Hi,
I want to use a few global register variables (r2, r3, r4) to implement several timers with a single hardware timer:
// System Timer Resolution Unit = ms
#define SYSTEM_TIMER_RESOLUTION 10
#define set_ms_timer(timer, ms) {timer = ((ms + SYSTEM_TIMER_RESOLUTION / 2) / SYSTEM_TIMER_RESOLUTION);}
register uint8_t isr_temp asm("r2");
register uint8_t shtxx_response_timeout asm ("r3");
register uint8_t measurement_cycle asm ("r4");
void init_system_timer (void)
{
shtxx_response_timeout = 0;
measurement_cycle = 0;
// Set Timer1 To Generate Compare Interrupt With The Intervall Time Defined In
// SYSTEM_TIMER_RESOLUTION
OCR1A = (F_CPU / 8 + 500 / SYSTEM_TIMER_RESOLUTION) / (1000 / SYSTEM_TIMER_RESOLUTION) - 1;
// Enable Interrupt On Compare A Match
TIMSK1 = (1 << OCIE1A);
// Set CTC Mode, Run Timer With F_CPU / 8
TCCR1A = 0;
TCCR1B = (1 << WGM12) | (1 << CS11);
}
ISR (TIMER1_COMPA_vect, ISR_NAKED)
{
asm volatile ("in %0, %1" : "=r" (isr_temp) : "I" (_SFR_IO_ADDR(SREG)));
// Decrement register variable shtxx_response_ms_timeout If It Is Not Zero
asm volatile
(
"cpse %0, __zero_reg__" "\n\t"
"dec %0"
: "+r" (shtxx_response_timeout)
: "0" (shtxx_response_timeout)
);
// Decrement register variable measurement_cycle If It Is Not Zero
asm volatile
(
"cpse %0, __zero_reg__" "\n\t"
"dec %0"
: "+r" (measurement_cycle)
: "0" (measurement_cycle)
);
asm volatile ("out %0, %1" :: "I" (_SFR_IO_ADDR(SREG)), "r" (isr_temp));
reti();
}
This should enable me to start a software timer this way:
set_ms_timer (measurement_cycle, 100);
And then I can simply poll the register variable if a timeout had occured:
if (measurement_cycle == 0)
{
// Do Something
}
The problem is that the GCC optimizer detects that the register variable is never changed inside the code (only inside the ISR) and thus deletes the if (measurement_cycle == 0) relevant stuff.
Is there a way to tell the compiler to not delete that code? Of course without using a volatile declared SRAM variable which will blow up my sweet tight ISR.
Regards
Sebastian
