7.4 Exception handling in embedded systems

Exception handling in embedded systems is crucial for maintaining stability and reliability. It involves managing unexpected events that disrupt normal program flow, such as hardware interrupts, software interrupts, and traps. Proper handling ensures smooth operation and recovery from errors.

Implementing robust exception handling mechanisms requires careful design and testing. This includes setting up exception vector tables, writing efficient handlers, and incorporating system protection techniques like watchdog timers and memory protection units. These measures help create fail-safe systems that can gracefully handle errors.

Exception Handling Fundamentals

Types and Causes of Exceptions

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• Exceptions are events that disrupt normal program flow and require special handling
• Types of exceptions include hardware interrupts (external events), software interrupts (special instructions), and traps (error conditions)
• Hardware interrupts are triggered by external devices (timers, peripherals) and handled by interrupt service routines (ISRs)
• Software interrupts are explicitly invoked by the program using special instructions (system calls, breakpoints)
• Traps are caused by error conditions during program execution (division by zero, invalid memory access, undefined instructions)

Exception Handling Mechanisms

• Exception vector table stores the addresses of exception handlers for each type of exception
• When an exception occurs, the processor saves the current state, looks up the appropriate handler in the vector table, and jumps to the handler code
• Exception handlers, also known as fault handlers, are special routines that execute when an exception occurs
• Handlers determine the cause of the exception, perform necessary actions (logging, cleanup), and decide how to recover or terminate the program
• Common recovery mechanisms include retrying the failed operation, using default values, rolling back to a previous state, or gracefully shutting down the system

Implementing Exception Handlers

• Exception handlers are typically written in assembly or low-level language for performance and direct access to system resources
• Handlers must save and restore any registers they modify to avoid corrupting the program state
• Handlers should be as concise as possible to minimize the time spent in the exception context
• Nested exceptions can occur if an exception is triggered while handling another exception, requiring careful design and resource management
• Testing and debugging exception handlers is crucial to ensure the system can gracefully handle and recover from various error scenarios (invalid inputs, resource exhaustion, hardware failures)

System Protection Mechanisms

Monitoring and Recovery Techniques

• Watchdog timers are hardware or software components that monitor the system for hangs or malfunctions
• If the watchdog is not periodically reset by the program, it triggers a system reset or other recovery action
• Stack overflow protection detects when the stack grows beyond its allocated space, preventing corruption of other memory areas
• Memory protection units (MPUs) enforce access controls on memory regions, preventing unauthorized reads or writes
• MPUs can define permissions (read, write, execute) for different memory sections and generate exceptions on violations

Ensuring System Integrity and Reliability

• System resets, both hardware and software, can be used to restart the system in a known good state after a failure
• Brownout detection circuits monitor the power supply voltage and trigger a reset if it drops below a safe threshold
• Redundant hardware components (dual processors, backup memory) can provide fault tolerance and continued operation in case of failures
• Secure boot processes verify the integrity of firmware and prevent tampering or unauthorized modifications
• Encryption and secure storage protect sensitive data and prevent unauthorized access or leakage

Fail-safe Design Principles

• Fail-safe design ensures that the system remains in a safe state or gracefully degrades in the presence of failures
• Timeouts and error detection mechanisms prevent the system from hanging or operating in an undefined state
• Assertions and sanity checks validate assumptions and detect logic errors during development and runtime
• Graceful degradation allows the system to continue operating with reduced functionality or performance in case of partial failures (sensor malfunction, communication loss)
• Redundancy and diversity in design (multiple sensors, different algorithms) increase resilience against single points of failure