
206
4378C–AVR–09/08
AT90PWM1
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.
21.1
Boot Loader Features
Read-While-Write Self-Programming
Flexible Boot Memory Size
High Security (Separate Boot Lock Bits for a Flexible Protection)
Separate Fuse to Select Reset Vector
Code Efficient Algorithm
Efficient Read-Modify-Write Support
Note:
during programming. The page organization does not affect normal operation.
21.2
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 21-2). The size of the different sections is configured by the
have different level of protection since they have different sets of Lock bits.
21.2.1
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
Loader code since the SPM instruction is disabled when executed from the Application section.
21.2.2
BLS – Boot Loader Section
While the Application section is used for storing the application code, the The Boot Loader soft-
ware 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
21.3
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 soft-
ware 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 71When erasing or writing a page located inside the RWW section, the NRWW section can be
read during the operation.
When erasing or writing a page located inside the NRWW section, the CPU is halted during
the entire operation.
Note that the user software can never read any code that is located inside the RWW section dur-
ing a Boot Loader software operation. The syntax “Read-While-Write section” refers to which