Bio-RV: Low-Power Resource-Efficient RISC-V Processor for Biomedical Applications

1. Problem Statement

Existing RISC-V processors are generally optimized for either high-performance computing (e.g., BOOM, CVA6) or general low-power embedded use (e.g., Ibex, PicoRV32). However, these designs often fail to meet the specific, stringent requirements of safety-critical biomedical applications, particularly implantable devices like pacemakers. Key challenges include:

• Lack of Explicit Control: Medical ASICs require the ability to halt execution, load external instructions, and monitor memory states without relying on complex debugging hardware or clock gating, which is crucial for post-silicon testing and firmware validation.
• Resource Overhead: High-performance cores utilize deep pipelines, speculation, and virtual memory, leading to excessive silicon area and power consumption unsuitable for battery-operated, always-on implantable devices.
• Determinism vs. Throughput: Biomedical control workloads prioritize predictable, deterministic execution and ultra-low power over raw instruction throughput or instruction-level parallelism.

2. Methodology

The authors propose Bio-RV, a compact, multi-cycle RV32I RISC-V processor designed specifically for biomedical control applications. The design philosophy prioritizes architectural simplicity, minimal resource utilization, and explicit controllability over peak performance.

• Micro-Architecture:

  • Non-Pipelined, Multi-Cycle Design: Bio-RV uses a single-cycle data path executed over multiple clock cycles (5 cycles for load, 3 for branch, 4 for arithmetic/jump/store). This eliminates the complexity of pipeline hazards and reduces switching activity.
  • Von Neumann Memory: It utilizes a unified instruction/data memory to save area and reduce address generation logic, accepting a trade-off in cycles per instruction (CPI).
  • Instruction Set: Supports a basic RV32I subset (R-type, I-type, load, store, jump, and branch).

• Key Architectural Features:

  • Explicit Execution Control: The processor features an external Instruction Enable (IE) signal. This allows the system to halt execution (IE=0) while keeping the clock running, enabling safe external instruction loading and memory observation without accidental execution.
  • External Instruction Loading: A dedicated 32-bit WriteData interface allows direct writing of instructions and data into the internal memory. This supports "firmware deployment" and "post-silicon bring-up" without needing complex debug probes.
  • Pure RTL Implementation: The design is built using standard RTL logic without vendor-specific primitives, DSP blocks, or FPGA-specific memories, ensuring portability from FPGA prototypes to ASIC.

• Implementation:

  • The design was prototyped on FPGAs and synthesized for ASIC using SCL's 180 nm CMOS technology, a mature node often used for mixed-signal and biomedical ASICs.

3. Key Contributions

1. Explicit Execution Control for Medical ASICs: Bio-RV introduces a mechanism to turn execution on/off and load external instructions deterministically. This is critical for the certification, long-term reliability testing, and field diagnostics of implantable devices.
2. Ultra-Minimal Controller-Oriented Architecture: The design sacrifices instruction throughput for deterministic control flow and minimal hardware state. It is optimized for "always-on" tasks where predictability and energy efficiency are paramount.
3. Portable Host Controller for Heterogeneous SoCs: Bio-RV serves as a lightweight host for biomedical SoCs containing domain-specific accelerators (e.g., TinyML, signal processing). It manages configuration, scheduling, and data transfer via high-throughput interfaces (like AXI) while maintaining a small footprint.
4. FPGA-to-ASIC Validation: The paper provides comprehensive validation across both FPGA and post-layout ASIC (180 nm) results, demonstrating the design's viability for safety-critical, resource-constrained environments.

4. Results

The performance of Bio-RV was evaluated against state-of-the-art RISC-V cores and commercial MCUs:

• Resource Utilization (FPGA):
  • LUTs: 708 (Significantly lower than BOOM's ~49k or even compact cores like M RISC-V's ~1.8k).
  • Flip-Flops: 235.
  • DSPs: 0 (No reliance on specialized blocks).
• ASIC Performance (180 nm CMOS):
  • Frequency: 50 MHz.
  • Area: ~697,039 µm² (approx. 0.84 mm²).
  • Power: 15 mW (Dynamic power).
  • Energy Efficiency: 17.18 pJ/cycle. This is significantly better than comparable designs (e.g., Riscy at 63.68 pJ/cycle and Mriscv at 850 pJ/cycle).
• Comparative Analysis:
  • While Bio-RV has a higher Average CPI (5.0) compared to pipelined cores (e.g., 1.27 for Riscy), this trade-off results in superior energy efficiency and a smaller silicon footprint.
  • Compared to the Siwa core (also 180 nm), Bio-RV operates at 2.5× higher frequency (50 MHz vs. 20 MHz) with only a 3.7% increase in area, while offering enhanced controllability features.

5. Significance

Bio-RV addresses a critical gap in the design of implantable medical devices. By prioritizing deterministic execution and explicit controllability over raw speed, it provides a robust solution for:

• Post-Silicon Validation: The ability to load instructions and halt execution externally simplifies the testing and certification of medical-grade ASICs.
• Long-Term Reliability: The simplified, non-pipelined architecture reduces the risk of timing errors and ensures predictable behavior over the multi-year lifespan of an implantable device.
• System Integration: It acts as an efficient, low-overhead host controller for complex biomedical SoCs, freeing up resources for specialized accelerators (like neural networks for signal processing) while managing critical system functions (pacing, telemetry, battery management).

In conclusion, Bio-RV demonstrates that a deliberately constrained, multi-cycle RISC-V architecture is highly effective for ultra-low-power, safety-critical biomedical applications, offering a unique balance of flexibility, low power, and testability that high-performance cores cannot provide.