NAME

       avr_fuse - <avr/fuse.h>: Fuse Support

SYNOPSIS

Detailed Description

       Introduction

       The Fuse API allows a user to specify the fuse settings for the specific AVR device they
       are compiling for. These fuse settings will be placed in a special section in the ELF
       output file, after linking.

       Programming tools can take advantage of the fuse information embedded in the ELF file, by
       extracting this information and determining if the fuses need to be programmed before
       programming the Flash and EEPROM memories. This also allows a single ELF file to contain
       all the information needed to program an AVR.

       To use the Fuse API, include the <avr/io.h> header file, which in turn automatically
       includes the individual I/O header file and the <avr/fuse.h> file. These other two files
       provides everything necessary to set the AVR fuses.

       Fuse API

       Each I/O header file must define the FUSE_MEMORY_SIZE macro which is defined to the number
       of fuse bytes that exist in the AVR device.

       A new type, __fuse_t, is defined as a structure. The number of fields in this structure
       are determined by the number of fuse bytes in the FUSE_MEMORY_SIZE macro.

       If FUSE_MEMORY_SIZE == 1, there is only a single field: byte, of type unsigned char.

       If FUSE_MEMORY_SIZE == 2, there are two fields: low, and high, of type unsigned char.

       If FUSE_MEMORY_SIZE == 3, there are three fields: low, high, and extended, of type
       unsigned char.

       If FUSE_MEMORY_SIZE > 3, there is a single field: byte, which is an array of unsigned char
       with the size of the array being FUSE_MEMORY_SIZE.

       A convenience macro, FUSEMEM, is defined as a GCC attribute for a custom-named section of
       '.fuse'.

       A convenience macro, FUSES, is defined that declares a variable, __fuse, of type __fuse_t
       with the attribute defined by FUSEMEM. This variable allows the end user to easily set the
       fuse data.

       Note
           If a device-specific I/O header file has previously defined FUSEMEM, then FUSEMEM is
           not redefined. If a device-specific I/O header file has previously defined FUSES, then
           FUSES is not redefined.

       Each AVR device I/O header file has a set of defined macros which specify the actual fuse
       bits available on that device. The AVR fuses have inverted values, logical 1 for an
       unprogrammed (disabled) bit and logical 0 for a programmed (enabled) bit. The defined
       macros for each individual fuse bit represent this in their definition by a bit-wise
       inversion of a mask. For example, the FUSE_EESAVE fuse in the ATmega128 is defined as:

       #define FUSE_EESAVE      ~_BV(3)

       Note
           The _BV macro creates a bit mask from a bit number. It is then inverted to represent
           logical values for a fuse memory byte.

       To combine the fuse bits macros together to represent a whole fuse byte, use the bitwise
       AND operator, like so:

       (FUSE_BOOTSZ0 & FUSE_BOOTSZ1 & FUSE_EESAVE & FUSE_SPIEN & FUSE_JTAGEN)

       Each device I/O header file also defines macros that provide default values for each fuse
       byte that is available. LFUSE_DEFAULT is defined for a Low Fuse byte. HFUSE_DEFAULT is
       defined for a High Fuse byte. EFUSE_DEFAULT is defined for an Extended Fuse byte.

       If FUSE_MEMORY_SIZE > 3, then the I/O header file defines macros that provide default
       values for each fuse byte like so: FUSE0_DEFAULT FUSE1_DEFAULT FUSE2_DEFAULT FUSE3_DEFAULT
       FUSE4_DEFAULT ....

       API Usage Example

       Putting all of this together is easy. Using C99's designated initializers:

       #include <avr/io.h>

       FUSES =
       {
           .low = LFUSE_DEFAULT,
           .high = (FUSE_BOOTSZ0 & FUSE_BOOTSZ1 & FUSE_EESAVE & FUSE_SPIEN & FUSE_JTAGEN),
           .extended = EFUSE_DEFAULT,
       };

       int main(void)
       {
           return 0;
       }

       Or, using the variable directly instead of the FUSES macro,

       #include <avr/io.h>

       __fuse_t __fuse __attribute__((section (".fuse"))) =
       {
           .low = LFUSE_DEFAULT,
           .high = (FUSE_BOOTSZ0 & FUSE_BOOTSZ1 & FUSE_EESAVE & FUSE_SPIEN & FUSE_JTAGEN),
           .extended = EFUSE_DEFAULT,
       };

       int main(void)
       {
           return 0;
       }

       If you are compiling in C++, you cannot use the designated intializers so you must do:

       #include <avr/io.h>

       FUSES =
       {
           LFUSE_DEFAULT, // .low
           (FUSE_BOOTSZ0 & FUSE_BOOTSZ1 & FUSE_EESAVE & FUSE_SPIEN & FUSE_JTAGEN), // .high
           EFUSE_DEFAULT, // .extended
       };

       int main(void)
       {
           return 0;
       }

       However there are a number of caveats that you need to be aware of to use this API
       properly.

       Be sure to include <avr/io.h> to get all of the definitions for the API. The FUSES macro
       defines a global variable to store the fuse data. This variable is assigned to its own
       linker section. Assign the desired fuse values immediately in the variable initialization.

       The .fuse section in the ELF file will get its values from the initial variable assignment
       ONLY. This means that you can NOT assign values to this variable in functions and the new
       values will not be put into the ELF .fuse section.

       The global variable is declared in the FUSES macro has two leading underscores, which
       means that it is reserved for the 'implementation', meaning the library, so it will not
       conflict with a user-named variable.

       You must initialize ALL fields in the __fuse_t structure. This is because the fuse bits in
       all bytes default to a logical 1, meaning unprogrammed. Normal uninitialized data defaults
       to all locgial zeros. So it is vital that all fuse bytes are initialized, even with
       default data. If they are not, then the fuse bits may not programmed to the desired
       settings.

       Be sure to have the -mmcu=device flag in your compile command line and your linker command
       line to have the correct device selected and to have the correct I/O header file included
       when you include <avr/io.h>.

       You can print out the contents of the .fuse section in the ELF file by using this command
       line:

       avr-objdump -s -j .fuse <ELF file>

        The section contents shows the address on the left, then the data going from lower
       address to a higher address, left to right.

Author

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