How to generate us and ms incremental counters?

[kmorri2004]

Posted on February 27, 2011 at 03:50

How to generate us and ms incremental counters?

[fggnrc2]

Posted on May 17, 2011 at 15:12

Your question has no solution, if it's literally taken.

Even Arduino milli() or micro() functions use an interrupt routine...

To make things clear, the following code is taken from:

arduino-0022-src.tar.gz\arduino-0022\hardware\arduino\cores\arduino\wiring.c

// the prescaler is set so that timer0 ticks every 64 clock cycles, and the

// the overflow handler is called every 256 ticks.

#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))

// the whole number of milliseconds per timer0 overflow

#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)

// the fractional number of milliseconds per timer0 overflow. we shift right

// by three to fit these numbers into a byte. (for the clock speeds we care

// about - 8 and 16 MHz - this doesn't lose precision.)

#define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) >> 3)

#define FRACT_MAX (1000 >> 3)

volatile unsigned long timer0_overflow_count = 0;

volatile unsigned long timer0_millis = 0;

static unsigned char timer0_fract = 0;

SIGNAL(TIMER0_OVF_vect)

{

    // copy these to local variables so they can be stored in registers

    // (volatile variables must be read from memory on every access)

    unsigned long m = timer0_millis;

    unsigned char f = timer0_fract;

    m += MILLIS_INC;

    f += FRACT_INC;

    if (f >= FRACT_MAX) {

        f -= FRACT_MAX;

        m += 1;

    }

    timer0_fract = f;

    timer0_millis = m;

    timer0_overflow_count++;

}

unsigned long millis()

{

    unsigned long m;

    uint8_t oldSREG = SREG;

    // disable interrupts while we read timer0_millis or we might get an

    // inconsistent value (e.g. in the middle of a write to timer0_millis)

    cli();

    m = timer0_millis;

    SREG = oldSREG;

    return m;

}

unsigned long micros() {

    unsigned long m;

    uint8_t oldSREG = SREG, t;

   

    cli();

    m = timer0_overflow_count;

#if defined(TCNT0)

    t = TCNT0;

#elif defined(TCNT0L)

    t = TCNT0L;

#else

    #error TIMER 0 not defined

#endif

 

#ifdef TIFR0

    if ((TIFR0 & _BV(TOV0)) && (t < 255))

        m++;

#else

    if ((TIFR & _BV(TOV0)) && (t < 255))

        m++;

#endif

    SREG = oldSREG;

   

    return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());

}

As you can see, the millis() and micros() functions use timer0_overflow_count and read the TIMER0 value.

If you want to replicate them, TIM4 is alike Arduino's timer0.

As regards your code, you have to follow what wiring.c does: handle TIM4 overflow interrupt and read TIM4 in millis() and micros() code.

regards

EtaPhi