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8331B–AVR–03/12
Atmel AVR XMEGA AU
application table section can be selected by the boot lock bits for this section. The possibilities
for different protection levels on the application section and the application table section enable
safe parameter storage in the program memory. If this section is not used for data, application
code can reside here.
4.3.3
Boot Loader Section
While the application section is used for storing the application code, the boot loader software
must be located in the boot loader section because the SPM instruction can initiate program-
ming when executing from this section. The SPM instruction can access the entire flash,
including the boot loader section itself. The protection level for the boot loader section can be
selected by the boot loader lock bits. If this section is not used for boot loader software, applica-
tion code can be stored here.
4.3.4
Production Signature Row
The production signature row is a separate memory section for factory programmed data. It con-
tains calibration data for functions such as oscillators and analog modules. Some of the
calibration values will be automatically loaded to the corresponding module or peripheral unit
during reset. Other values must be loaded from the signature row and written to the correspond-
ing peripheral registers from software. For details on calibration conditions such as temperature,
voltage references, etc. refer to device datasheet.
The production signature row also contains an ID that identifies each microcontroller device type
and a serial number for each manufactured device. The serial number consists of the production
lot number, wafer number, and wafer coordinates for the device.
The production signature row cannot be written or erased, but it can be read from application
software and external programmers.
4.3.5
User Signature Row
The user signature row is a separate memory section that is fully accessible (read and write)
from application software and external programmers. It is one flash page in size, and is meant
for static user parameter storage, such as calibration data, custom serial number, identification
numbers, random number seeds, etc. This section is not erased by chip erase commands that
erase the flash, and requires a dedicated erase command. This ensures parameter storage dur-
ing multiple program/erase operations and on-chip debug sessions.
4.4
Fuses and Lockbits
The fuses are used to configure important system functions, and can only be written from an
external programmer. The application software can read the fuses. The fuses are used to config-
ure reset sources such as brownout detector and watchdog, startup configuration, JTAG enable,
and JTAG user ID.
The lock bits are used to set protection levels for the different flash sections (i.e., if read and/or
write access should be blocked). Lock bits can be written by external programmers and applica-
tion software, but only to stricter protection levels. Chip erase is the only way to erase the lock
bits. To ensure that flash contents are protected even during chip erase, the lock bits are erased
after the rest of the flash memory has been erased.
An unprogrammed fuse or lock bit will have the value one, while a programmed fuse or lock bit
will have the value zero.
Both fuses and lock bits are reprogrammable like the flash program memory.
