Direct Synthesis of Hardware Designs Using a SAT Solver

1
Direct Synthesis of Hardware Designs Using a SAT
Solver
Dr David Greaves University of
Cambridge Computer Laboratory http//www.cl.cam.a
c.uk/users/djg
RSP 2004 Geneva
(C) 2004 IEEE Computer Society
2

• System Design is a planning problem
• We deploy resources in time and space
• Meet the design constraints
• Dont use too many resources
• Formal proof of correctness nice to have
• Synthesis from Formal Spec ?
• Good idea provided we can freely mix with
  conventional design approaches
• When designing a transmitter/receiver pair less
  actually needs to be specified.

3
Two Major Approaches

• Stepwise refinement from Formal Spec ?
• Oft been mooted
• Hard to control in practice (mixing ?)
• Ultimately Syntax-directed
• Automatic Assembly of Parts ?
• Much larger search space
• A huge potential spectrum of algorithms
• Modular approach assists engineering.

4
H/W Design Using SAT ?

• SAT solvers today handle
• 1000s of clauses
• 100s of variables
• Take a few minutes to run
• Generate a stream of 1s and 0s that meet a goal
• An FPGA
• Embodies a hardware design
• Is programmed by a stream of 1s and 0s

5
A first approach...

• Define an FPGA in RTL with many free variables -
  the fuses.
• Express a design as any mixture of
• Regular RTL
• Further RTL that refers to nets in the FPGA
• Logical assertions over the nets and their future
  states
• Form the conjunction of the assertions viewing
  the RTL as a macro expansion language.
• SAT-solve the resulting formula quantified over
  all free variable settings.
• Back-substitute the solution into the FPGA
• Simplify the result using logic trimming
  identities
• Disconnected logic
• Boolean identities

6
InitialDesign Flow
7
FPGA Definition
X(s1) XFUN(xs1, s1, s2, , sn, D) X(s2)
XFUN(xs2, s1, s2, ., sn, D)
X(sn) XFUN(xsn, s1, s2, ., sn, D)
XFUN macro expands to a LUT of 2(n D)
fuses. D denotes inputs to this local
structure. Global clock is implied.

• Can use a full crossbar FPGA
• Not constrained by 2-D wiring

8
Future State Projection

• Assertions can refer to
• current state v
• next state X(v)
• future state X(v, n) n gt 2

X(ab, n) is expanded to X(a, n) X(b,
n) X(s1, n1) is expanded to XFUN(xs1, X(s1,n),
X(D, n)) X(D) is not constrained so is further
free vars for universal quantification.
9
Conversion to CNF

• Normally an Exponential Blow Up Problem!
• Avoid with new intermediate variables (nv)

(a b l.r) can be converted with three new
clauses (a b nv) (a !l !r) (!nv l)
(!nv r)
This first new clause can be dropped without
affecting SAT. Preserves NNF.
10
Universal Quantification

• Our assertions must hold for all settings of the
  free variables (current state and current and
  future inputs).
• Form conjunction over all settings
  . A new exponential blow up!
• Also the nv variables need to be skolemised.
• Perhaps some input dont cares or wont happen
  vectors reduce the expansion.
• Divide clauses into disjoint support sets and
  quantify each set separately.

11
Example 1 Two Rail Signaling

• Examples Cambridge Ring, Transputer.
• One bit per baud over two wires
• Here we let SAT chose the protocol

12
Transmitter
//-------------------------- // Transmitter
encoder // Encoder state node bool s0, tx1,
tx2 X(tx1) XFUN("xtx1", din, tx1, s0)
X(tx2) XFUN("xtx2", din, tx2, s0)
X(s0) XFUN("xtxs", din, tx1, tx2, s0) //
Transmitter Constraints - we require a change on
at least //one of the two lines. always tx1
! X(tx1) \/ tx2 ! X(tx2)
13
Channel Model
//-------------------------- // Channel
Model // This two wire channel may be
permanently // swapped and/or inverted in one
half or the other. node bool rx1, rx2,
ch_inv1, ch_inv2, ch_swap X(ch_inv1)
ch_inv1 // These assigns mean that
X(ch_inv2) ch_inv2 // the initial value of
this variable is not known X(ch_swap)
ch_swap // but that it will not change.
X(rx1) ((ch_swap) ? tx1tx2) ch_inv1
X(rx2) ((ch_swap) ? tx2tx1) ch_inv2
14
Receiver
//-------------------------- // Receiver
decoder node bool srx, sry X(dout)
XFUN("xDOUT", rx1, rx2, dout, srx, sry)
X(srx) XFUN("xSRX", rx1, rx2, dout, srx,
sry) X(sry) XFUN(xSRY, rx1, rx2, dout,
srx, sry) node bool match0, match1, match2,
match3, working match1 X(dout,1) din
match2 X(dout,2) din match3
X(dout,3) din node bool xdel0, xdel1
working (xdel1) ? match3 (xdel0) ? match1
match2 always working
15
Solvers Tried

• Chaff Version Spelt 3
• Walksat
• A homebrew BDD package

SAT DIMACS file size approximately 500
kilobytes. All three solvers solved or failed
identically on all problems tried.
16
A Conventional Solution

• As used on CR82.

17
Solution from SAT
18
and with channel inverted
19
Example 2 MFM RLL Coding
// Run length limitation - // max of three
consecutive zeros. always A gt (X(A) \/ X(A,
2) \/ X(A, 3))

• Encoder and Decoder are again simple FPGAs
• Data needs only be transmitted every-other baud
  interval (clock cycle).

20
MFM Receiver Spec
node bool phaser X(phaser)
XFUN("xphix", phaser) always X(phaser)
phaser node bool match0, match1, match2,
match3, match4 match0 din dout
match1 din X(dout) match2 din
X(dout, 2) match3 din X(dout, 3)
match4 din X(dout, 4) working .. as
before always (phaser \/ working)
MFM only needs convey a data bit every other baud
interval.
21
Example 2 Failure

• The MFM design could not be solved
• There is no solution that does not have a start
  up transient ?
• Need a method of modeling system start.

22
StartupIssuesExaminedExample NRZI
Implementation A - One cycle of delay
Implementation B - Not valid at SOD.
23
NRZI Illustation

• Implementation A
• This works, but has one cycle of delay.

• Implementation B
• No solution can be found.
• X(N, -1) cannot be taken to be equal to another
  occurrence of X(N, -1)

24
Adding A Phantom Reset Guard
node bool reset_0, reset_1, resetting
initial reset_0 1 reset_1 1
X(reset_0) 0 X(reset_1) reset_0
resetting reset_0 reset_1
... guts of design are unchanged ... always
resetting \/ working
We can now use a model checker to generate an
expression for SAT solving. The SAT solution
holds in all reachable states. The reset nets are
purely phantoms - they do not appear in the real
hardware. (Also, we might support error-recovery
specifications)
25
Conclusions

• A novel approach for a few hundred fuses.
• But perhaps constrained to small glue-logic
  situations ?
• Designs larger than the dual rail fail during
  conversion to CNF (heap explodes)
• Better algorithms ?
• Use the approach at each tier in a tree ?

Thankyou!