Firmware Design
Summary
The Firmware Design document defines the low-level software that directly controls hardware components in your medical device. This document covers embedded software architecture, real-time requirements, hardware interfaces, and safety-critical firmware functions essential for reliable device operation.
Why is Firmware Design important?
Firmware design documentation is essential for patient safety because firmware directly controls hardware functions and real-time operations that can immediately affect patient care. Poor firmware design can lead to device malfunction, timing failures, or hardware control errors that compromise patient safety. Regulatory authorities require detailed firmware specifications to ensure devices operate reliably and safely under all conditions.
The firmware design also demonstrates that you have systematically considered real-time constraints, hardware safety, and fault handling. This documentation supports risk management, software verification, and hardware-software integration throughout the development process.
Regulatory Context
Under 21 CFR Part 820.30 (Design Controls) and IEC 62304 (Medical Device Software):
- Firmware specifications must be documented as design inputs
- Software lifecycle processes per IEC 62304 for firmware development
- Risk management for firmware hazards per ISO 14971
- Cybersecurity considerations for connected firmware
Special attention required for:
- Real-time safety requirements and timing constraints
- Hardware abstraction layer documentation
- Firmware update mechanisms and security
- Integration with higher-level software systems
Under 21 CFR Part 820.30 (Design Controls) and IEC 62304 (Medical Device Software):
- Firmware specifications must be documented as design inputs
- Software lifecycle processes per IEC 62304 for firmware development
- Risk management for firmware hazards per ISO 14971
- Cybersecurity considerations for connected firmware
Special attention required for:
- Real-time safety requirements and timing constraints
- Hardware abstraction layer documentation
- Firmware update mechanisms and security
- Integration with higher-level software systems
Under EU MDR 2017/745 and IEC 62304:2006:
- Firmware must comply with IEC 62304 software lifecycle processes
- Software safety classification (Class A, B, or C) applies to firmware
- Risk management per EN ISO 14971 for firmware-related risks
- Essential Requirements compliance for firmware safety functions
Special attention required for:
- Software safety classification impacts firmware documentation requirements
- Real-time performance verification and validation
- Firmware security and update mechanisms
- Hardware-software interface verification
Guide
Understanding Firmware Design
Firmware is low-level software that directly interfaces with hardware components and provides the foundation for higher-level software applications. Firmware typically includes:
- Hardware Abstraction Layer (HAL): Software interface to hardware peripherals
- Real-Time Operating System (RTOS): Task scheduling and resource management
- Device Drivers: Software that controls specific hardware components
- Communication Protocols: Low-level data transmission and reception
- Safety Monitoring: Watchdog timers, fault detection, and error handling
Firmware Architecture
Document your firmware structure systematically:
- System Architecture: Overall firmware organization and module relationships
- Task Structure: Real-time tasks, priorities, and scheduling requirements
- Memory Management: RAM and flash memory allocation and usage
- Interrupt Handling: Interrupt service routines and priority levels
- Communication Interfaces: Hardware communication protocols and data flow
Real-Time Requirements
Firmware must meet strict timing requirements:
- Response Times: Maximum allowable delays for critical functions
- Task Priorities: Priority assignment for real-time tasks
- Interrupt Latency: Maximum time to respond to hardware interrupts
- Scheduling Requirements: Task scheduling algorithms and timing constraints
- Deterministic Behavior: Predictable timing under all operating conditions
Hardware Interface Specifications
Document how firmware controls hardware:
- Peripheral Configuration: Setup and configuration of hardware peripherals
- Register Mapping: Memory-mapped register access and control
- Signal Timing: Hardware signal timing requirements and constraints
- Power Management: Low-power modes and power state transitions
- Fault Detection: Hardware monitoring and fault response mechanisms
Safety and Reliability Features
Firmware must implement safety mechanisms:
- Watchdog Timers: System monitoring and automatic reset capabilities
- Error Detection: Hardware and software error detection mechanisms
- Fault Handling: Response to hardware faults and error conditions
- Safe States: Default safe operating modes during fault conditions
- Data Integrity: Memory protection and data validation mechanisms
Development and Testing Considerations
Plan for firmware verification:
- Development Environment: Tools, compilers, and debugging interfaces
- Testing Strategies: Unit testing, integration testing, and hardware-in-the-loop testing
- Code Coverage: Testing coverage requirements for safety-critical functions
- Version Control: Firmware versioning and configuration management
- Update Mechanisms: Secure firmware update procedures and rollback capabilities
Example
Scenario: You’re developing firmware for an insulin pump that must precisely control motor-driven insulin delivery while monitoring battery levels, detecting occlusions, and communicating with a smartphone app. Your firmware must ensure patient safety through reliable real-time operation.
