<avr/fuse.h>: Fuse Support(3avr) | avr-libc | <avr/fuse.h>: Fuse Support(3avr) |
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<avr/fuse.h>: Fuse Support - IntroductionIf 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
#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.
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