Power Management

Power Management for Battery-Operated Medical Devices

Purpose

Define safe power management patterns: sleep modes, wake sources, power budget, battery monitoring, graceful shutdown, and data preservation.

When to Apply

• Battery-operated or portable devices; low-power modes; brownout handling.
• Adding/adjusting sleep policies or power domains.

Requirements (testable)

1. Sleep/Wake Policy: Define allowed sleep states and wake sources; ensure critical monitoring preserved. Rationale: avoid missing safety events.
2. Power Budget: Establish budget per mode; verify high-load features fit within limits. Rationale: battery life and thermal safety.
3. Battery Monitoring: Monitor voltage/SOC with thresholds; annunciate low/critical; throttle or safe shutdown when critical. Rationale: prevent unsafe operation on low power.
4. Brownout Handling: Detect brownout; enter safe state and preserve critical data; avoid unstable operation. Rationale: prevent corruption/unsafe outputs.
5. Persistent Data Integrity: Use CRC/ECC and atomic writes for retained data; verify on startup. Rationale: data reliability.
6. Wake Handling: Validate wake source; debounce to avoid rapid cycles; reinitialize peripherals safely. Rationale: stable resume.

Recommended Practices

• Isolate safety monitoring from aggressive sleep (keep minimal always-on domain).
• Stagger high-current events; avoid simultaneous radio + motor where possible.
• Log power events and thresholds crossed for diagnostics.
• Test across temperature/voltage corners.

Patterns

Low battery handling:

// REQ-PWR-BATT-01; TEST-PWR-04
void handle_battery(float vbat) {
    if (vbat < VBAT_CRIT) {
        annunciate_low_batt();
        save_state_crc();
        enter_safe_shutdown();
    } else if (vbat < VBAT_WARN) {
        annunciate_low_batt();
    }
}

Brownout interrupt:

// REQ-PWR-BROWN-02; TEST-PWR-06
void BOD_IRQHandler(void) {
    clear_bod_irq();
    save_state_crc();
    enter_safe_shutdown();
}

Atomic retained write:

// REQ-PWR-DATA-03; TEST-PWR-08
bool save_state_crc(void) {
    state_t s = collect_state();
    s.crc = crc32(&s, sizeof(s) - sizeof(s.crc));
    return write_retained_atomic(&s, sizeof(s));
}

Anti-Patterns (risks)

• Deep sleep disabling critical monitoring/alarms -> risk: missed safety events.
• No brownout handling -> risk: corrupted state, unsafe outputs during voltage sag.
• Writing retained data without integrity checks -> risk: silent corruption.
• Aggressive radio/motor use without budget -> risk: brownout or thermal issues.

Verification Checklist

• Sleep/wake policy defined; critical monitoring maintained.
• Power budget documented; high-load features assessed.
• Battery monitoring thresholds set; warnings/critical handling implemented.
• Brownout detection and safe shutdown implemented/tested.
• Persistent data uses CRC/ECC and atomic writes; verified on startup.
• Wake source validation and peripheral reinit implemented.
• Power event logging and corner testing performed.

Traceability

• Use REQ-PWR-### and TEST-PWR-###; link to risk controls for power loss hazards.

References

• IEC 60601-1 (power safety considerations; informative).
• IEC 62304 for safe state handling.

Changelog

• 1.0.0 (2026-01-04): Initial power management guidance with brownout, battery, and data integrity patterns.

Audit History

• 2026-01-04: Audit performed. Verified:
  • IEC 60601-1 reference appropriate as informative for power safety
  • Brownout and battery monitoring patterns technically accurate
  • Atomic write patterns for data integrity are sound