<avr/lock.h>: Lockbit Support

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<avr/lock.h>: Lockbit Support

    \par Introduction

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

    Programming tools can take advantage of the lockbit information embedded in
    the ELF file, by extracting this information and determining if the lockbits
    need to be programmed after 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 Lockbit API, include the <avr/io.h> header file, which in turn
    automatically includes the individual I/O header file and the <avr/lock.h>
    file. These other two files provides everything necessary to set the AVR
    lockbits.

    \par Lockbit API

    Each I/O header file may define up to 3 macros that controls what kinds
    of lockbits are available to the user.

    If __LOCK_BITS_EXIST is defined, then two lock bits are available to the
    user and 3 mode settings are defined for these two bits.

    If __BOOT_LOCK_BITS_0_EXIST is defined, then the two BLB0 lock bits are
    available to the user and 4 mode settings are defined for these two bits.

    If __BOOT_LOCK_BITS_1_EXIST is defined, then the two BLB1 lock bits are
    available to the user and 4 mode settings are defined for these two bits.

    If __BOOT_LOCK_APPLICATION_TABLE_BITS_EXIST is defined then two lock bits
    are available to set the locking mode for the Application Table Section
    (which is used in the XMEGA family).

    If __BOOT_LOCK_APPLICATION_BITS_EXIST is defined then two lock bits are
    available to set the locking mode for the Application Section (which is used
    in the XMEGA family).

    If __BOOT_LOCK_BOOT_BITS_EXIST is defined then two lock bits are available
    to set the locking mode for the Boot Loader Section (which is used in the
    XMEGA family).

    The AVR lockbit modes have inverted values, logical 1 for an unprogrammed
    (disabled) bit and logical 0 for a programmed (enabled) bit. The defined
    macros for each individual lock bit represent this in their definition by a
    bit-wise inversion of a mask. For example, the LB_MODE_3 macro is defined
    as:
    @code
    #define LB_MODE_3  (0xFC)

`

    To combine the lockbit mode macros together to represent a whole byte,
    use the bitwise AND operator, like so:
    @code
    (LB_MODE_3 & BLB0_MODE_2)
    \endcode

    <avr/lock.h> also defines a macro that provides a default lockbit value:
    LOCKBITS_DEFAULT which is defined to be 0xFF.

    See the AVR device specific datasheet for more details about these
    lock bits and the available mode settings.

    A convenience macro, LOCKMEM, is defined as a GCC attribute for a
    custom-named section of ".lock".

    A convenience macro, LOCKBITS, is defined that declares a variable, __lock,
    of type unsigned char with the attribute defined by LOCKMEM. This variable
    allows the end user to easily set the lockbit data.

    \note If a device-specific I/O header file has previously defined LOCKMEM,
    then LOCKMEM is not redefined. If a device-specific I/O header file has
    previously defined LOCKBITS, then LOCKBITS is not redefined. LOCKBITS is
    currently known to be defined in the I/O header files for the XMEGA devices.

    \par API Usage Example

    Putting all of this together is easy:

    @code
    #include <avr/io.h>

    LOCKBITS = (LB_MODE_1 & BLB0_MODE_3 & BLB1_MODE_4);

    int main(void)
    {
        return 0;
    }
    \endcode

    Or:

    @code
    #include <avr/io.h>

    unsigned char __lock __attribute__((section (".lock"))) =
        (LB_MODE_1 & BLB0_MODE_3 & BLB1_MODE_4);

    int main(void)
    {
        return 0;
    }
    \endcode

    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 LOCKBITS macro defines a global variable to store the lockbit data. This
    variable is assigned to its own linker section. Assign the desired lockbit
    values immediately in the variable initialization.

    The .lock 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 .lock section.

    The global variable is declared in the LOCKBITS 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 the lockbit variable to some meaningful value, even
    if it is the default value. This is because the lockbits default to a
    logical 1, meaning unprogrammed. Normal uninitialized data defaults to all
    locgial zeros. So it is vital that all lockbits are initialized, even with
    default data. If they are not, then the lockbits may not programmed to the
    desired settings and can possibly put your device into an unrecoverable
    state.

    Be sure to have the -mmcu=<em>device</em> 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 .lock section in the ELF file by
    using this command line:
    @code
    avr-objdump -s -j .lock <ELF file>
    \endcode

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