4. MQX RTOS Handing Interrupts -1

MQX handles HW interrupts and exceptions with interrupt service routines (ISR). Please note the sentence ‘exceptions are handled by ISR’. In due course I would discuss my experience with some very interesting problem involving exception ISR.

What an ISR? What goes inside an ISR?

ISR is not a task. It’s a small piece of code which is executed whenever an interrupt occurs. The main functionally of ISR may include

a)      Servicing a device

b)      Clearing an error condition

c)       Singling a task

MQX allows to pass a parameter to the ISR handler whenever application is installing an ISR. This parameter is a pointer. Please note that this pointer should not be at task stack, as it can be called even before task is created.

People keep on saying ISR should be precise. Well, that makes lots of sense as during ISR other interrupts may be disabled. And you definitely not want to miss the interrupts. Are you asking for example?

Hmm, let’s say whenever aeroplane’s engine is getting too hot, cooling procedure should be initiated immediately.  Now, this is how it would work, a sensor would trigger an HW interrupt, and ISR would ask you application to run the Cooling Task. But wait, you application is handling a pervioius ISR which has disabled other interrupts. And you are playing ‘print Hello word’ a zillion time game there in ISR(just kidding!). So you would miss the interrupt.( And lets hope you are not flying on the plane )

Back to serious stuff.

What is Kernel ISR?

MQX provides a kernel ISR(_init_kernel_isr()) which is written in assembly language. The kernel ISR runs before any other ISR. And it

a)      saves the context of the active task.

b)      switches to the interrupt stack .

c)       calls the appropriate ISR .

d)      after the ISR has returned, restores the context of the highest-priority ready task

When MQX starts it installs kernel ISR to all interrupts.

Initializing interrupt handling/Installing interrupts and Default ISR

       When MQX starts, it initializes its ISR vector table which has entry for all interrupts. Each entry consists of  a)   pointer to ISR  b)data to pass as parameter c)   Default exception handler.

Initially ISR for each entry is ‘_int_default_isr()’.

All of this is performed by a MQX kernel function called _int_init(), which loops for all interrupts and does what I mentioned above.

continued in next post. Click here

Good article on ARM exception handling
http://www.csie.nctu.edu.tw/~wjtsai/EmbeddedSystemDesign/Ch3-1.pdf