Lpc2000

At 02:50 PM 2/22/04 -0500, you wrote:
>Robert Adsett wrote:
>
> > At 02:22 PM 2/22/04 -0500, you wrote:
> > >At 10:13 AM 2/22/04 -0600, you wrote:
> > > >As a side note.  The disabling and restoring of the global interrupt
> > > >flag in the cpsr can has a similar problem.
> > >
> > ><snip>
> > >
> > > >For more info on this, Atmel has
> > > >an APP note discribing it on their web site in the AT91 ARM7 section.
> > >
> > >Thanks for the pointer.
> >
> > I just had a chance to read the app note (I presume this is the one
> > http://www.atmel.com/dyn/resources/prod_documents/DOC1156.PDF)
> > <http://www.atmel.com/dyn/resources/prod_documents/DOC1156.PDF%29> and it
> > appears to be warning about faults that can happen when using a style of
> > code I would consider very error prone to begin with.  Unless I'm missing
> > something (I've only had a little while to consider it) the only time a
> > problem occurs if an interrupt routine actually modifies the cpu
> > status of
> > the interrupted code.  What would ever prompt anyone to do that in the
> > first place?
> >
> > [snip]
>
>     It's a common method of creating interrupt prioritization.  If you
>have a "slow" interrupt routine, or something more critical, it's common
>to re-enable interrupts inside the interrupt routines.

That I can understand (I've done it myself), but even in that case you 
don't modify the status register of the process you are interrupting but 
only your own copy.  The usual interrupt entry in either SW or HW goes 
something like

save processor status
save interrupt return address

and then on exit

return to saved return address
restore processor status

The issue in the appnote involves modifying the saved processor 
status.  (ie that of the interrupted code)  I don't understand why you 
would want to do that.  In the best case it will do nothing and in the 
worst....


>     I kinda miss the days of the 8051 and such when an arbitrary
>peripherial that generates interrupts could be specified to be high or
>low priority.

Or the ST10/C166 where they are almost infinitely variable :)

The VIC would seem to provide at least some of that but I haven't played 
with it yet.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III