Datasheet

233
7734Q–AVR–02/12
AT90PWM81/161
20. Boot Loader Support – Read-While-Write Self-Programming
In AT90PWM81/161, the Boot Loader Support provides a real Read-While-Write Self-Program-
ming 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 configured 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.
20.1 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
• Optimized Page
(1)
Size
• Code Efficient Algorithm
• Efficient Read-Modify-Write Support
Note: 1. A page is a section in the Flash consisting of several bytes (see Table 21-12 on page 254)
used during programming. The page organization does not affect normal operation.
20.2 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 20-2 on page 236). The size of the different sections is configured by
the BOOTSZ Fuses as shown in Table 20-7 on page 246 and Figure 20-2 on page 236. These
two sections can have different level of protection since they have different sets of Lock bits.
20.2.1 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 20-2 on page 237. The Application section can never store any
Boot Loader code since the SPM instruction is disabled when executed from the Application
section.
20.2.2 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
Lock bits (Boot Lock bits 1), see Table 20-3 on page 237.
20.3 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