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Cracking the Analog-Design Nut

Since a large percentage of system-on-a-chip (SoC) designs
include some analog circuitry, designers are faced with the problem
of integrating analog design into the predominantly digital SoC
design flow. This has been difficult to do because:

  1. While digital design incorporates a top-down design flow,
    designing analog circuits is basically a bottom-up operation.
  2. An analog block on a chip generally has more dimensions in its
    design space (in other words, has more design variables and
    constraints) than does a digital block, adding to the block's
    design complexity.
  3. For chip development, there are far fewer good analog designers
    than there are digital designers.
  4. For design reuse, digital portions of a chip may use hard or
    soft silicon IP (SIP). Designers only have hard SIP available for
    analog portions of a chip that comprise more than basic functions.
    Process-specific hard IP provides known performance, but cannot be
    modified for slightly different functions or other processes.
  5. Analog/mixed-signal chip-design tools are less mature than
    their digital counterparts.

The design-tool problem is of particular interest since one
tool, logic synthesis, demonstrates the large gap between
chip-based digital and analog design capabilities. Logic synthesis,
which commercially debuted in the 1980s, introduced a completely
new paradigm for digital-chip design. The ability to go from a
chip's architectural description in RTL to a library-dependent
gate-level description allowed digital-chip design to become
top-down. Designers now had an automated mechanism (albeit one that
requires good designer knowledge of the synthesis tool and the
design) for going from an architectural to a structural
representation of a digital chip and the blocks within that
chip.

A complete synthesis tool for generating the analog portions of
a chip is not in mainstream design. Analog synthesis is a much more
difficult problem. Even if you have the correct circuit to use for
a particular analog function, such as a PLL or an ADC, you still
have to optimize many component parameters to meet that circuit's
design constraints. These parameters include component values, such
as resistance and capacitance, as well as transistor sizes and
geometries.

The lack of adequate analog synthesis points out the need for a
good analog-circuit optimization tool for SoC designers—the
optimizing problem is really the limiting factor to higher
analog/mixed-signal design productivity. Circuit configurations for
many analog functions are well known; however, optimizing the
function's circuitry for a particular application can take many
weeks or even months, depending on circuit complexity and design
constraints. Furthermore, once a designer has optimized a circuit's
parameter values, the layout of that optimized circuit is another
lengthy process.

To successfully integrate analog design into SoC chip design, we
need a new breed of analog-design tools, focused on circuit
optimization and physical implementation. There are a handful of
EDA companies working on such tools, including Barcelona Design, ADA, Neolinear, and
Antrim Design Systems.

Some of these companies offer analog/mixed-signal SIP, as do
some SIP/library vendors such as NurLogic,
specializing in communication-centric mixed-signal cores. Barcelona
(Picasso, Dali, Miro, Goya), Antrim (MSS), Neolinear (NeoCircuit),
and ADA (AMS Genius) all have products with some degree of
analog/mixed-signal circuit synthesis. Barcelona (synthesis tools
include floorplan placement now, with routing under development)
and Neolinear (placement and routing with NeoCell) also have tools
for automated analog layout, although the companies' products
different in terms of degree of automation during layout and
complexity of the circuits the tools handle.

Ideally, what SoC designers need for integrating analog
functions on a mostly digital chip are design tools and SIP
for:

  1. Analog synthesis that will produce an optimized circuit based
    on that circuit's design specification. Since there are often many
    different analog architectures for a given function, such as an op
    amp or an ADC, the design tool must also help the designer choose
    the best architecture for a given set of specifications.
  2. Analog/mixed-signal layout that considers complex details such
    as transistor geometry, device matching, and device
    orientation.
  3. A broad range of analog SIP that includes behavior models,
    preferably in analog/mixed-signal versions of Verilog or VHDL,
    along with traditional Spice models. Soft SIP, for all but basic
    analog functions, is still a ways away. All hard SIP must be
    verified on the silicon process that the designer is
    targeting.
  4. Analog/mixed-signal tools must be able to fit into an existing
    SoC design methodology.

While there are not yet commercial design tools available to
completely accomplish all of these needs, the tools that are around
have made progress in solving some of the analog-design problems.
Look for EDA companies such as the ones mentioned in this article
to come out with vastly improved tools for analog/mixed-signal chip
designs within the next 12 to 18 months, if not sooner.

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