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Introduction to HW/SW Co-Design

Lectures 8–9 · System Level Design; Co-design and Partitioning

What is HW/SW Co-Design?

Hardware/Software Co-Design is a methodology for designing embedded systems where hardware and software components are developed concurrently, with the goal of optimising system performance, cost, and power consumption.

The Design Challenge

In embedded systems, functionality can be implemented in either:

  • Hardware (HW): Custom logic circuits, FPGAs, ASICs
  • Software (SW): Programs running on processors/microcontrollers

Each approach has distinct trade-offs:

Aspect Hardware Software
Performance Custom parallel datapaths for selected operations Depends on processor architecture, code and workload
Flexibility ASIC logic is fixed; FPGA logic can be reconfigured Software can be replaced within platform constraints
Development cost Hardware design, verification and integration effort Software development, validation and toolchain effort
Unit cost Depends on technology, volume and device resources Depends on the processor, memory and other platform needs
Energy Specialisation can reduce energy per operation Depends on execution time and processor power states
Time to market Influenced by synthesis, verification and fabrication Influenced by reuse, integration and software complexity

These are design considerations, not universal rankings. A hardware accelerator that saves computation can still lose overall performance through transfers and synchronisation. Compare the complete implementation against the same requirements.

Co-Design Flow

The typical HW/SW co-design flow involves:

  1. System Specification — Define requirements and constraints
  2. Modelling — Create a system-level model of behaviour
  3. Partitioning — Decide which functions go to HW vs SW
  4. Synthesis — Generate HW and SW implementations
  5. Co-Simulation — Verify the combined system
  6. Integration — Bring HW and SW together on the target platform

Key Insight

The partitioning step is critical — it determines the fundamental architecture of the system and has the greatest impact on meeting design constraints.

Why Co-Design Matters

Traditional approaches design hardware and software separately, leading to:

  • Sub-optimal partitioning decisions
  • Integration problems discovered late
  • Missed performance or cost targets

Co-design addresses these issues by considering hardware and software as a unified design space from the start.

Next Steps