Re: [avr-gcc-list] 32-bits time

[David Kelly]

On Oct 12, 2005, at 3:22 PM, Eric Pasquier wrote:

> //================================================
> // timer32_now
> // Retrieve the current time from the global clock in Timer1,
> // disabling interrupts to avoid stomping on the TEMP register.
> //------------------------------------------------
> inline uint32_t timer32_now(void)
> {
>   int8_t    sreg;
>   uint16_t  tempL, tempH;
>
>   sreg=SREG;
>   cli();
>   tempL=TCNT1;
>   tempH=hiword_time;
>   SREG=sreg;
>
>   return (((uint32_t)(tempH)<<16) + tempL);
> }

As others have said with the prescaler set to 1 TCNT1 is running too  
fast. However I think below will not misbehave as badly as the above.  
Specifically the test for TOV1. The union/struct thing just makes  
things easier to read and write. Avr-gcc handles it very efficiently:

inline uint32_t timer32_now(void)
{
    int8_t sreg;
    union {
        struct {
            uint16_t lo, hi;
        } w;
        uint32_t l;
    } temp;

    sreg = SREG;
    cli();
    temp.w.lo = TCNT1;
    temp.w.hi = hiword_time;

    //  check for unserved Timer1 Overflow
    if( TIFR & (1<<TOV1) )
        temp.w.hi++;        // do what hasn't yet been done in  
SIG_OVERFLOW1

    SREG = sreg;

    return( temp.l );
}

One more thought is to make use of the temporary latch register  
associated with 16 bit registers such as TCNT1:

inline uint32_t timer32_now(void)
{
    int8_t sreg;
    union {
        struct {
            uint8_t lo, mid;
            uint16_t hi;
        } part;
        uint32_t l;
    } temp;

    sreg = SREG;

    temp.part.lo = TCNT1L;    //  TCNT1H is latched
    cli();                    //  disable IRQ
    temp.part.mid = TCNT1H;   //  fetch TCNT1H from the latch
    temp.part.hi = hiword_time;

    SREG = sreg;

    return( temp.l );
}

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.