Imagecraft c compiler and development environment for the atmel avr

ICCV8 for AVR – C Compiler for Atmel AVRIf you access a global variable inside an interrupt handler, be sure that anymodifications of the global variable in the main application cannot be interrupted.Non-atomic access (i.e., access that may require multiple machine instructions)includes access to 16- or 32-bit variables, bit operations and non-basic C types(i.e., array).Spurious or unexpected interrupt behaviors can crash your program: You should always set up a handler for “unused” interrupts. An unexpectedinterrupt can cause problems. Beware that accesses to variables larger than the natural data size of theCPU require multiple accesses. For example, writing a 16-bit value on an 8-bitCPU probably requires at least two instructions. Therefore, accessing thevariable in both the main application and interrupt handlers must be done withcare. For example, the main program writing to the 16-bit variable may getinterrupted in the middle of the 2-instruction sequence. If the interrupt handlerexamines the variable value, it would be in an inconsistent state. Most CPU architectures do not allow nested interrupts by default. If youbypass the CPU mechanism and do use nested interrupts, be careful not tohave unbound nested interrupts. On most systems, it is best to set your interrupt handlers to execute as fast aspossible and to use as few resources as possible. You should be carefulabout calling functions (your own or a library) inside an interrupt handler. Forexample, it is almost never a good idea to call such a heavy-duty library functionas printf inside an interrupt handler. With few exceptions, our compilers generate reentrant code. That is, yourfunction may be interrupted and called again as long as you are careful withhow you use global variables. Most library functions are also reentrant, withprintf and related functions being the main exceptions.Test your external memory interface carefully. For example, do not just walk theentire external RAM range and verify write a few patterns in a single loop, as itmight not detect the case where the high address bits are not working correctly.The compiler may be doing something unexpected, even though it is correct. Forexample, for RISC-like targets such as the Atmel AVR, TI MSP430 and the ARMCPU, the compilers may put multiple local variables in the same machine registeras long as the usage of the local variables does not overlap. This greatly improvesthe generated code, even though it may be surprising when debugging. Forexample, if you put a watch window on two variables that happen to be allocatedto the same register by the compiler, both variables would appear to be changing,even though your program is modifying only one of them.39