Re: [avr-gcc-list] On getting an ISR leaner with GCC

[dlc]

Thanks all for the suggestions.  I was tempted by the "naked" function, 
but that required a total re-write mostly in assembly to get it right 
and I have found that sometimes the "register" variables get slammed by 
something else, so something I was doing wasn't reliable.  The code 
generated by gcc was so bloated in the ISR that I was tempted also to 
rewrite sections in assembly to fix it up, but again balked at getting 
the C and assembly sides to agree on symbolic variable names/locations.

I ended up simply changing IO lines and moving a set of polled lines to 
the INT0 and INT1 IRQ's and counting level changes.  That reduced the 
use of the 10us IRQ significantly since the polled lines would change 
only every few dozen milliseconds.  They really needed their own ISR's, 
which ran pretty quickly regardless.  I was simply trying to do too much 
in one ISR and overloaded it.  My change sucked about 100 bytes out of 
the ISR.

I have been spoiled by the PIC 18F series autosave of status and pointer 
registers and defining certain variables as only changed in the ISR, 
this allowed very lean ISR code with little overhead.  Gcc and the AVR 
may allow better C code than the PIC, but ISR's have to be much more 
carefully considered!

have fun,
DLC

Bob Paddock wrote:
> Look up:
>
> __attribute__((naked))
>
> That should get you down to the bare metal where you have to do it all.
>
> On Sat, Dec 27, 2008 at 2:52 AM, dlc <address@hidden 
> <mailto:address@hidden>> wrote:
>
>     The subject says it all.  I've an ISR that is using too many cycles
>     for something that kicks every 10us.  I'm very experienced with the
>     PIC and know how to specify registers that are not touched outside
>     of the ISR to avoid lots of "push-n-pop" actions.  Is there
>     something like that in the AVR toolchain or am I going to have to
>     resort to assembly to get this thing leaned down?  I'd like to
>     create some memory that is dedicated to the ISR such that the
>     compiler knows that it doesn't need to be saved. Also a way to get
>     scratchpad memory assigned to it for some basic math that I don't
>     want to have to use the overhead of push/pop.  Can anyone offer
>     insight into how I can lean an ISR down or what sorts of operations
>     or memory types to avoid there with avr-gcc?
>
>     many thanks,
>     DLC
>     -- 
>     -------------------------------------------------
>     Dennis Clark          TTT Enterprises
>     www.techtoystoday.com <http://www.techtoystoday.com>
>     -------------------------------------------------
>
>
>     _______________________________________________
>     AVR-GCC-list mailing list
>     address@hidden <mailto:address@hidden>
>     http://lists.nongnu.org/mailman/listinfo/avr-gcc-list
>
>
>
>
> --
> http://www.wearablesmartsensors.com/
> http://www.softwaresafety.net/
> http://www.designer-iii.com/
> http://www.unusualresearch.com/

--
-------------------------------------------------
Dennis Clark          TTT Enterprises
www.techtoystoday.com
-------------------------------------------------