STM32 Interrupts


 

1. Processor Mode

The processor mode can change when exceptions occur. And it can be in one of the following modes:

  • Thread Mode: Which is entered on reset.
  • Handler Mode: Which is entered on all other exceptions.

 


 

2. Exception Types

Exceptions can be fired by various events including:

  • Internal Exceptions
  • External Exceptions
 

3. Internal Exceptions

Exceptions that get fired by an internal source to the system and not by any external hardware or peripherals. And this includes:

  • Reset: This is the handler routine that gets executed when the processor gets out of a reset state whatever the source is.
  • System Service Call (SVC): Similar to SVC instruction on other ARM cores. it allows non-privileged software to make system calls. This provides protection for critical system functionalities.
  • Pended System Call (PendSV): Operates with SVC to ease RTOS development as it’s intended to be an interrupt for RTOS use.
  • Non-Maskable Interrupt (NMI): As the name suggests, this interrupt cannot be disabled. If errors happen in other exception handlers, an NMI will be triggered. Aside from the reset exception, it has the highest priority of all exceptions.

 4. Fault Exceptions

Some of the system exceptions are used to signal and handle specific faults. There are several categories for fault exceptions which include:

  • Bus Faults
  • Usage Faults
  • Memory Management Faults
  • Hard Faults
5. Exception States

Each exception can be in one of the following states:

  • Inactive:  Not pending nor active.
  • Pending: Exception event has been fired but the handler is not executed yet.
  • Active: The exception handler has started execution but it’s not over yet. Interrupt nesting allows an exception to interrupt the execution of another exception’s handler. In this case, both exceptions are in the active state.
  • Active And Pending:  The exception is being serviced by the processor and there is a pending exception from the same source.
 


6. an Interrupt controller

An interrupt controller is a separate peripheral like other peripherals. This is used to manage the multiple interrupts from the other peripherals or interrupt sources. Actually, the processor will have only one or a few interrupt lines. Using those few interrupt lines, we cannot control all the interrupts. So, this controller will help to manage those interrupts and tell the processor who has triggered the interrupt and which function (ISR) should the processor call.
ARM processors have been using multiple interrupt controllers in their products (licensed). And vendors also can use their own controller but mostly they use ARM’s interrupt controller. We will see some of them in this post.
7. Nested Vectored Interrupt Controller (NVIC)controller 
A nested vectored interrupt controller is used to manage the interrupts from multiple interrupt sources.


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