UnBlock and RcBack: Modeling Custom Blocks
UnBlock is a library generator for Structured Custom Layout
Blocks. These blocks are usually handcrafted, don't use an ASIC library,
and usually take advantage of circuit and layout techniques which are
normally avoided in standard cell designs. The main problem with these
blocks is the difficulty of functional and timing verification. Transistor
level timing and functional simulators are the only available tools
designers can rely on. It is difficult to prepare inputs for them,
simulation times are too long and are more buggy compared to standard
ASIC design tools. UnBlock removes this handicap of the custom choice,
by generating a cell library with which the structured custom block
can be simulated with. With a standard cell type library, it becomes
possible to use timing analysis and simulation tools at a higher level
than transistor level.
The user identifies the generic functional units inside the block as ACDL
templates. The structured SPICE netlist is scanned by UnBlock looking
for variations of the generic function, and writes out circuit files for
each of the functional instantiations, and a specific ACDL file for the
new cell. With these two outputs, library generators in Libtech's tool
suite can take over to generate a custom library for the custom block.
RcBack takes as its input an extracted flat netlist along with the CDL
file and splits it into extracted cells identified by UnBlock and DSPF
or lumped capacitance files for the interconnect wiring between the cells.
Back-annotated circuit files can be re-characterized for maximum accuracy
and new libraries can be rebuild using MakeLib. The user can optionally
select to have the interconnect be merged inside the cells, essentially
creating a new cell out of every instance.
UnBlock also generates a new Verilog netlist, which references the cells
in the custom library. With the availability of a gate level library,
one can speed up block functional verification process by orders of
magnitude compared to transistor level tools.
Also timing analysis can be performed faster using the more accurate
Using UnBlock various custom layouts can be modeled, including
memories, state machines, various hard cores etc. By unifying the
verification flow around the gate level simulators, users can save time,
and streamline the design flow, and achieve a uniform accuracy across
the chip. It automatically monitors the changes in the input netlist,
incrementally updating its outputs. It has been used effectively in
porting and verifying a RISC engine, an MPEG decoder, various complex
cells. For memories, it is better to use LibChar directly along with
a fast spice simulator.
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