Synthesis from HDL Other synthesis paradigms Introduction to asynchronous circuit design: specification and synthesis Synthesis from HDL Other synthesis paradigms

Outline Disclaimer: this is NOT a comprehensive review Synthesis from standard HDL (Verilog) [L. Lavagno et al Async00] Subset for asynchronous specification Data-path/control partitioning Circuit architecture. Control generation Synthesis from asynchronous HDL (CSP, Tangram) CSP for control generation [A. Martin et al, Caltech] Tangram for silicon compilation [K. van Berkel et al, Philips] Control synthesis using FSMs [K. Yun, S. Nowick] Burst-mode machines Comparison with STGs Disclaimer: this is NOT a comprehensive review

Motivation Language-based design key enabler to synchronous logic success Use HDL as single language for specification logic simulation and debugging synthesis post-layout simulation HDL must support multiple levels of abstraction

Control-data partitioning Splitting of asynchronous control and synchronous data path Automated insertion of bundling delays CONTROL UNIT request DATA PATH delay acknowledge

Design flow HDL specification Synthesizable HDL (data) Control/data splitting STG (control) Synthesis (Synopsys) Logic delays Synthesis (petrify) Timing analysis (Synopsys) Logic implementation HDL implementation Delay insertion

Asynchronous Verilog subset by example always begin wait(start); R = SMP * 3; RES = SMP * 4 + R; if(RES[7] == 1) RES = 0; else begin if(RES[6] == 1) RES = 1; end; done = 1; wait(!start); done = 0; end SMP R R E S RES C.U. start done begin-end for sequencing, fork-join for concurrency, if-else for input choice Only structured mix of sequencing, concurrency and choice can be specified

Controller design flow HDL Syntax-directed translation Petri Net Transformations Reductions Trace Expressions Synthesis Circuit

Trace expressions: example || ;  a b c d e ( a || ( b ; c) ) || (d  e)

Reduction Example a d;a; ( b || f ) f b e c c h g; h;e d g

Transformation: concurrency reduction ; || a b c d f Concurrency in TE: b and f have a common parallel father f b c d

Transformation: concurrency reduction ; || a b c d f f and b are ordered f b ; c d

Synthesis Place-based encoding ( based on a David-cell approach) Transformations to improve area and performance Structural methods to derive a circuit [Pastor et al.] Transactions on CAD, Nov’98

Place-based encoding ER(t1) = 111- ER(t2) = --11 p2+ p1+ p1 p2 1100 0010 p4+ t2 ER(t2) = --11 t2 p3- p4 0001 p4-

Synthesis example: VME bus ldtack+ p2+ p1- LDS+ p8- p11- p3+ lds+ D+ LDTACK+ DSr+ LDTACK- p1+ p2- p7- p4+ p10- dsr+ dtack+ D+ DTACK- LDS- ldtack- p8+ p3- Place encoding p11+ p5+ DTACK+ D- p9- p6- dsr- lds- dtack- p4- DSr- p6+ p9+ p10+ p7+ D- p5-

VME bus spec after transforms ldtack+ p2+ ldtack+ p1- p8- p11- d+ lds+ p3+ lds+ D+ dtack+ dsr+ p1+ p2- p7- dsr- p9+ ldtack- p9- p4+ p10- dsr+ dtack+ ldtack- p8+ lds- dtack- Reductions Transforms p3- p11+ p5+ d- p9- p6- dsr- lds- dtack- p4- p6+ p9+ p10+ p7+ D- p5-

Deriving Next state function z+ z- y- x- y+ p1 p2 p3 p4 p5 p6 p7 000 1-0 1-1 0-1 -0- -1- 010 Next-state function of signal y ?

