Macro Placement in VLSI Design

Macro Placement in Floorplan

Macro placement is a physical step in the design of the VLSI circuits where the large functional components known as macros including the memory blocks, IP blocks, and the analog blocks are placed in the design layout. Macro placement is also critical as its relationship with other elements defines the impact of a design on performance, congestion, and power.

Objectives of Macro Placement

• Minimize Interconnect Length: Shrink the gap of connected macros with standard cells to improve signal timing and minimize delay.
• Optimize Performance: Place macros to satisfy timing requirements and enhance the general chip performance.
• Reduce Routing Congestion: However, do not use placement strategies that result in dense routing regions in the FPGAs since they will cause signal integrity problems and complicate the manufacturing process.
• Enhance Power Distribution: Also, ensure that the placement of macros complements its power distribution and architecture aspect in minimizing IR drop.

Guidelines for Macro Placement

Minimize Interconnect Length

• Use Fly Lines: On the plane, flylines can be imagined to represent the connectivity of the netlist which would help to indicate the length of interconnections between macros and other cells.
• Cluster-Related Macros: Components that require a lot of communication between them should be positioned in an area with short connecting wires to minimize the delay in signal transmission.   

Place Near Core Boundary 

• Core Boundary Placement: To minimize the routing, those macros that are passing signals frequently to the core or to I/O pads should be located as close to the core boundary as possible to get better signal integrity.
• Peripheral Macros: Place macros that control the external signals close to the chip edge so that routing to I/O pads is easier.
• Do Not Use Notches and Any Kind of Irregular Shape
• Regular Shapes: Be certain macros are not put in closely clustered provisions to form notches or other shapes that are hard to route.
• Contiguous Blocks: Place macros in a row to avoid the connection of many separated diverse macros which would increase the complexity of the design and could make routing tougher.

Maintain Sufficient Channel Width

• Calculate Channel Width: Calculate the number of signal connections and the routing layers to identify the minimum distance between macros needed, according to the DRC.
• Avoid Congestion: Make sure that there is adequate space between the identified macro locations so that the requisite routing channels shall not be crowded.

Use Keep-out Margins

• Define Keep-out Zones: Set buffer zones around macros that nothing other than standard cells or other components of the design can occupy to reduce densities and hence improve QoR.
• Buffer Zones: Provide thermal and power management around some of the active macros using buffer zones.

Balance Power Distribution

• Power-aware Placement: Place macros concerning the power distribution network to reduce the effects of IR drop and implement a good power distribution across…the chip.
• Decouple Capacitors: Locate decoupling capacitors close to high power macros to ensure a stable power supply and diminishing noises.

Consider Thermal Management

• Thermal-aware Placement: Supply macros that produce a high amount of heat all across the surface of the chip to prevent any region from getting extremely hot, therefore preventing the formation of hot zones.
• Heat Sinks and Vias: Incorporate extra heat sinks or thermal via for thermal reasons in case this is essential.

Timing Optimization

• Critical Path Consideration: As for the macros, they should be placed on the critical path where timing requirements need to be fulfilled and where signal signals have to propagate.
• Buffer Insertion: Because of the movements in and out of the buffer, decide on the locations of the buffer insertion points about macros so that enough signal can be achieved to meet the timing requirements.

Use Hierarchical Design

• Hierarchical Blocks: It is essential to split the design into successive blocks, which contain macros; it will help in the parallel work of the design and verification teams.
• Reuse Strategies: Save reuse strategies for the most frequently used macro placements for better workflow.

Iterative Refinement

• Placement Refinement: Thus, it is advisable to use such iterative refinement approaches to modify the macro placement according to the results of the timing analysis, power analysis, and the reports on the congestion of the routing channels.
• Optimization Tools: Use EDA tools specifically in the placement and routing of the macro cells in such a way that the placement results in the best placement layout.

Conclusion

Optimization of macro placement at a global level has a significant influence on the introduction of new designs in the VLSI chip, quality of interconnects, and power consumption. By adhering to the mentioned guidelines and applying a set of macro placement-based manual and automated tools, designers’ operation can maximize the macro placement result that achieves the design objectives and contributes to the improvement of chip quality.