24. Boot Loader Support – Read-while-write Self-Programming
ATmega16/32/64/M1/C1
In ATmega16/32/64/M1/C1, the boot loader support provides a real read-while-write self-programming mechanism for
downloading and uploading program code by the MCU itself. This feature allows flexible application software updates
controlled by the MCU using a flash-resident boot loader program. The boot loader program can use any available data
interface and associated protocol to read code and write (program) that code into the Flash memory, or read the code from
the program memory. The program code within the boot loader section has the capability to write into the entire flash,
including the boot loader memory. The boot loader can thus even modify itself, and it can also erase itself from the code if
the feature is not needed anymore. The size of the boot loader memory is configurable with fuses and the boot loader has
two separate sets of boot lock bits which can be set independently. This gives the user a unique flexibility to select different
levels of protection.
24.1 Boot Loader Features
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Read-while-write self-programming
Flexible boot memory size
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High security (separate boot lock bits for a flexible protection)
Separate fuse to select reset vector
Optimized page(1) size
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Code efficient algorithm
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Efficient read-modify-write support
Note:
1. A page is a section in the flash consisting of several bytes (see Table 25-12 on page 260) used during pro-
gramming. The page organization does not affect normal operation.
24.2 Application and Boot Loader Flash Sections
The flash memory is organized in two main sections, the application section and the boot loader section (see Figure 24-2 on
page 243). The size of the different sections is configured by the BOOTSZ fuses as shown in Table 24-7 on page 251 and
Figure 24-2 on page 243. These two sections can have different level of protection since they have different sets of lock bits.
24.2.1 Application Section
The application section is the section of the Flash that is used for storing the application code. The protection level for the
application section can be selected by the application boot lock bits (boot lock bits 0), see Table 24-2 on page 244. The
application section can never store any boot loader code since the SPM instruction is disabled when executed from the
application section.
24.2.2 BLS – Boot Loader Section
While the application section is used for storing the application code, the The boot loader software must be located in the
BLS since the SPM instruction can initiate a programming when executing from the BLS only. The SPM instruction can
access the entire flash, including the BLS itself. The protection level for the boot loader section can be selected by the boot
loader lock bits (boot lock bits 1), see Table 24-3 on page 244.
24.3 Read-while-write and no Read-while-write Flash Sections
Whether the CPU supports read-while-write or if the CPU is halted during a Boot Loader software update is dependent on
which address that is being programmed. In addition to the two sections that are configurable by the BOOTSZ Fuses as
described above, the flash is also divided into two fixed sections, the read-while-write (RWW) section and the no read-while-
write (NRWW) section. The limit between the RWW- and NRWW sections is given in Table 24-8 on page 252 and Figure 24-
2 on page 243. The main difference between the two sections is:
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When erasing or writing a page located inside the RWW section, the NRWW section can be read during the
operation.
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When erasing or writing a page located inside the NRWW section, the CPU is halted during the entire operation.
ATmega16/32/64/M1/C1 [DATASHEET]
241
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
