Demystifying the Vector Table in Microcontrollers

The vector table is a critical component in microcontrollers, serving as the roadmap for handling exceptions and interrupts. In this article, we explore what the vector table is, why it’s essential, and how it is structured and used to manage both system exceptions and peripheral interrupts.

What Is the Vector Table?

At its core, the vector table is simply a table of pointers—addresses that tell the processor where to jump when an exception or interrupt occurs. Think of it as a list of directions: each entry in the table points to the corresponding exception or interrupt handler routine. This table is fundamental because, without it, the processor wouldn’t know which function to execute when something goes wrong or when an external event occurs.

How Is the Vector Table Organized?

The vector table starts at the very beginning of the Flash memory (also known as ROM) and is populated during the startup phase of the microcontroller. Its structure is fixed and predetermined by the processor architecture, ensuring consistency across devices. Here’s what typically resides in the vector table:

• Initial Stack Pointer:
  The very first entry stores the initial stack pointer. When the microcontroller resets, it loads this value to set up the main stack before executing any code.

• Exception Handlers:
  Immediately following the stack pointer is the address of the Reset_Handler, the function that gets called right after a reset. This is followed by addresses for system exceptions like NMI (Non-Maskable Interrupt) and Hard Fault, among others. These handlers have fixed priorities and are crucial for system stability.

• Interrupt Handlers:
  After the system exceptions, the vector table contains the addresses of various external interrupt handlers. For example, if you have implemented a handler for an I2C or CAN interrupt, its address will be stored in the appropriate slot in the vector table. The ordering of these entries corresponds to the IRQ numbers, which are used by the NVIC (Nested Vectored Interrupt Controller) to determine priority and handling order.

How the Vector Table Works

When an exception or interrupt occurs, the processor automatically references the vector table to fetch the address of the corresponding handler. For instance, if a watchdog interrupt occurs, the processor looks up the vector table entry corresponding to that interrupt’s IRQ number, loads the address, and jumps to that handler routine. This mechanism ensures a fast and efficient response to critical events.

The vector table is defined in the startup code of your project, often written in assembly or C. The startup file not only defines the vector table but also includes default implementations (often as weak functions) for interrupt handlers. This allows you to override only the handlers you need in your application, while leaving the rest to a default “dummy” implementation.

Moreover, the vector table is placed in a special section (commonly named something like isr_vector) by the linker script, ensuring that it resides at the correct starting address in Flash. This precise placement is vital, as the processor expects the vector table to be at a known location immediately after reset.

Why Is It Important?

Understanding the vector table is crucial for embedded system developers because:

• Interrupt Management: It defines how the processor responds to both system exceptions and peripheral interrupts.
• Debugging: Knowing how the vector table works aids in troubleshooting unexpected behavior during exceptions.
• Customization: It allows you to customize and optimize interrupt handling by overriding default handlers, ensuring that your application responds quickly to critical events.

Conclusion

The vector table is the backbone of a microcontroller’s interrupt and exception handling system. By providing a structured list of pointers to various handler routines, it ensures that the processor can quickly and reliably respond to both internal exceptions and external interrupts. Whether you’re debugging your startup code or configuring custom interrupt service routines, a solid understanding of the vector table is key to mastering microcontroller programming.

Written By: Musaab Taha

This article was improved with the assistance of AI.