gervasi timing

8/21/2019 Gervasi Timing

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Mark Sherwood and Associates All rights reserved Copyright JANUAR

When Timing-Is-Everything …

Charge Transfer Model for

Input Signaling & Referencing

!ill "ervasi

#echnology Analystw$gervasi%att&i'co$

(w$gervasi%co$cast'net)


2/25



Agenda

* Review traditional set+p hold ti$e

* -.a$ine i$pact o/ signal slew rate

* Co$pare single ended to di//erential

* hat a&o+t non1$onotonic signals

* 3ooking to the /+t+re* Call to action


3/25



Why a New Model?

* 4aria&le 5re6+ency syste$s e.panding 7 5i.ed /re6+ency syste$s 8+st needed a

yes9no answer on signal integrity 7 4aria&le /re6+ency syste$s need $ore:

calc+lated &ased on availa&le data eye

Availa&le operating /re6+ency ;

< (re6+ired data eye = crap)


4/25



Traditional Input Timing Model

>N?U#

Set+p @old

C3CB

C3CB

#i$ing derating &y inp+t signal slew rate:


5/25



!ro"lems with Traditional Model

* Hoes not acco+nt /or di//erences &etween

single ended and di//erential

* Ass+$es set+p hold re$ain &alanced* No analytical way to pro8ect derating

* No g+idance to package developers

* No $echanis$ to handle non1$onotonicsignal transitions

* 3eaves i$portant picoseconds wasted


6/25



Charge Transfer Model #CTM$

>N?U#

Charge #rans/er #i$ing Model 7 All inp+ts have a slew rate dependent aspect t-I# and

an independent aspect t>N# 7 S+$$ing t-I# = t>N# gives inp+t transition ti$e t#

7 #ransition ti$e t# has $in and $a. val+es

7 Hi//erential inp+t transitions inherently di//erent

t>N#t-I#

t#

T h e N e w

W a y


7/25



%actors Affecting Transition

* -.ternal /actors that are edge ratedependent 7 ?ackage parasitics 7 >np+t stage < characteristics incl+ding

inp+t type sat+ration

* >nternal /actors that are edge rate

independent 7 >nternal loading $is$atch 7 n1chip ro+ting 7 ?rocess, voltage, te$perat+re

t & ' T

t I N T


8/25



Standards (ersus !roprietary

* C#M was developed inside J-H-C /or HHR2

* Standards re6+ire an envelope within which $+ltiple

s+ppliers specs operate, there/ore:

7 > +se 4R-5 as an acc+$+lation start threshold

7 > +se 4>3AC and 4>@AC as sat+ration thresholds

7 #hese do not represent all inp+t types, but

7 #hey do de/ine a standard way to de/ine the envelope

* ?roprietary syste$s analyses co+ld +se real

thresholds /or increased application speci/ic acc+racy


9/25



t'T for Slow Slew Rate) Single nded) simple model

VREF

VIHAC

=

VREF

+ VSAT

VIHDC

VILDC

VILAC

=

VREF

- VSAT

t'T

slew

At

T

EXT

*2=

tINT

tT

AT * Charge

to Transition


10/25



t'T for %ast Slew Rate) Single nded) simple model

VREF

VIHAC

=

VREF

+ VSAT

VIHDC

VILDC

VILAC

=

VREF

- VSAT

t'TtSAT

ASAT * Charge

to Saturation

AA++ * Charge

after Saturation

AT * ASAT , AA++

SAT

SAT T

SAT EXT

V

A At t

)( −+=

(SAT * Saturation

(oltage ffset

tINT

Internal factorstT


11/25



t'T for Slow Slew Rate) +ifferential) simple model

VREF

VIHAC

=

VREF

+ VSAT

VIHDC

VILDC

VILAC

=

VREF

- VSAT

AT * Charge

to Transition

t'T tINT

tT


12/25



t'T for %ast Slew Rate) +ifferential) simple model

VREF

VIHAC

=

VREF

+ VSAT

VIHDC

VILDC

VILAC

=

VREF

- VSAT

AT * ASAT , AA++

t'T tINTtT


13/25



Applying CTM to Setup & -old

* Relate inp+t signals (data, address,

co$$and) to re/erence (clock, stro&e)

* Assert t#$a. and t#$in characteristics* Ass+$e inp+ts and re/erences $ay have

di//erent inp+t slew rates

* +tco$e is a $atri. o/ deratings


14/25



CTM Setup Time +efinition

>N?U#

Set+p

C3CB

C3CB

Set+p ; t#$a. o/ inp+t 7 t#$in o/ re/erence

t#$a.

t#$in


15/25



CTM -old Time +efinition

>N?U#

@old

C3CB

C3CB

@old ; t#$a. o/ re/erence 7 t#$in o/ inp+t

t#$in

t#$a.


16/25



-ow does this help.?

#he Charge #rans/er Model gives a higher acc+racy /orset+p and hold relationships

>t also provides a way to acc+rately descri&e derating/or inp+t slew rate

Charge #rans/er Model gives a $ethod /or analyFing

non1$onotonic transitions


17/25



/ample0 ++R1 Input +erating Ta"les

2'00'

0'


18/25



+erating 2sing Charge Transfer

* Users contin+e to +se 4R-5 crossing /or single endedand crosspoint o/ di//erential pair /or $eas+re$ent

* Herating ta&les correlate $eas+re$ent to re6+iredset+p hold ti$es

* !oot1ti$eK slew rate calc+lations $ay &e needed,e'g' $od+les st+//ed and total loading

∴#he Charge #rans/er Model red+ces syste$develop$ent cost &y ena&ling $ore co$ple. ti$inganalysis


19/25



Non3Monotonic Transitions

* 3ine re/lections in co$ple. syste$ loading or end1

+ser con/ig+ra&le syste$s ca+se shelves or ring&ack,

/or e.a$pleG

Memory

Controller

Hata &+s


20/25



Charge Transfer Model for Non3Monotonic Transitions

VREF

VIHAC

=

VREF

+ VSAT

VIHDC

VILDC

VILAC

=

VREF

- VSAT

t'T tINT

tT

Accumulated chargereaches m(3ps needed

to trigger input

Ring"ac4 hereshould not matter


21/25



Ring"ac4 "elow accumulation threshold

VREF

VIHAC

= VREF

+ VSAT

VIHDC

VILDC

VILAC

=

VREF

- VSAT

t'T

A5

A1

A6

Charge Accumulation *

A5 7 A1 , A6

tINT

tT


22/25



Ne/t 8eneration Models

* CharacteriFe and co$&ine inp+t types

7 here t-I# is the sa$e and only t>N# is di//erent

* More co$ple. geo$etrical area calc+lations

* More acc+racy in acc+$+lation sat+ration points

* Non1linearities in charge acc+$+lation

* Model /or gro+nd &o+nce

* Model /or power s+pply noise


23/25



Summary

* Charge #rans/er Model separates inp+t signalcharacteristics 7 Slew rate dependent

7 Slew rate independent

* Single ended and di//erential deratedi//erently

* Matri. derating ta&les needed to handle C#M* Non1$onotonics can &e dealt with


24/25



Call to Action

* -.tend >!>S $odels with C#M* Need logic analysis tools /or calc+lating

charge acc+$+lation 7 Acc+$+lation threshold 7 Sat+ration threshold 7 Co$prehend set+p hold ti$ing derating

* $41ps acc+$+lation area

* 3ine +p or $ark t#($in) t#($a.)* Single ended vers+s di//erential acc+$+lation

* 3ooking ahead 7 Algorith$ic acc+$+lation $odel


25/25



Than4 9ou

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