The bounded problem of placemenr and rout ing within a gate array was easy to solve in com parison to the module designs. H ere the con stra ints p l aced by designers, the l i mi tations of tools, and the complexities of design requi red extensive human i n tervention .
Ana lysis tools were used extensively tO assist in determi n i ng the quali ty of design at the two design leve l s : gate a rrays and m odu les . These tools analyzed such factors as thermal d issi pa t i o n , s i g n a l i n tegri ty, and crossta l k . The con strai n ts defined i n these tools and in t he exten s i v e d e s i g n - r u l e c h e c k e rs w e re m e t , t h u s ensuri ng a h i gh-qual i ty design .
Most of the tools used for the physical design were developed w i t h i n D i g i ta l . Those deve l oped outside t h e VAX 8800 CAD group were modi fi e d , sometimes extensively, to meet the needs of the project.
Physical Design and Man ufacturing Interface
A proprietary physical design system , cal led the VAX layout system (VLS) , was used for the final p hys i c a l design tasks . VLS rook t h e phys i c a l design , a s given b y t h e p l acement a n d rou ting
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tools. and added the data requ ired to manufac ture the design. A layou t designer, through the VLS i nteractive graphics system , could manually complete the rou ting that could not be hand.led by the autOmat i c roo l s . Some add i tional parts that were necessary for fabrication , such as han d les for modules , were also added at this t i m e . The n e t resu lt was a complete design , specified so t ha t it cou l d be used to m a n u fa c tu re t h e product.
The design data was then col lected ro form a release package. To keep track of the formal release of design data. a system cal led POST was deve l oped by the CAD group. POST provided an on-l ine database , which any member of the pro ject team cou ld query ro determine the release status of a design.
Problems Imposed by the
Design Methodology
Up to this point, we have described the basics of the design m etho d o l ogy used to develop the VAX 8 8 0 0 syst e m a n d some h i g h l i gh ts of the CAD t o o l s su p p o rt i n g t h a t m e t h o d o l ogy. As mentioned earlier, t he CAD process was p laced d i rectly i n to the hands of the designers . Thus a tight coupl ing was establ ished between the pro cess of clesign and the design process. This cou p l i n g posed several major probl e m s , as now descri bed , for the CAD group .
Train ing
With direct control of a process or tool given to t he desi gners, t hey a l l now needed extensive t ra i n i n g . O n p re v i o u s p r o j e c t s , o n e h i g h l y knowl edgea b l e i n d iv i d u a l cou l d r u n a roo l ; now, there were 3 0 or so novice users a l l learn ing to use that same too l . Extensive support for those users, in terms of both trainers and docu mentation , had to be provided .
I n most cases t h e designers q u i ckly learned how to u t i l i z e the tools . In a few cases - the placement of modules in particular - p lacement experts were needed owing tO the spec i a l i zed narure of the task. I n sum mary, the extent of the su pport r e q u i red by u s e rs w a s g r e a t e r t h a n anti c ipated .
State Maintenance
The t a s k o f s t a te m a i n te n a n c e proved to be extremely complex owing to the freedom given to designers to make changes at almost any poin t
Digital Technical journal
i n the design process. To ensure that the logical and physica l designs matched , it was necessary to do a complete isomorphic comparison of the physical topology agai nst the logical topol ogy of the design.
Logical Prints
The sche m a t i cs genera ted by the designers a t t h e i r w o r k s t a t i o n s r e p re s e n t e d t h e l og i ca l design, not the physical one . Certa i n features avai lable in the SCAlDSystem tools, such as vcc torized signals and gates, aiJowed it to prod uce a concise representation of the logic. This came, however, at the expense of not putting physical data back onto the print set . For reasons of state maintenance. we were also u nable to restruc t u re a p r i n t set o n c e m a pp e d t o a p h ys i c a l i mplementation . Both these factors contri buted to a print set that appeared q u i te d i fferent from those generated by previous projects .
Logical print sets, w h i l e i n i t i a l l y envisioned as being benefi c i a l , later caused problems i n documenting the design s . This was particula rly true for module - l evel designs for which training was needed so that groups outside the project team cou ld interpret the new symbology.
Cross References
U s i n g l og i c a l p r i n t sets a l o n e , a t e c h n i c i a n cou ld not probe a p i n o f t he p hysica l board s . Since an abstract mapping took place i n t he CAD process. i t was necessary to develop an exten sive set of cross references s howing t he map ping of the logical to the physical design . These cross references proved to be cumbersome and , when printed , consumed vast a mounts of paper.
Libraries
CAD tools run on l i braries, and each major tool h a s i rs o w n fo r m a t f o r l i b r a ry d a t a . T h ese l i braries m u s t be consistent a cross the e n t i re process. Despite a l l the safeguards bui l t i n to the process , we fo u n d t h a t i nc o n s i s t e n c i e s s r i J J crept back i n to t h e database . D iscovering and e l i m i n a t i n g those i n co n s i s te n c i e s , m a n y of which were fou nd late in the project, consumed a lor of time.
Summmy
Both the design methodology and the CAD pro cess su pport i ng t he VAX 8 8 0 0 project were q u i te successfu l . The fi rst protOtype hardware
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delivered r o u s worked a s expected. We fou n d only a sma l l number o f h ardware problems dur i n g the prototype debug phase of the project. Most of those problem s were i n areas that had not had extensive simu lation or t i m i ng verifica tion.
Some genera l conclusi ons reached from the VAX 8800 project can help future CAD design ers to i mprove their tools.
• A close cou p l i ng from the start, both phys i c a l l y a n d o r ga n i z a t i o n a l l y , b e t we e n a l l groups associ ated w i t h the project leads to the development of a smooth process flow.
• The design methodology has a d i rect and far rea c h i ng i m pact on t h e CAD p rocess . T h e capabi l i ties o f CAD tools d i rectly affect t h e design methodology.
• Extensive s i m ulation and t i m i ng veri fication
before fabrication can help to achieve a high quality product.
• The i m pact of radi c a l changes ( e . g . , in the data content of schematics) must be appreci ated and then taken i n to account b y a l l pro ject members .
In future projects w e w i l l focus on reducing the process- loop ti mes and e n hancing the capa b i l ities of the simu lation and t i m i n g verification too l s . I t w i l l be e a s i e r to fu n c t i on in fu t u re design enviro n m e nts, and m o re tools w i l l be p laced d i rectly i n to the hands of the designers. The design methodology w i l l be mod i fi e d to make the reso lution of the design state easier and therefore faster.
References
1 . Structured Computer Aided Logi c Design was developed at Lawrence Livermore Labora t o r i e s a n d a p p l i ed t h e re to t h e design o f t h e S l computer.
2 . C . Wiecek, "The Simu lation o f Processor Performance for the VAX 8800 Fam i ly , " D igital Tec h n ical jo u r n a l ( Fe b r u a ry
1 98 7 , this issu e ) : 1 00- 1 1 0 .
3 . A . M a t t hews . " O n - l i ne M a n u fa c t u r i n g Data Access on t h e VAX 8 8 0 0 Project , "
Digital Te c h n ical jo u rn a l ( Fe b r u a ry 1 98 7 , t h is issue) : 1 3 6- 1 4 1 .
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Andrew]. Matthews