RE: [avr-gcc-list] interrupt optimization

[MuRaT KaRaDeNiZ]

Hi David,

To save cpu time to the extreme,
I would use a timer in external event count mode, and in the interrupt service 
toggle the  clk pin to the  event counter by sbi and cbi instructions, so

   ISR(...)
    {
        sbi that_port, that_pin
        cbi that_port, that pin
    }

    and may be one of this two instructions can be dropped too, there are bit 
toggling registers and if the timer supports counting every toggle :)

Regards

Murat Karadeniz
http://www.onlinefavbar.com/mukas



-----Original Message-----
    From: "David Bourgeois" <address@hidden>
    Sent: 01.02.06 22:39:36
    To: "address@hidden" <address@hidden>
    Subject: [avr-gcc-list] interrupt optimization
    
    Hi,
    
    I have an interrupt which is called a lot of times but only really do  
    something very rarely:
    
    ISR(...)
    {
         if (sampling-- == 0)
         {
             sampling = 0x7F;
             callInlineFunction();
         }
    }
    
    The problem is that I end up with a lot of push/pop that just eat up all  
    my CPU cycles most of the time. Using an inline function helped removing  
    all registers that were not used by the function at all (don't know why  
    the optimization doesn't do this already) but I still get a lot of push  
    not necessary by the "if" and that I would like to move before the  
    function call inside the "if".
    
    What's the way to go in this situation? I thought of using a naked ISR and  
    adding the necessary push by inline asm but what if I later change my code  
    and the compiler use different registers? I'm afraid to end up with broken  
    code.
    
    Thanks,
    David Bourgeois
    
    asm output of the ISR with inline function call:
      334:      1f 92           push    r1
      336:      0f 92           push    r0
      338:      0f b6           in      r0, 0x3f        ; 63
      33a:      0f 92           push    r0
      33c:      11 24           eor     r1, r1
      33e:      2f 93           push    r18
      340:      3f 93           push    r19
      342:      8f 93           push    r24
      344:      9f 93           push    r25
      346:      ef 93           push    r30
      348:      ff 93           push    r31
    
      34a:      80 91 0f 01     lds     r24, 0x010F             ; if 
(sampling-- == 0)
      34e:      81 50           subi    r24, 0x01       ; 1
      350:      80 93 0f 01     sts     0x010F, r24
      354:      88 23           and     r24, r24
      356:      51 f5           brne    .+84            ; 0x3ac 
<__vector_16+0x78>
    
    
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