Введение
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 macro, FUSEMEM, is
defined as a GCC attribute for a custom-named section of ".fuse".
Finally, a 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.
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 EESAVE
fuse in the ATmega128 is defined as:
#define EESAVE ~_BV(3)
Примечание:
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:
(BOOTSZ0 & BOOTSZ1 & EESAVE & SPIEN & 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.
Пример
Использования API
Putting all
of this together is easy:
#include <avr/io.h>
FUSES = { .low = LFUSE_DEFAULT, .high = (BOOTSZ0 & BOOTSZ1 & EESAVE & SPIEN & JTAGEN), .extended = EFUSE_DEFAULT, }; 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.
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Automatically generated by Doxygen 1.5.2 on 21 Dec 2007.