Hi all,
 
If I'd want a 1 mSec delay and smclk/mclk = 8MHz
What should mS be??
 
void delay_ms(int del) {                   //This routine is approx 1mS
delay @ 8 MHz
   unsigned int ctr;
   while(del--)
      for (ctr=0; ctr < mS; ++ctr)
            ;
}
 
thanks,
Martijn
 

Martijn,

> If I'd want a 1 mSec delay and smclk/mclk =
8MHz
> What should mS be??
>  
> void delay_ms(int del) {                   //This routine is 
> approx 1mS
> delay @ 8 MHz
>    unsigned int ctr;
>    while(del--)
>       for (ctr=0; ctr < mS; ++ctr)
>             ;
> }

Depends upon how your compiler codes the loop, for a start, and whether
you want to take interrupts into account or not...  One assumes that you
can time the loop for a given value of mS using a scope, or
hand-calculate the value of mS given the instructions in the loop.

-- Paul.

Hi,

it depends on the compiler, compiler options, interrupts etc. and therefore you
should test it by measuring the time 10,000 delay_ms needs:

const unsigned int mS24;
unsigned long int i;
for (i=0;i<10000;i++)
{
  delay(1);
}

You can estimate the correct value of mS with binary search or calculating, but
you have to take into account the time a function call needs (and temperature
and voltage dependence of the clock) if you need the best fitting value.

Regards

Rolf

> > If I'd want a 1 mSec delay and smclk/mclk = 8MHz
> > What should mS be??
> >  
> > void delay_ms(int del) {                   //This routine is 
> > approx 1mS
> > delay @ 8 MHz
> >    unsigned int ctr;
> >    while(del--)
> >       for (ctr=0; ctr < mS; ++ctr)
> >             ;
> > }

> Depends upon how your compiler codes the loop, for
a start, and 
whether
> you want to take interrupts into account or not...
One assumes that 
you
> can time the loop for a given value of mS using a
scope, or
> hand-calculate the value of mS given the instructions in the loop.


Yes, a good way to get the value is measuring it with a scope.
But if you change something, for example
 - more or longer interrupts
 - clock frequency
 - different compiler (or new version with better optimization,
   some compilers even optimize empty loops totally away)
you may have to re-adjust the loop.

So, sometimes it is better to use a timer (if you have one).
Assume Timer_A counting in "Continuous Mode" with 32768Hz:

void delay_ms(unsigned short ms)
{
   unsigned short tar, tar0, clk;

   clk = ms * 32;  // 32 Timer_A clocks are ca. one ms; ms < 2048!
   // or exactly: clk = (32768ul*ms) / 1000; // ms < 2000!
   while((tar0=TAR) != TAR);     // read TAR until stable
   do
      while((tar=TAR) != TAR);   // read TAR until stable
   while((unsigned short)(tar-tar0) < clk);
}

Another way is to let a compare register (TACCRx) set a flag when 
time is over.

Wolfgang