Firmware Architecture Overview
The insulin pump firmware is built on a real-time operating system (FreeRTOS) running on an ARM Cortex-M4 microcontroller. The architecture consists of five main modules: Motor Control, Safety Monitoring, Communication, User Interface, and Power Management. Each module operates as separate tasks with defined priorities and inter-task communication through message queues.
Task Structure and Priorities
| Task | Priority | Period | Function |
|---|---|---|---|
| Safety Monitor | 1 (Highest) | 100ms | Occlusion detection, battery monitoring, fault detection |
| Motor Control | 2 | 50ms | Insulin delivery control, motor positioning, flow rate regulation |
| Communication | 3 | 500ms | Bluetooth communication with smartphone app |
| User Interface | 4 | 200ms | Button input processing, display updates, alarm management |
| Power Management | 5 (Lowest) | 1000ms | Battery monitoring, low-power mode transitions |
Hardware Interface Implementation
Motor Control Interface: PWM signal generation at 20 kHz for stepper motor control with microstepping capability. Position feedback through optical encoder with 1000 pulses per revolution. Motor current monitoring through ADC for stall detection.
Sensor Interfaces: Pressure sensor monitoring via 12-bit ADC at 100 Hz sampling rate for occlusion detection. Battery voltage monitoring with 0.1V resolution. Temperature sensor for environmental monitoring and compensation.
Communication Hardware: UART interface to Bluetooth module at 115200 baud with hardware flow control. SPI interface to external flash memory for data logging at 8 MHz clock rate.
Safety and Fault Handling
Watchdog Timer: 500ms timeout with automatic system reset if not refreshed. Safety monitor task refreshes watchdog every 100ms during normal operation.
Occlusion Detection: Pressure sensor monitoring with 3-sigma threshold detection. Motor stall detection through current monitoring. Automatic delivery stop and alarm generation upon detection.
Battery Safety: Low battery warning at 20% capacity, critical battery alarm at 10% capacity with delivery suspension. Battery voltage monitoring every 100ms with trend analysis.
Fault Response: Three-level fault classification (Warning, Alarm, Critical). Critical faults result in immediate delivery stop and safe state entry. All faults logged to non-volatile memory with timestamp.
Real-Time Performance Requirements
Motor Control Timing: Motor step commands must be generated within ±1ms of scheduled time to maintain delivery accuracy. Maximum jitter of 100μs for consistent flow rate.
Safety Response Time: Occlusion detection must trigger delivery stop within 200ms of threshold crossing. Battery critical alarm must activate within 500ms of detection.
Communication Latency: Bluetooth commands must be processed within 1 second. Status updates transmitted every 5 seconds during active delivery.
Memory Management
Flash Memory Usage: 256KB total with 180KB for application code, 32KB for bootloader, 32KB for configuration data, and 12KB reserved for firmware updates.
RAM Allocation: 64KB total with 32KB for RTOS heap, 16KB for stack space, 8KB for communication buffers, and 8KB for data logging buffers.
Non-Volatile Storage: External 1MB flash for delivery history logging with wear leveling. Configuration parameters stored in internal EEPROM with redundancy.