The NVIC (Nested Vectored Interrupt Controller) is a key component in the STM32F407 microcontroller that handles interrupt requests. When multiple interrupts occur simultaneously, the NVIC uses a priority resolution mechanism to determine which interrupt should be serviced first. Understanding how this mechanism works is essential for efficient interrupt handling in your applications.
The NVIC in the STM32F407 supports up to 240 external interrupts, each with its own priority level. The priority level determines the order in which interrupts are serviced. The NVIC uses a priority grouping mechanism to divide the 240 interrupt lines into several priority groups, with each group having a different number of priority levels.
The priority grouping is configured using the SysTick_Config() function, which is part of the CMSIS (Cortex Microcontroller Software Interface Standard) library. The SysTick_Config() function takes a parameter that specifies the number of bits used for the preemption priority and the number of bits used for the subpriority. The remaining bits are used for the group priority.
Once the priority grouping is configured, the NVIC assigns a priority level to each interrupt source. The priority level is set using the NVIC_SetPriority() function, which takes two parameters: the interrupt number and the priority level. The interrupt number corresponds to the specific interrupt source, while the priority level is a value between 0 and the maximum priority level supported by the NVIC.
When an interrupt occurs, the NVIC compares the priority level of the interrupt with the priority level of the currently active interrupt. If the new interrupt has a higher priority, it preempts the currently active interrupt and starts executing its interrupt service routine (ISR). If the new interrupt has a lower or equal priority, it is queued and waits for the currently active interrupt to finish.
In case multiple interrupts with the same priority level occur simultaneously, the NVIC uses a mechanism called tail-chaining to determine the order in which they are serviced. The tail-chaining mechanism ensures that the interrupt that occurred first is serviced first. This mechanism is particularly useful when handling interrupts that have the same priority level and need to be processed in a specific order.
Understanding the priority resolution mechanism in the NVIC is crucial for efficient interrupt handling in your applications. By configuring the priority grouping and setting the appropriate priority levels for your interrupt sources, you can ensure that critical interrupts are serviced promptly and in the desired order.
| References |
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| 1. STM32F407 Reference Manual |
| 2. CMSIS Documentation - NVIC |