Lpc2000

Magnus,

This will certainly give an improvement, but even the shortest 
possible loop will have a built-in jitter that cannot be removed, 
particularly if you take into account the additional cycles needed to 
access the timer peripheral.

Unfortunately, I don't have figures for this, as they've long since 
been discarded (we analysed the o/p signal for noise in various 
frequency bands), but from memory even a couple of clock cycles 
jitter had a significant effect.

Of course, a lot depends on what your trying to achieve: just reduce 
some annoying hiss in a self-build project it might be fine.

Brendan

-- In lpc2000@yahoogroups.com, "Magnus Lundin" <lundin@m...> wrote:
>
> Hi
> 
> Another way to remove jitter is to read the timer in the interrupt 
> routine and loop on this until a predefined value, the number of 
> clock ticks when there is maximal latency at this point in the
> interrupt routine, has passed since the timer rollover.  
> This can be done just before outputing the samples or at the 
beginning.
> 
> This uses the interrupt to place the clock at almost the right time
> and the a wait loop on the timer counter for precise adjustment.
> 
> Magnus
> 
> --- In lpc2000@yahoogroups.com, "brendanmurphy37"
> <brendan.murphy@i...> wrote:
> > To get rid of jitter, you should code your o/p interrupt so that 
the 
> > code from the start of the interrupt through to where the sample 
is 
> > o/p has no "if" statements or similar (i.e. it must execute the 
same 
> > number of instructions for each interrupt). You then have to try 
and 
> > ensure that the program being interrupted is always on an 
instruction 
> > that takes the same number of cycles to execute. One way of doing 
> > this is to set up a timer interrupt to happen just before the o/p 
> > time. We use this approach, and jump to a (lot of "nops"). 
Karrl's 
> > suggestion (go to idle mode) is probably better than this, as it 
> > saves code space). The "real" interrupt can then be used to knock 
you 
> > out of this to o/p the sample. This will not remove jitter 
entireley, 
> > but it makes a significant difference.
> >
>