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STM32: enter and exit correctly to/from an ISR (assembly code)

Question asked by engkyr on Nov 28, 2015
Latest reply on Nov 30, 2015 by engkyr
Hello everybody,

At the moment I'm studying for an exam based on microcontrollers and I have choose the stm32 family, but I need to program this microcontroller in assembly.
In the last meeting with my professor, he had a doubt on how I handled the interrupt routine. I had explain that my code doesn't fail and doesn't saturate the stack, but maybe I'm wrong and I wrote a "bad code".
Could someone enlighten me about this?
I wrote this interrupt routine:
        push    {LR}
        BL      get_char              
        BL      checking
        BL      put_char
        pop     {PC}

I need to save the LR, because the code will jump in others subroutines. Am I right?
With the instruction:
pop     {PC}
Does it exit correctly from the ISR?

Another question. In one subroutine I want to exit from the ISR and return directly to "main" and wait for another interrupt in sleep mode, if a counter is less than 3.
        section .text:CODE:NOROOT(2)
        ldrb    r3, [r2, #4]           
        strb    r3, [r5, r4]           
        add     r4, r4, #1              ; count++
        cmp     r4, #3                  ; if r4 >= 3
        BCS     complete            
        pop     {PC}                    ; return to main 
        mov     r4, #0                  ; count=0
        BX      LR                      ; return from function call
Am I doing everythig right?
I will appreciate any tip!