Re: [avr-gcc-list] A Question Of Optmization

[Ruud Vlaming]

Sorry,

i meant first

   volatile unsigned char TimerTick;

and then
 
  TimerTick = 10;

Since you need the variable in the ISR routine.

Ruud.



On Tuesday 15 April 2008 20:12, Karl O. Feger wrote:
> Dear all,
> 
> I'm relatively new to AVR-GCC and try to do something,
> 
> that is standing procedure in C51 for me. I want to
> 
> program a delay based on a timer-ISR. The code looks
> 
> like this:
> 
> *********************************
> 
> //isr-test, very basic
> 
> // cpu: ATMega128
> 
> // speed: 16 mhz
> 
>  
> 
>  
> 
> #include <avr/io.h>
> 
> #include <avr/interrupt.h>
> 
>  
> 
>  
> 
> unsigned char TimerTick;
> 
>  
> 
>  
> 
> //---------------------------------------
> 
> // Timer/Counter1 Compare Match A
> 
> //---------------------------------------
> 
>  
> 
> ISR(TIMER1_COMPA_vect) // timer_compare_a
> 
> {
> 
>  
> 
>         TimerTick ? TimerTick-- : 0;
> 
> }
> 
>  
> 
> //---------------------------------------
> 
> // main
> 
> //---------------------------------------
> 
>  
> 
> int main()
> 
> {
> 
>  
> 
>  
> 
>         unsigned char a;
> 
>  
> 
>         // --- TIMER1_COMPA irq ---
> 
>         // selected time = 10 ms (160000 ticks)
> 
>         // prescaler = 8 (8 ticks ... 32.768 ms)
> 
>         TCCR1B = (1<<WGM12)|(1<<CS11);
> 
>         OCR1AH = 0x4e;
> 
>         OCR1AL = 0x20;
> 
>         TIMSK |= (1<<OCIE1A);
> 
>  
> 
>         // main loop
> 
>         sei();
> 
>  
> 
>         for (;;)
> 
>         {
> 
>         TimerTick = 10;
> 
>         while(TimerTick);
> 
>         a = ~a;
> 
>         }
> 
>         return(0);
> 
> }
> 
> ********************************
> 
>  
> 
> When I compile this without optimization (-O0)
> 
> Everything is just fine and as expected. When I select
> 
> ANY other optimization level, the resulting assembler
> 
> code looks like this (starting at "TimerTick = 10;"):
> 
> **********************
> 
> 45:            TimerTick = 10;
> 
> +00000086:   E08A        LDI     R24,0x0A         Load
> 
> immediate
> 
> +00000087:   93800100    STS     0x0100,R24      
> 
> Store direct to data space
> 
> +00000089:   CFFF        RJMP    PC-0x0000       
> 
> Relative jump
> 
> ***********************
> 
> so the program stops here and loops.
> 
>  
> 
> Where is my thinking wrong? I tried all sources I could
> 
> imaging.
> 
>  
> 
> Thanks for any help
> 
> Karl
> 
>  
> 
>  
> 
>