#15 Startup Code Part-3: Vector table initialization, exception handlers, interrupt handlers

Summary

This tutorial provides an in-depth guide on setting up startup code for embedded systems using the Tiva-C microcontroller. It covers essential topics like initializing the stack pointer, handling exceptions with a custom vector table, and using fault injection to test system responses. The focus is on creating a robust, deployable solution for production environments, ensuring error handling is efficient and reliable. Additionally, the lesson emphasizes the use of a common error handler, the `assert_failed()` function, and introduces the importance of testing infrequent events in embedded programming.

Takeaways

• 😀 The importance of initializing a custom vector table in embedded systems is emphasized, particularly for microcontroller programming.
• 😀 Correctly setting up the stack pointer and ensuring the vector table includes the appropriate reset handler (e.g., __iar_program_start) is critical for system initialization.
• 😀 The use of CMSIS-compliant names for exception handlers (e.g., HardFault_Handler, NMI_Handler) ensures consistency and compatibility with the ARM Cortex-M architecture.
• 😀 Fault injection is a recommended technique for testing infrequent events, such as HardFaults or stack overflows, to ensure the system behaves as expected.
• 😀 A single common error handler (assert_failed) can be used to catch all faults and assertions during debugging, streamlining the debugging process.
• 😀 By setting breakpoints in the assert_failed function, developers can easily catch any issues during testing, helping to track down the root cause of errors.
• 😀 Interrupt handlers for supported interrupts must be correctly mapped in the vector table, and unused interrupts should be aliased to a default handler to prevent errors.
• 😀 Fault injection can simulate error conditions deliberately, allowing developers to test the system's response to unusual or edge-case scenarios.
• 😀 The vector table must be carefully constructed with the correct layout, preserving reserved entries as outlined in the processor's datasheet.
• 😀 Developing the habit of testing all code, including rare scenarios, ensures that the system is thoroughly validated and can handle various edge cases in production.

Q & A

What is the purpose of setting up a custom vector table in embedded systems?

The custom vector table in embedded systems is essential for defining the addresses of exception and interrupt handlers. It provides the system with the correct functions to call when specific exceptions or interrupts occur, ensuring proper system behavior and fault recovery.

How does the reset handler (__iar_program_start) work in the startup code?

The reset handler, `__iar_program_start`, is the entry point for the system after a reset. It initializes the system by setting up the necessary hardware and software configurations and ultimately begins the main execution of the program.

What is the significance of the `CSTACK$$Limit` symbol in the vector table?

The `CSTACK$$Limit` symbol defines the address where the stack pointer should point after a reset. It is typically linked to the top of the stack, ensuring that the system starts with the correct stack pointer address for proper function execution.

Why is the `assert_failed` function crucial for embedded system debugging?

The `assert_failed` function is used as a central error handler that can catch and handle errors, faults, and assertions. By setting a breakpoint at this function, developers can debug all issues in the system, ensuring the system is robust and can recover from errors.

What is fault injection, and how is it useful in embedded systems testing?

Fault injection is the intentional introduction of errors into the system to simulate faults. It allows developers to test how the system reacts to unexpected failures (like stack overflows) and ensures that the system can recover gracefully without crashing.

How does the system react during a HardFault handler triggered by fault injection?

When a HardFault handler is triggered (for example, by a stack overflow), the system jumps to the HardFault exception handler. If the handler is correctly implemented, it can call the `assert_failed` function and reset the system, ensuring that the error does not cause a denial of service.

What is the role of weak aliasing in interrupt handlers?

Weak aliasing is used to provide default behavior for interrupt handlers that are not explicitly defined. For interrupts that are not utilized, the default handler (such as `Unused_Handler`) is called, which prevents the system from malfunctioning due to undefined interrupt behaviors.

Why is it important to maintain reserved slots in the vector table?

Maintaining reserved slots in the vector table is critical to preserving the correct layout of the table as defined by the microcontroller's datasheet. These reserved entries ensure compatibility with the hardware and prevent unexpected behavior due to misalignment in the vector table structure.

What are the benefits of using a single common error handler like `assert_failed`?

Using a single common error handler like `assert_failed` simplifies the debugging process by consolidating all error handling into one function. This allows developers to set breakpoints and trace all types of errors, making it easier to manage and fix issues across the system.

What is the next step after completing the custom startup code and vector table setup?

After completing the custom startup code and setting up the vector table, the next step is to introduce interrupts. This involves configuring and enabling interrupt handlers for the system's various interrupt requests (IRQs), which will handle external events like timers, input signals, and more.