Accurate static analysis and complete
coverage of the functional space remain very challenging for mixed-signal
designs. The functional verification of mixed -signal designs has never
been completely possible.
It is very common to use behavioral
models of analog/mixed-signal blocks during the full chip functional
verification stage, and to use .lib timing models
during the physical implementation stage. There are multiple issues and
challenges with this approach:
Creating a .lib
timing model of a complex analog/mixed-signal block is sometimes very difficult
or impossible due to the complexity of the block.
approximate/estimated numbers in a quick timing model leads to a lot of
guard-banding, and acts as a hindrance in achieving the most optimized design.
Having multiple views
of the analog/mixed-signal block (such as design specification, behavioral
model, .lib model, design schematic, physical layout, physical abstract view,
etc.) might lead to erroneous designs, because all the different views are
never verified against each other.
To create a more accurate .lib timing
model of a complex analog/mixed-signal block, the Encounter Digital
Implementation system can generate a Full Timing Model (FTM). These models enable
a more accurate static timing analysis (STA) of timing paths that cross analog
and digital boundaries.
An FTM contains the complete
netlist and the parasitics of the analog/mixed-signal block, which helps the
designer get a "glass box" view of the block. This not only breaks the
complexity of creating the .lib timing model for complex analog/mixed-signal
blocks, but also opens the door to using STA methods for verifying the design.
Full Timing Model (FTM) of complex analog/mixed-signal block
The creation of FTM models becomes much
easier in an OpenAccess-based physical implementation environment, where full
custom analog design is visible in the digital implementation space and vice
versa. The connectivity-driven layout implementation flow in Virtuoso, and the
capability to understand and visualize complete design in the Encounter Digital
Implementation System, enables the designers to use STA on a complete chip with
a full view of signals crossing the analog and digital boundaries.
Due to the abstracted single-level view
of the physical design in the digital implementation environment, designers can
overcome the capacity limitation imposed by the full custom environment and
make use of many other statistical and static analysis tools which are far
faster than SPICE or mixed-SPICE time domain simulations.
For example, one of the most useful
capabilities of the digital environment is signal integrity (SI) analysis,
which uses the worst case "all aggressors" method where all the nets in the
design are analyzed one by one, while assuming that the rest of all the nets in
the design are aggressors. Aggressors are eliminated based on the coupling
capacitance that each of them have with the victim net. This method of SI
analysis is very useful for analyzing top chip-level nets in the design,
including long nets that sometimes run from one corner of the chip to the
other, such as busses. SI also helps analyze the transition time and signal
delays on long nets.