Re: [avr-gcc-list] Best practice for external variable (newbie-ish question)

[Russell Shaw]

Matthew Smith wrote:
> Hi David
>
> <snip />
>
> > The only time I use "extern" is for prototyping in #include files.  Even
> > so, the variable has to be declared somewhere without "extern"  and
> > outside of a function in order to create the allocation. Make  sure you
> > #include the prototype file in the source file which creates  the
> > storage for the variable because avr-gcc will bellyache if the  two do
> > not match. This is one of the ways one makes the compiler  double-check
> > one's work.
>
> Thanks for that; I do have everything in one file and I'm getting some
> very, very odd and unpredictable actions.  The system is a pump
> controller that reads pressure via an ADC and uses the AVR timer to
> determine a minimum run time (unless a critical pressure is reached, in
> which case the timer is ignored).

Beware that when reading or writing a 16-bit timer register, it is done
in two 8-bit chunks using a hidden global hardware register that is shared
between any peripheral with 16-bit registers. Therefore, you should block
ISRs accessing any other peripheral that is likely to use this register.

> I've got an LED showing me when the timer is running and another on a
> solid state relay showing what the pump is supposed to be doing.
> Substituting a potentiometer for the pressure transducer, I've observed
> it working as planned, but sometimes the timer never expires (or won't
> start) and sometimes the "virtual" pump turns on, but never off.
>
> Having been musing about what to try to rectify this, I'll remove the
> extern declarations and see if this helps.
>
> Cheers
>
> M