[avr-libc-dev] volatie with winavr2010 ?

[Leiu]

here is my code:
        volatile uint8_t first_edge = 'A';
        .................................
        ISR(TIMER1_OVF_vect)
        {
        if(first_edge <= 'Z')
        {
                avr2pc(first_edge); //avr2pc to transmit character to PC
                ++first_edge;
        }else
        {
                first_edge = 'A';
        }

        }

my result:
        I only show 'A' in hyperterminal :
        => first_edge can't update values;
        Please tell me why?