oracular (3) porting.3avr.gz

NAME

       porting - Porting From IAR to AVR GCC

Introduction

       C language was designed to be a portable language. There two main types of porting activities: porting an
       application to a different platform (OS and/or processor), and porting to a different compiler. Porting
       to a different compiler can be exacerbated when the application is an embedded system. For example, the C
       language Standard, strangely, does not specify a standard for declaring and defining Interrupt Service
       Routines (ISRs). Different compilers have different ways of defining registers, some of which use non-
       standard language constructs.

       This chapter describes some methods and pointers on porting an AVR application built with the IAR
       compiler to the GNU toolchain (AVR GCC). Note that this may not be an exhaustive list.

Registers

       IO header files contain identifiers for all the register names and bit names for a particular processor.
       IAR has individual header files for each processor and they must be included when registers are being
       used in the code. For example:

       #include <iom169.h>

       Note
           IAR does not always use the same register names or bit names that are used in the AVR datasheet.

       AVR GCC also has individual IO header files for each processor. However, the actual processor type is
       specified as a command line flag to the compiler. (Using the -mmcu=processor flag.) This is usually done
       in the Makefile. This allows you to specify only a single header file for any processor type:

       #include <avr/io.h>

       Note
           The forward slash in the <avr/io.h> file name that is used to separate subdirectories can be used on
           Windows distributions of the toolchain and is the recommended method of including this file.

       The compiler knows the processor type and through the single header file above, it can pull in and
       include the correct individual IO header file. This has the advantage that you only have to specify one
       generic header file, and you can easily port your application to another processor type without having to
       change every file to include the new IO header file.

       The AVR toolchain tries to adhere to the exact names of the registers and names of the bits found in the
       AVR datasheet. There may be some descrepencies between the register names found in the IAR IO header
       files and the AVR GCC IO header files.

Interrupt Service Routines (ISRs)

       As mentioned above, the C language Standard, strangely, does not specify a standard way of declaring and
       defining an ISR. Hence, every compiler seems to have their own special way of doing so.

       IAR declares an ISR like so:

       #pragma vector=TIMER0_OVF_vect
       __interrupt void MotorPWMBottom()
       {
           // code
       }

       In AVR GCC, you declare an ISR like so:

       ISR(PCINT1_vect)
       {
           //code
       }

       AVR GCC uses the ISR macro to define an ISR. This macro requries the header file:

       #include <avr/interrupt.h>

       The names of the various interrupt vectors are found in the individual processor IO header files that you
       must include with <avr/io.h>.

       Note
           The names of the interrupt vectors in AVR GCC has been changed to match the names of the vectors in
           IAR. This significantly helps in porting applications from IAR to AVR GCC.

Intrinsic Routines

       IAR has a number of intrinsic routine such as

       __enable_interrupts() __disable_interrupts() __watchdog_reset()

       These intrinsic functions compile to specific AVR opcodes (SEI, CLI, WDR).

       There are equivalent macros that are used in AVR GCC, however they are not located in a single include
       file.

       AVR GCC has sei() for __enable_interrupts(), and cli() for __disable_interrupts(). Both of these macros
       are located in <avr/interrupt.h>.

       AVR GCC has the macro wdt_reset() in place of __watchdog_reset(). However, there is a whole Watchdog
       Timer API available in AVR GCC that can be found in <avr/wdt.h>.

Flash Variables

       The C language was not designed for Harvard architecture processors with separate memory spaces. This
       means that there are various non-standard ways to define a variable whose data resides in the Program
       Memory (Flash).

       IAR uses a non-standard keyword to declare a variable in Program Memory:

       __flash int mydata[] = ....

       AVR GCC uses Variable Attributes to achieve the same effect:

       int mydata[] __attribute__((progmem))

       Note
           See the GCC User Manual for more information about Variable Attributes.

       avr-libc provides a convenience macro for the Variable Attribute:

       #include <avr/pgmspace.h>
       .
       .
       .
       int mydata[] PROGMEM = ....

       Note
           The PROGMEM macro expands to the Variable Attribute of progmem. This macro requires that you include
           <avr/pgmspace.h>. This is the canonical method for defining a variable in Program Space.

       To read back flash data, use the pgm_read_*() macros defined in <avr/pgmspace.h>. All Program Memory
       handling macros are defined there.

       There is also a way to create a method to define variables in Program Memory that is common between the
       two compilers (IAR and AVR GCC). Create a header file that has these definitions:

       #if defined(__ICCAVR__) // IAR C Compiler
       #define FLASH_DECLARE(x) __flash x
       #endif
       #if defined(__GNUC__) // GNU Compiler
       #define FLASH_DECLARE(x) x __attribute__((__progmem__))
       #endif

       This code snippet checks for the IAR compiler or for the GCC compiler and defines a macro
       FLASH_DECLARE(x) that will declare a variable in Program Memory using the appropriate method based on the
       compiler that is being used. Then you would used it like so:

       FLASH_DECLARE(int mydata[] = ...);

Non-Returning main()

       To declare main() to be a non-returning function in IAR, it is done like this:

       __C_task void main(void)
       {
           // code
       }

       To do the equivalent in AVR GCC, do this:

       void main(void) __attribute__((noreturn));

       void main(void)
       {
           //...
       }

       Note
           See the GCC User Manual for more information on Function Attributes.

       In AVR GCC, a prototype for main() is required so you can declare the function attribute to specify that
       the main() function is of type 'noreturn'. Then, define main() as normal. Note that the return type for
       main() is now void.

Locking Registers

       The IAR compiler allows a user to lock general registers from r15 and down by using compiler options and
       this keyword syntax:

       __regvar __no_init volatile unsigned int filteredTimeSinceCommutation @14;

       This line locks r14 for use only when explicitly referenced in your code thorugh the var name
       'filteredTimeSinceCommutation'. This means that the compiler cannot dispose of it at its own will.

       To do this in AVR GCC, do this:

       register unsigned char counter asm("r3");

       Typically, it should be possible to use r2 through r15 that way.

       Note
           Do not reserve r0 or r1 as these are used internally by the compiler for a temporary register and for
           a zero value.

           Locking registers is not recommended in AVR GCC as it removes this register from the control of the
           compiler, which may make code generation worse. Use at your own risk.