Ultra-Reliable, Low-Power Implantable Medical Devices (IMDs)

Prompt

Background context: You are designing the embedded firmware and hardware architecture for a next-generation Implantable Cardioverter-Defibrillator (ICD). The device uses an ultra-low-power ARM Cortex-M4F microcontroller, running a safety-certified RTOS. It relies on a lithium-silver vanadium oxide battery with a strict energy budget. The scenario: During clinical trials, a edge-case phenomenon is discovered: when a patient experiences a specific type of polymorphic ventricular tachycardia (PVT) alongside high electromagnetic interference (EMI) from everyday appliances, the digital signal processing (DSP) pipeline suffers from periodic register corruption (bit-flips due to localized transient faults), causing the ventricular fibrillation (VF) detection algorithm to intermittently miss the shock window. Exam questions: Part 1 fault-tolerant architecture and memory integrity The system currently uses standard SRAM without hardware Error-Correcting Code (ECC). Due to physical space constraints, you cannot change the MCU. - Propose a software-implemented fault tolerance (SIFT) strategy to guarantee the integrity of the VF detection state machine variables without increasing latency beyond the 50ms deadline. - Quantify the worst-case execution time (WCET) overhead of your proposed software redundancy technique compared to hardware ECC. Part 2 real-time scheduling and energy constraints The ICD must operate for 10 years. The DSP filtering task (\(T_{1}\)) runs every 10ms with a execution time \(C_1 = 2\text{ms}\). The wireless telemetry logging task (\(T_{2}\)) runs every 100ms with \(C_2 = 15\text{ms}\). - If a telemetry sync occurs exactly when the PVT anomaly is triggered—requiring an immediate high-priority shock calculation task (\(T_{3}\), \(C_3 = 8\text{ms}\)) to preempt the system—determine if the task set is schedulable under Rate Monotonic Scheduling (RMS). - Prove whether a deadline miss occurs, and design a dynamic Dynamic Voltage and Frequency Scaling (DVFS) profile that minimizes energy consumption during baseline monitoring while ensuring zero deadline misses during \(T_{3}\) execution. Part 3 hazard analysis and fail-safe modes A complete depletion of the telemetry buffer triggers a watchdog timer (WDT) reset. - Map out the failure mode using Failure Modes and Effects Analysis (FMEA) or a Fault Tree for the scenario where the WDT resets the MCU precisely during the 200-millisecond cardiac vulnerability window (T-wave phase) when a shock command is armed but not yet delivered. - Define the hardware-software boundary architecture required to ensure the high-voltage capacitors safely discharge without inducing an accidental iatrogenic arrhythmia during this reboot loop.