As semiconductor technology continues to scale, the complexity of modern SoCs has grown dramatically. Today’s designs integrate multiple processors, heterogeneous accelerators, independent voltage islands, and several clock domains, each with its own performance and power targets. In such environments, floorplanning becomes a strategic, architecture-driven activity that strongly determines downstream success in physical design. A well-crafted floorplan directly impacts timing closure, routing efficiency, power integrity, and thermal behavior.
This blog presents the essential floorplanning techniques for multi-core, multi-voltage, and multi-clock SoCs that every physical design engineer should master.
1. Hierarchical, Architecture-Aligned Floorplanning
A hierarchical floorplan aligns the physical structure closely with the RTL organization, making it easier to manage large designs. Partitioning the SoC into well-defined blocks, such as compute cores, GPU subsystems, memory controllers, and peripheral clusters and it helps to maintain modularity and simplifies integration. High-interaction or high-traffic units should be placed closer together to reduce routing delays and minimize latency. By ensuring early synchronization between the logical and physical hierarchies, engineers gain better predictability and improved control over physical implementation constraints.
2. Multi-Core Placement for Performance and Thermal Efficiency
Multi-core architectures demand floorplans that balance performance and thermal dissipation. Cores are often grouped symmetrically or placed around shared resources like last-level caches or interconnect switches to achieve uniform access latency. Adequate spacing is essential to accommodate power grid structures, clock networks, and local routing without causing congestion. Early thermal estimation must guide placement decisions so that high-power cores do not cluster together or sit near temperature-sensitive analog or RF IP, thereby preserving both performance stability and reliability.
3. Managing Multi-Voltage Domains
Multi-voltage islands improve power efficiency but introduce unique floorplanning challenges. Each voltage domain requires a clean physical boundary to simplify power grid design and reduce IR-drop complications. Shapes should remain contiguous and avoid fragmentation to maintain a robust local power distribution. Special cells, such as level shifters, isolation cells, and retention registers must be allocated space near voltage boundaries to avoid timing overhead and ensure clean domain crossings. For power-gated blocks, additional area must be reserved for power switches and retention structures, enabling efficient low-power operation without compromising performance.
4. Designing for Multiple Clock Domains
Independent clocking structures are fundamental in modern SoCs and must be carefully planned. Logic belonging to the same clock domain should remain physically clustered to limit clock tree complexity and reduce skew. When asynchronous domains are inevitable, the floorplan must reserve space for synchronizers, handshake logic, and clock domain crossing modules near the interface points. For critical domains, locating clock roots centrally helps balance clock insertion delays and supports more predictable timing closure. Proper domain isolation and alignment improve both power efficiency and clock network robustness.
5. Interconnect-Aware Floorplanning
High-speed interconnects such as network on chips (NoC)s, AXI/ACE buses, or proprietary fabrics are critical performance enablers. These structures benefit from centralized placement relative to the major masters and slaves they connect. Routing corridors must be planned early to support high-bandwidth traffic, reducing the chance of congestion during detailed routing. Early global routing feedback helps refine the placement of interconnect switches or routers and identifies hotspots needing adjustments. A fabric-aware floorplan ensures efficient bandwidth utilization and better system-level performance.
6. Power Planning and IR-Drop Optimization
Power distribution becomes increasingly challenging as designs grow in complexity. Macro orientation and block placement must consider local PDN access to ensure clean power delivery. High-current blocks, especially CPU clusters or AI accelerators require wide stripes, additional metal layers, and generous via ladders to meet current density limits. For Multi threshold (MT) CMOS or power-gated designs, space planning for power switches is essential. Decoupling capacitors should be strategically placed across the floorplan to reduce dynamic IR-drop and stabilize voltage during peak activity. These measures help maintain electrical integrity throughout the SoC.
7. Accommodating Clock Tree Requirements
Clock tree design requires foresight during floorplanning to avoid skew issues and maintain optimal timing. Reserved regions or “buffer islands” allow for smooth buffer insertion during CTS without causing placement congestion. Keeping related logic blocks physically close helps reduce clock detours and insertion delays. In high-frequency applications, designers may opt for mesh or hybrid mesh-tree structures; these require additional planning for grid alignment and power support. By integrating clock considerations early, teams ensure fewer surprises during CTS and timing closure.
8. Optimal Macro Placement and Channel Planning
Large macros such as SRAMs, register files, and analog IP significantly shape the overall floorplan. Placing memories near compute blocks minimizes access latency, while macros needing external connectivity are better positioned at the periphery. Consistent channel spacing ensures predictable routing and avoids choke points. Keep-out margins and halos around macros prevent standard cell density from overwhelming routing near macro boundaries, enabling smoother timing convergence and improved routing quality. Proper macro planning directly translates to a more manufacturable layout.
9. Timing-Driven and Congestion-Driven Optimization
Modern tools provide early feedback on timing and routing congestion, even before full placement. Using this feedback, floorplan refinements can be made by adjusting block positions, applying density screens, or adding soft blockages. Iterative ECO-driven improvements help manage long critical paths, unconstrained nets, and regions of excessive routing demand. This proactive approach reduces the need for late-stage, high-effort fixes and enhances overall design predictability.
10. Early Verification and Sign-Off Alignment
A successful floorplan is one that anticipates sign-off requirements from the start. Early EM/IR simulations ensure that power grid structures are adequate. Preliminary timing analysis with virtual clocks helps identify paths requiring attention. DRC considerations guide macro spacing, routing channels, and block boundaries. By validating these elements early, the physical design flow experiences fewer late-stage disruptions and retains schedule and quality targets.
Conclusion
Floorplanning for modern multi-core, multi-voltage, and multi-clock SoCs requires a balance of architectural understanding, physical design expertise, and forward-looking planning. Each decision from domain boundaries to interconnect placement shapes the path to successful timing closure, power integrity, and manufacturability.
With the right techniques and a disciplined methodology, physical design engineers can create efficient, scalable, and high-performance SoCs. At StarVLSI, we are committed, to empowering engineers with the knowledge and hands-on skills required to excel in these advanced design challenges in our Physical design course.
