Setting the Vector Table Offset Register (VTOR) on a Cortex-M4 microcontroller allows relocating the interrupt vector table to a custom memory location. However, there are limitations to consider when configuring the VTOR to ensure system stability and prevent errors.
Address Restrictions: The VTOR only accepts the starting address of the interrupt vector table. This address must be aligned to a power of two, with a minimum size of 128 words (512 bytes) for the Cortex-M4. This alignment ensures the vector table resides at an efficiently accessible location for the processor. Failure to adhere to these alignment restrictions can lead to a Hard Fault.
Vector Table Size: The interrupt vector table must be large enough to accommodate all interrupt vectors used in the system. The number of required vectors depends on the microcontroller’s design and utilized peripherals. An insufficiently sized vector table can lead to out-of-bounds interrupts overwriting critical data in memory, causing severe consequences.
Memory Access Permissions: The memory location pointed to by the VTOR must have appropriate access permissions. In privileged mode, the processor can access all memory regions. However, in user mode, access is restricted by the Memory Protection Unit (MPU). If the vector table resides in a memory region inaccessible to user mode, the system will encounter errors during interrupt handling. Proper MPU configuration is crucial for ensuring secure and reliable system operation.
System Reset: After modifying the VTOR value, a system reset is required for the changes to take effect. Resetting ensures the processor reloads the interrupt vector table from the new location. This step is critical to prevent interrupt handling errors after changing the VTOR.
Security Considerations: When using the VTOR to relocate the interrupt vector table, security implications must be considered. Placing the table in an unprotected memory region could allow malicious code to modify interrupt vectors, potentially leading to system compromise. Appropriate security measures, such as using the MPU to restrict access to the memory region containing the vector table, should be implemented to mitigate security risks.
Performance Impact: Relocating the interrupt vector table can impact system performance, particularly interrupt response time. If the vector table resides in external flash memory, access times may be longer compared to internal SRAM. A balance between memory layout flexibility and system performance should be considered when deciding on the vector table’s location. Optimizing the vector table placement can significantly improve the system’s interrupt handling performance.
