Page 204 - A Practical Guide from Design Planning to Manufacturing

Logic Design 177

Cell-based design (CBD) flows make use of some automation but
allow human designers more input. In these types of designs, structural
RTL is created by hand using only cells from an already prepared library.
This library may be the same as or separate from the synthesis library.
The designer gains more control over the design by choosing the cells
to be used rather than leaving it up to automation. This may allow the
designer to create a smaller and faster implementation. Extra time and
effort are required to generate structural RTL, but automated tools can
still place and route together the cells from the library to automatically
generate layout. This approach requires more effort than automated syn-
thesis but still makes use of a reusable cell library.
Custom designs use the least amount of automation. Acircuit designer
creates by hand a transistor-level circuit schematic, which implements the
desired logic, and a mask designer creates layout by hand to implement
the desired circuit. The circuit schematic has the same basic information
as structural RTL but is not limited to use preexisting cells. Instead, the
circuit designer may create any combination of transistors needed, includ-
ing those that cannot be modeled in RTL. This allows for the highest-
performance circuit but also requires the most care to prevent failures due
to human error in the design process. A mask designer working by hand
can also create layout that combines the different gates of a block more
compactly than an automated assembly of library cells that were created
without the knowledge of how they would be used. However, this means
the layout created will typically be used in this one block alone.
It is possible to design an entire processor using only one of these three
design flows, but many modern processors are a combination of all three.
Different portions of the die use different levels of automation as needed.
Synthesis is best suited to complex “random” logic, like instruction decode,
without regular patterns of which a human designer could take advantage.
These are areas that human designers have the most difficultly with and
in some cases the result of synthesis will be better than any design done
by hand. Synthesis is also typically used where area or speed are not crit-
ical or there are likely to be last minute changes to the logic, since syn-
thesis provides the fastest time from a logic change to new layout.
CBD might be used in functional units like an adder where there are
regular patterns of which a human designer can take advantage. These
functional units are also more likely to limit the processor frequency.
Custom design is best used on memory arrays like caches. In these
blocks, small circuits are repeated thousands or millions of times. Very
small improvements in these circuits have a large impact on the over-
all chip, so these are areas worth optimizing by hand. Custom design
may also be used to improve the speed of the slowest circuit paths. If
these circuits limit the overall frequency of the processor, extra design
time may be worth the effort.