In sanity synthesis, we do not consider physical information such as, how cells are placed and how wires are routed. But in production synthesis, physical information has to be taken into account, and DC Topographical mode helps to achieve the goal.
Topographical mode performs coarse placement with congestion-aware routing estimates during synthesis. The resulting RCs in topographical mode correlate closely with post-layout results
Topographical mode requires physical data, including:
- Reference libraries from ASIC vendor, defining standard cells, IP cells, macro cells, IO pad cells information, such as cell size and pin locations
- Technology data from ASIC vendor, such as technology file, TLUPlus files, and Layer Mapping files. Technology file defines routing layers, vias and layout DRC rules; TLUPlus file defines parasitic net RC models; Layer mapping file matches same layers with different names
- Floorplan information from layout team, which constrains placement of standard cells and macros
- Design library created by DC during setup. Design library is a container to hold all physical design and technology data
We will cover how to setup topographical mode in next post.
DC recommends a two-pass synthesis flow, where the 1st netlist is used by layout tool to design the floorplan, and 2nd netlist will be placed and routed based the floorplan. The two-pass synthesis flow is shown below.
Of course, design teams may choose to skip the 1st pass synthesis if the project is evolved from previous generation, and the “legacy” floorplan can be reused.