minimized while TNS is not worsen.

Linear Programming Based Optimization

Inserting delay into wires to remove hold violations A linear programming formulation Extend such formulation for the complex timing

constraints Graph-reduction approach

Input Combinational circuit C* s.t. for any pin p of C*,

hold_slackp < 0 and setup_slackp > 0 C* can then be represented as a directed

acyclic graph G(V,E) V is the pins of C* (i, j) ∈ E represents an edge

I : the zero in-degree pins O : the zero out-degree pins for each pin i in V

three real-value variables, xi(delays inserted at pin i for hold-time constraints), hai, and sai

Hold-time constraints

For buffer library characterization is necessary in order to get an empirical ratio such that

we assume that the buffer only affects the driver cell and the sink cells of the buffer

Delays introduced by inserting the buffer is

(a) (b)

Setup constraints

Objective :

The setup constraints limit the delays that can be inserted

ri is only necessary when there is no feasible solution

Some pins with positive setup slacks and positive hold slacks that are not included

Graph Reduction

Bottom-up Buffer Insertion

Given: a pin i, hold delay DH and setup delay DS

Find a buffering solution at pin i from a buffer library B: hold delays introduced by the chosen buffers are

as close to DH

setup delays introduced by the chosen buffers are not larger than DS

Minimize the area of the chosen buffers

DP based algorithm A set of buffering candidates C(L, dh, ds, A)

is kept during the process For each buffer in B, we insert it to any of

the existing candidates

New buffering candidates (1) if d′s > DS, C′ is removed immediately (2) if d′h <= dh, C′ is removed as well

d′h > DH + margin where margin is a parameter, then C′ is removed too

(3) C′ is dominated by any existing candidate C*(I*, d*h, d*s, A*) if d′h < d*h and A′ > A*

Chose the candidate that has the largest ratio of dh/A as the buffering solution

Bottom-up Methodology process the pins by the bottom-up topological

ordering (i.e. from PO to PI) DP algorithm cannot realize the exact

amount of hold delays/setup delays by inserting buffers(extra delays)

Suppose now we are processing pin p, collected extra delays cur_setup_reqp = setup_reqp − ds_delay extra delays = cur_setup_reqp – sap

Ds = xp + cur_setup_reqp − sap

Similarly to get Dh

Optimization Flow

Experimental Results

Concluding Remarks

First propose a linear programming based approach that minimizes the number of inserted delays

A bottom-up buffer insertion and the flow of optimizing are presented