[En-Nut-Discussion] Problems when NutGetMills() is close to overflow?

[Andreas Helmcke]

Erik Lindstein wrote:
> First of all the documentation for the NutGetMills() should be updated.
> 
> I guess there are two errors in there:
> --------- From 4.4.0 -------
> This function returns the value of a counter, which is incremented
> every system timer tick. During system start, the counter is cleared
> to zero and will overflow with the 32 bit tick counter (4294967296).
> With the default 1024 ticks/s this will happen after ##### 7.9 years.
> ##### The resolution is also given by the system ticks.
> 
If I am not totally mistaken the tickcounter will overflow after 49 days.
> 
> I have some timeouts i use together with NutGetMillis().
> Lets say i use it like this.
> 
> uint32_t timeout = NutGetMillis() + 10000;
> 
> for(;;)
> {
>        if(timeout <= NutGetMillis())
>        {
>                printf("Timeout - 10 Sec");
>                timeout = NutGetMillis() + 10000;
>        }
>        NutSleep(100);
> }
> 
> If timeout overflows this timeout will occur directly in the next if
> statement instead of the 10sec delay i expect.
> 
> First i thought that NutGetMillis() would overflow after 7.9 years but
> as that is not true i need to rewrite some of my code.
> Anyone have some nice and clean code to handle this?

Unsigned subtractions will do the trick.
For example:

uint32_t lastTick = NutGetMillis();

for(;;)
{
        if(NutGetMillis() - lastTick > 10000)
        {
                printf("Timeout - 10 Sec");
                lastTick = NutGetMillis();
        }
        NutSleep(100);
}

Greetings,
	Andreas