Page 205 - A Practical Guide from Design Planning to Manufacturing

178 Chapter Six

TABLE 6-1 Steps between RTL and Layout
Synthesis Cell-based design (CBD) Custom
Full Autoplaced Autorouted Cell-based Full
synthesis cells cells custom custom
Automatic logic Yes No No No No
minimization
Automatic placement Yes Yes No No No
Automatic routing Yes Yes Yes No No
Reusable cell library Yes Yes Yes Yes No

In summary, synthesis provides the fastest design time but the least
optimized result. By performing all the circuit and layout design by hand,
custom design produces the most optimal result but requires the most
time. CBD provides a middle ground, with better speed and area than
synthesis in less time than full custom design.
Even within a single logic block, levels of automation may be mixed
together. A designer may choose some cells by hand but allow synthe-
sis tools to pick the rest. Each of the steps between RTL and layout can
be automated or performed by hand (see Table 6-1).
To create layout from RTL, first the logic gates to be used must be
chosen. This logic minimization is the heart of automated synthesis
tools. The final layout also requires each of the cells to be placed rela-
tive to the others and wires routed to connect them. Cell-based design
flows may perform these operations automatically or allow the user to
control them. What synthesis and CBD have in common is working
from a reusable cell library. As automation tools become more sophisti-
cated, full custom design is becoming less common, but more use of syn-
thesis and CBD requires larger and more sophisticated cell libraries. The
individual cells that make up the library are typically created using
custom design, so the work of custom design is not disappearing but
instead shifting from creating large functional blocks to cell libraries in
support of synthesis and CBD. In the end, automation will never com-
pletely replace custom circuit design and layout.

Pre-silicon validation
Manufacturing a new processor design is expensive. Masks must be cre-
ated to transfer the layout of the different material layers onto the wafer.
This cost does not change even if only a few test prototypes are needed.
Manufacturing new chips also takes time. Changing from producing one
revision of the design to the next takes weeks or months. This makes it
critically important that the design be tested as thoroughly as possible
before any chips are actually made. This is the job of pre-silicon
validation.