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Imagine you are days away
from completing the implementation of a fairly complex mixed-signal design, and
you are already day-dreaming about the vacation you have planned in a few weeks.
Then it happens -- the dreaded change in the design requiring ECOs to digital
or analog content, or worse yet, implementation problems that need fixing.
Should you call the travel agent to see if you can still buy travel insurance?
The fact is, for design
engineers, ECOs are a fact of life. Many digital design environments have implemented
fast and efficient ECO flows, which can accommodate pre and post mask ECOs. In the older generation of mixed signal
designs, the analog portion of the design was partitioned off, creating
physical boundaries between the digital and analog content. This made it easier
to incorporate changes in the digital or analog content without worrying too
much about creating problems in the interfaces between the two.
Challenges of ECO Management
Today's mixed signal
designs are implemented using a hierarchical methodology, and it is not
uncommon to find analog blocks residing inside digital functions that control
analog signals. What used to be a purely digital block is now incorporating
such things as power management features, which introduce analog content in
Two of the questions most
often asked in the context of mixed signal designs are:
The implementation of a
mixed-signal design may incorporate both netlist and schematic driven
methodologies. This is where parts of the design are represented by a netlist
while others are represented as schematics. For large mixed signal SoCs, it is very
important that the results from analysis of the top level floorplan be used to
drive implementation of the many blocks in the design, regardless of whether a
particular block is represented by a schematic or a netlist.
In addition, a methodology
is needed to allow for better communication between the top level designer and
those in charge of implementing the many blocks in the design. Without this
methodology, there is always a chance that the design of blocks end up
invalidating the analysis done at the top level.
How OpenAccess Supports ECOs
The Cadence mixed-signal
solution offers a concurrent mixed-signal floorplanning solution, which allows
for efficient, frequent and clear exchange of data between the top level designer
and those designing the lower level blocks. The solution is implemented on the
OpenAccess (OA) database, and allows for the exchange of floorplanning, placement
and routing constraints. In addition, the environment offers the capability to
imbed mixed-signal IP blocks with constraints that are used when instantiating such
IPs within a design, regardless of the environment used for implementing the
top level of the design.
To address the challenges
of performing ECOs on mixed signal designs, Cadence has implemented powerful
ECO functionalities that perform ECOs on the actual OA database,
containing both custom and P&R objects. For example a design containing
P-cells can be ECO'ed in the Encounter environment without the need to separate
out the custom objects in the design. The following figure depicts the pre-mask
ECO flow that implements changes made to the netlist.
The first step in this
flow is to open the pre-ECO database in the Encounter Digital Implementation
System (EDI) using the restoreOaDesign
command. As you can see, there is no need for a LEF technology library,
as a common OA PDK can be used for the EDI and Virtuoso environments,
containing all technology information needed for the mixed-signal
A Verilog netlist is then
extracted from the OA database, which can contain P-cells, if they existed in
the design. The Verilog netlist can now be edited to make the necessary
changes. In EDI, the command ecoOaDesign can now be used to read in the changes
in the Verilog netlist, while maintaining the floorplanning, placement,
routing, and other aspects of the pre-ECO OA database.
If new instances have been
added to the design, and the design takes advantage of low power features, additional
steps are required to repair the power domain, using CPF. At this point the
design is ready to go through ECO placement, or the user can optionally use other
features of EDI to guide placement of the new unplaced cells in the design.
Once ECO routing is completed, and any DRC issues resolved, the user can save
the full OpenAccess database, with ECOs.
Similar flows exist for
post-mask ECOs, as well as ECOs to the OA database, using the Virtuoso
Very good material Ben.