Deriving Next State function z+ z- y- x- y+ p1 p2 p3 p4 p5 p6 p7 000 1-0 1-1 0-1 10- -01 11- -11 010 Next-state function of signal y ? y = x + z

Conclusion Initial prototype of automated flow without state explosion for ASIC design From HDLs (control / data splitting) Existing tools for data-path synthesis Direct synthesis guarantees implementation (HDL  Petri net, Petri-net-based encoding) Synthesis of large controllers by efficient spec models (Free-choice Petri nets + trace expressions) Exploration of the design space (optimization) by property-preserving transformations Logic synthesis by structural methods Quality of design often acceptable Timing post-optimization can be applied

Synthesis from asynchronous HDL CSP based languages CSP = communicating sequential processes [T. Hoare] Two synthesis techniques based on program transformations [Caltech] based on direct compilation [Philips] Tools are more mature than for asynchronous synthesis from standard HDL Complete shift in design methodology is required

Using CSP for control generation After li goes high do full handshake at the right, then complete handshake at the left and iterate. ro li Q element ri lo STG: li+ ro+ ri+ ro- ri- lo+ li- lo- CSP: *[[li];ro+;[ri];ro-;[not ri];lo+;[not li];lo-] “;” = sequencing operator ro+ = ro goes high; ro- = ro goes low [li] = wait until li is high; [not li] = wait until li is low

Using CSP for control generation *[[li];ro+;[ri];ro-;[not ri];lo+;[not li];lo-] weak ri Production rules: li -> ro+; ri -> ro- not ri -> lo+; not li -> lo- ro li Conflict: ro+ and ro- are not mutually exclusive (since ri+ and li+ are not) Eliminate conflict by state signal insertion (= CSC)

Conflict elimination CSP: *[[li];ro+;[ri];x+;[x];ro-;[not ri];lo+;[not li];x-;[not x];lo-] Production rules: not x and li -> ro+; x or not li -> ro- x and not ri -> lo+; not x or ri -> lo- ri -> x+; not li -> x- ro li x FF not x lo ri

Conclusions Generating circuits from CSP control program is similar to STG synthesis One can be reduced to the other Particular technique may vary. Direct CSP program transformations can be (and were) used instead of methods based on state space generation See reference list for more details

Buffer example in Tangram (a?byte & b!byte) begin x0: var byte | forever do a?x0 ; b!x0 od end a Buffer b passive port * active port Each circle mapped to a netlist ; Q element a T x T b Data path

Summary Tangram program is partitioned into data path and control Data path is implemented as dual or single rail Control is mapped to composition of standard elements (“;” “||” etc) Each standard element is mapped to a circuit Post-optimization is done Composing islands of control elements and re-synthesis with STG can give more aggressive optimization Philips made a few chips using Tangram, including a product: 8051 micro-controller in low-power pager Muna (25 wks battery life from one AAA battery) Similar approach used in Balsa (Manchester Univ., public domain)

Burst mode FSM Close to synchronous FSMs with binary encoded I/O Work in bursts: Input transitions fire Output transitions fire State signals change Mostly limited to fundamental mode: next input burst cannot arrive before stabilization at the outputs s1 b-/x- a+b+/y+ a-/x+y- s2 s4 c-/y+ c+/y- s3

Extended Burst mode Directed don’t cares (b*): some concurrency is allowed for input transitions that do not influence an output burst Conditional guards <b+> = “if b=1 then …” s1 b-/x- a+b*/y+ <b+>a-/x+y- s2 s4 c-/y+ <b+>c+/y- s3

Synthesis of XBM Next state and output functions free of functional and logic hazards Sequential feedbacks should not introduce new hazards State assignment one state of the BM spec to one layer of Karnaugh map compatible layers are merged layers are compatible if merging does not introduce CSC violations or hazards Layers are encoded using race free encoding

XBM and STG s1 s2 s4 s3 x- a+ b+ b-/x- a+b*/y+ y+ <b+>a-/x+y- c-/y+ <b+>c+/y- a- c+ s3 eps y- x+ y- c- y+ b-

Summary Specification: XBM is subclass of STGs Synthesis: techniques are extensions of synchronous state assignment and logic minimization Timing: environment is limited to fundamental mode (difficult for pipelined and highly concurrent systems) internals are delay insensitive See reference list for details