Post by thread:[PIC] C18 ISRs calling other functions?

As far as I can tell, the C18 compiler automatically takes care of the two different context saving methods for WREG, BSR and STATUS, depending on whether the interrupt priority is high (i.e. use shadow regs & fast return) or low (i.e. use stack). Great! Since ISRs use a temporary data section kept separate from normal C functions, does this create any issues with ISRs calling functions elsewhere in program memory? The C18 Compiler User's Guide (DS51288J-page 28) says the following, but I can't conclude much from it: "... Interrupt service routines use a temporary data section that is distinct from that used by normal C functions. Any temporary data required during the evaluation of expressions in the interrupt service routine is allocated in this section and is not overlaid with the temporary locations of other functions, including other interrupt functions. The interrupt pragmas allow the interrupt temporary data section to be named. If this section is not named, the compiler temporary variables are created in a udata section named fname_tmp. For example: void foo(void); .... #pragma interrupt foo void foo(void) { /* perform interrupt function here */ } The compiler temporary variables for interrupt service routine foo will be placed in the udata section foo_tmp. ...." In other words, do I have to name or move this ISR temporary area somehow to enable 'normal' execution of arbitrary functions? Thoughts welcome, Matt

Matt Rhys-Roberts wrote: > In other words, do I have to name or move this ISR temporary area > somehow to enable 'normal' execution of arbitrary functions? The part of the manual you quoted gives no hint this is even a issue. Manually placing a relocatable section is usually a bad idea, and giving it a name only means it won't have its default name. But in any case, I can't see any reason you should think the name or placement of the section will matter to runtime operation. Step back and think about what is going on. The location and name of the temporary data used by the interrupt routine doesn't matter from that point of view either. The compiler is merely making sure that temporary RAM locations used during interrupt are not reused anywhere else, since it has to assume a interrupt could happen when in any of the other routines. This also means you can't call any of these other routines from the interrupt routine unless those other routines are compiled in such a way as to not use static temporary variables. In other words, any routine called during interrupt must be reenterable. That means static temporary variables are out. All temporary variables need to be dynamically allocated, probably by using the stack (although there are other ways). Most likely your compiler defines a particular way of waving a dead fish over a routine (often called a pragma) to indicate it must be reenterable, or maybe that's the default when no dead fish is waved. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

On Tue, 27 Jul 2010, Matt Rhys-Roberts wrote: > Since ISRs use a temporary data section kept separate from normal C > functions, does this create any issues with ISRs calling functions > elsewhere in program memory? > In other words, do I have to name or move this ISR temporary area > somehow to enable 'normal' execution of arbitrary functions? No, you don't have to do anything special to call functions from ISRs. However, doing so will cause the ISR to save all of the .tmpdata section unconditionally, which can add hundreds of instruction cycles to your interrupt context save/restore time. There is a way to prevent this, but the easiest way is to not call functions from the ISR. -- John W. Temples, II